Weekly Review: April 3-9, 2013

Prepared by: Penny Goddard

Weather:

Mild weather prevailed at the start of the period, with temperatures remaining above zero below about 1700 m in many areas. Light snowfall was recorded in most areas on April 3 and 4. On April 5, a front arrived on the south coast, with heavy precipitation. This frontal system then pushed inland, bringing rain, snow and strong SW winds to most regions, with freezing levels near treeline. In the wake of the front, temperatures became somewhat cooler (dropping below 0 during overnight periods), with mainly unsettled conditions.

Snowpack:

At the start of the period, typical spring time conditions dominated. Surface snow was weak when wet during warm parts of the day, or at low elevations where no overnight refreeze occurred. In the NW Coastal region, a layer of surface hoar buried around March 9 was proving especially persistent. In other regions, this layer had become very unlikely to be triggered. Large cornices were prolific and became unstable during warm temperatures.

The light snowfall on April 3 or 4 buried a newly developed layer of surface hoar (‘Easter surface hoar’) on high-elevation north aspects in the Columbia Mountains. The main frontal system around April 5/6 led to new wind slabs and storm slabs at alpine elevations; and added rain to the snowpack below treeline. Below about 1800 m, the snowpack is reported to be isothermal, or heading that way, in many areas.

Avalanche Activity:

The snow and wind associated with the front brought a natural cycle of wind slabs and storm slabs, mainly on lee aspects. Most were in the size 1-2 range, with isolated events at size 3. With the onset of rain, numerous loose wet avalanches were reported at low elevations.

The ‘Easter surface hoar’ layer became reactive in the Columbia Mountains, with numerous skier-remote triggered slabs in specific areas (mostly high-elevation north aspects) from up to 150 m away, starting on April 7. By April 9, this type of activity had slowed, but persistent slab avalanches on this layer may linger as a concern in specific terrain.

Cornice fall continued to be reported throughout the period, in some cases triggering slabs on slopes below. Glide slabs to size 3 have also been failing, especially in coastal regions.

Incidents:

A skier was caught in a size 1 slab on a NE wind-loaded aspect in the backcountry near Revelstoke Mountain Resort on April 7. Although it was a small avalanche, there was serious exposure to a drop below and the skier was fortunate to escape the slide.

Skiers also triggered avalanches in the Duffey Lake area on April 7 and 8, in one case carrying a skier for about 20 m.

For more information on all reported incidents, please see our Incident Report Database.

Weekly Review: Mar 21 2013- Mar 27 2013

Prepared by: Joe Lammers

Weather:

At the beginning of the review period, some regions saw moderate to locally heavy snowfall while the Columbia Mountains were already under the influence of a drying ridge. From Saturday onward, most areas of the province saw mainly clear skies, strong diurnal temperature fluctuations and no precipitation.

Snowpack:

Snowfall from the beginning of the review period has now bonded well to old surfaces in most areas; however, in the far northwest a new round of wind and snowfall has created fresh windslabs at higher elevations.  In general, current snow surfaces include settled powder, facets and surface hoar on shaded slopes. On solar aspects and at lower elevations you are likely to find wet snow or a crust (depending on the time of day). Cornices are large and have become weak with solar warming.

Up to a metre below the surface is a surface hoar/crust interface buried between March 9^th and 10^th.  Although this layer is gaining strength, it remains a concern among professionals, particularly with warm temperatures and the higher probability of large cornice falls acting as a trigger.

With the exception of the Yukon and Bighorn regions, where significant basal facets remain, mid and lower snowpack layers in most areas are generally considered well bonded.

Avalanche Activity:

At the beginning of the period, there was a fairly widespread round of storm slab activity that occurred in response to new snow and wind.

In the northwest, the March 9^th surface hoar has consistently produced explosives controlled and skier triggered slab avalanches. Elsewhere, the general trend has shifted to cornice fall and wet loose avalanche activity; however, destructive persistent slabs have continued to fail in isolated terrain in the Columbia Mountains. In most cases they required a large trigger such as a cornice fall, although in isolated cases they were triggered by human activity.

Incidents:

There were two fatalities that occurred over the last week:

-On March 23^rd a snowmobile triggered a size 3 slab avalanche which took place in the Southern end of the Purcell Mtn. Range. The avalanche, which is thought to have failed on the March 10^th surface hoar, involved three people. The deceased was buried 1.5 metres.

-On March 24^th a skier triggered a size 2 slab avalanche in the Roger’s Pass area. This avalanche, which also released on a surface hoar/crust interface, resulted in the fatality of a backcountry skier. The avalanche was triggered from a thin, rocky spot on a shallow, scoured slope. The skier was buried 195cm down in a terrain trap.

For more information on all reported incidents, please see our Incident Report Database.

Weekly Review: Mar 14 2013- Mar 20 2013

Prepared by: James Floyer

Weather:

Through much of the past week, significant precipitation fell in many regions, first as a warm, wet storm, and then as heavy convective snowfall from the unstable air mass left behind after the storm. In some areas, the snowfall amounts experienced during the convective precipitation events were quite dramatic. Areas that saw particularly high amounts were the Coquihalla (80 cm), the Lizard range (80 cm), and Kootenay Pass (40 cm). Air temperatures started out warm and then very gradually cooled down. Many areas experienced good sunshine on Monday and/or Tuesday (18^th /19^th ).

Snowpack:

The warm, wet storm left behind moist snow in many areas below approximately 1900 m. In some areas, this froze fairly rapidly into a crust, that was initially breakable and weak, but gradually gained strength as the temperatures slowly cooled. In other areas, an insulating layer of snow further slowed the development of the crust. Due to the very high spatial variability of the snowfall this week, anywhere from 30 to 90 cm new snow was deposited on top of this interface. The snow on top of this interface was initially reactive, but the bond strengthened relatively quickly and does not appear to have left a persistent weak layer in most areas.

A previous weak layer of surface hoar from March 9^th or 10^th remained touchy, particularly at higher elevations. This layer is best preserved in the Northwest regions, where slightly lower amounts of colder snow fell. It can still be found in the alpine in most other regions too.

With the exception of the Yukon and Bighorn regions, where significant basal facets remain, mid and lower snowpack layers in most areas are generally considered well bonded.

Avalanche Activity:

An avalanche cycle was noted at the beginning of the week, with many regions reporting slab avalanches up to size 3. Most avalanches at the beginning of the week ran on the March 9^th / 10^th interface. Some large low elevation avalanches were observed running to ground due to an isothermal snowpack. Further natural activity was noted during the subsequent convective precipitation, generally soft slab and loose snow avalanches up to size 2.5. In most regions, this activity was associated with storm slabs above the moist snow interface, with the occasional event releasing or stepping down to the March 9^th / 10^th layer, particularly at higher elevations. However, in the Northwest region, the March 9^th / 10^th layer remained much more reactive, and avalanches continued to release through the period on this layer.

Incidents:

There were two very notable close calls involving human-triggered avalanches. In both cases, one group remotely triggered an avalanche onto another group who were grouped up in the runout zone at the bottom of the path.

• On March 17^th a size 3.5 avalanche was triggered in Cherry Bowl in the Shames backcountry near Terrace. Three skiers were fully buried and recovered alive with minor injuries. An account of the incident with pictures can be found here.
• On March 19^th a size 2.5 avalanche was triggered above the Spearhead Glacier in the Whistler/Blackcomb backcountry. Two people were partially buried and one person completely buried. All were recovered alive. Click here for an account of this incident.

Other events also occurred that resulted in near misses.

• Skier-triggered avalanche March 14^th in the Duffey Lake area – more details:
• Skier-triggered avalanche March 15^th in the Esplanades north of Golden – more details:

Weekly Review: March 7 – March 13, 2013

Prepared by: Peter Marshall

Weather:

A ridge of high pressure brought drier conditions and sunny skies to most regions for March 8-10. Some areas received convective snowfall, but accumulations were generally light. Temperatures were cool with periods of strong west or northwesterly winds. The pattern changed significantly by March 11 or 12. A strong and persistent southwesterly flow developed resulting in wet and mild conditions for the southern half of the province. At the same time, the northern regions were influenced by a weak northwesterly flow and an invading arctic front from the northeast. This led to drier conditions and much cooler temperatures than the southern half of the province.

Snowpack:

In the Northwest the snowpack has been generally strong with mainly wind slab issues from periods of strong southwest-northwest winds. 10-20 cm of new snow had fallen by March 13^th burying a widespread layer of surface hoar. At this point the low density new snow had not developed into a slab and was not reactive.

The southern part of the province saw little snow and at least periods of sunshine for the first part of the week. This allowed the Feb 20 (S. Coast) and Feb 12 (Interior) persistent weak layer, now down 100-150+ cm, to gain strength. Activity on these layers decreased significantly through the week, but they could wake up again with heavy loading.

The dry period also resulted in a new layer of surface hoar or facets being buried (March 9 and 11). This layer was reported to be very widespread throughout most regions. By March 13^th this weakness was buried by 30-60 cm of heavy new snow and was reported to be very touchy in most areas.

Avalanche Activity:

In general, avalanche activity tapered off throughout the period. Most natural activity was a result of solar radiation and/ or daytime warming. There were reports of loose wet activity on solar aspects, and natural cornice falls, some of which triggered slabs up to size 3.

Activity on the mid February persistent weak layer became much more sporadic. There were still a few reports of rider triggered avalanches on this layer from the South Coast Inland, Kootenays and Columbias, but in general this weakness became much more difficult to trigger.

Widespread natural and human triggered avalanches were reported again by March 12^th with a return to stormy and mild weather in the south. Most avalanches were releasing easily on a weak layer of surface hoar or facets that were buried around March 10^th.

Incidents:

There were a few incidents this week that resulted in partial burials and/ or injuries. Early in the period there were two incidents reported from the South Coast regions. In one case a rider triggered what was likely a thin wind slab, was caught, deployed their air bag, and ended up being buried up to their waist in the run out zone.

Around March 9^th there were also two reports of skier triggered avalanches in the South Columbia that resulted in injuries. In both these cases they were triggered on west-southwest aspects near treeline (when the sun was out) and likely released on the Feb 12 persistent weakness.

Click here for a summary of these incidents. 

Weekly Review: February 28 – March 6, 2013

Prepared by: Peter Marshall

Weather:

The most significant weather event of the period was a strong pineapple express (or atmospheric river) that blasted most regions with heavy precipitation and rising freezing levels beginning on February 28^th. Most regions received 50-100 mm by March 2^nd, with some regions reporting closer to 150 mm. Freezing levels in the south climbed to 1800-2000 m and 1400-1500 m in the north. The remainder of the periods enjoyed drier conditions, cooler temperatures and sunny breaks.

Snowpack:

The snowpack on the North Coast is primarily described as being well settled and strong with few persistent weaknesses. Outflow winds earlier this week resulted in some reverse loading. There is a late February interface that has popped up in isolated cases on steep rocky slopes. In the Bear Pass and further north there is some concern with large triggers stepping down to the basal weakness creating full depth avalanches.

On the South Coast the primary snowpack feature is a buried layer of surface hoar or a sun crust (Feb. 20) down 60-120 cm. This layer was the primary culprit in the large avalanche cycle last weekend. Thin new wind slabs have also formed from brief northerly winds. Cornices are large and ripe.

In the Interior the most active persistent weakness was the Feb. 12 surface hoar and/or sun crust. This layer is now down 60-120 cm. There are a series of other crusts and surface hoar layers between 100 and 200 cm deep but these have been primarily dormant. Cornices are also very large in most of the Interior.

Avalanche Activity:

A large and widespread natural avalanche cycle occurred in all regions in response to the strong system that pushed through between February 28 and March 2.

In the Northwest coastal region, widespread natural activity, including size 3-3.5 slabs which ran full path, was reported by Sunday. Conditions improved quickly after the storm passed but there were still isolated reported of size 3-3.5 glide slabs and cornice or ice fall triggered slabs in steep terrain.

In the Columbia and Rocky Mountains, numerous natural and human-triggered avalanches were observed, failing within the storm snow or on the Feb 12 interface at the base of the storm snow. Many were size 2-3.5 from a variety of aspects at all elevations. There were also a few reports of size 4-4.5 avalanches that redefined path boundaries and knocked over mature forest. After the storm passed there were a few reports of human triggered avalanches, including one size 3 skier triggered avalanche from Rogers Pass on March 5.

Similar avalanche activity occurred on the South Coast. Numerous avalanches up to size 4 were reported during and after the storm. Some of these ran full path and knocked over mature timber. Most of the larger events appeared to have failed on the Feb. 20 surface hoar or sun crust down 60-150 cm.

Incidents:

There were no new reports submitted to the incident database this week; however, there were a few notable stories trickling in. On the South Coast there was one report of a size 2 skier triggered avalanche in the Vantage Ridge area off the Duffey Lake road. This occurred the day after the storm passed (March 3) and resulted in a partial burial with injuries. There were also several reports of skier triggered avalanches up to size 2 in the Whistler-Blackcomb backcountry on this day.

On March 5 there was a report of a size 3 skier triggered avalanche off Cheops Mountain in Rogers Pass. This avalanche was triggered by a group while ascending. Fortunately no one was involved. 

Weekly Review: February 14 - 20, 2013

Prepared by: Penny Goddard

Weather:

An active series of frontal systems brought snow and wind to most regions this week. On Friday Feb 15, the North Coast received about 70 cm of snow, which was mainly isolated to the coast. Winds were moderate to strong and variable in direction. On Saturday, Sunday and Monday, a mixture of frontal and convective precipitation brought 20-60 cm of snow to the Coast, Columbia, and Rocky Mountains with periods of moderate to strong winds. The bulk of these storms were confined to north of the Kootenay Boundary region, while a ridge of high pressure kept the far southern regions much drier, sunnier and less windy.

Snowpack:

The North Coast developed a direct-action storm slab and wind slab problem in response to Friday’s rapid loading. In the Interior, light to moderate snowfall built a variable storm slab of around 20-60 cm above the touchy Feb 12 interface. This interface consists of surface hoar and/or a sun crust and seems persistent. Wind slabs also developed above this interface on a variety of slope aspects. New cornices developed in many areas.

Deeper weak layers (the Feb 3-4 interface, down about 70-80 cm; and the late Jan interface, down about 110 cm) are now generally considered unlikely to be triggered, expect perhaps with a large load like an avalanche in motion or cornice fall; or from a thin-spot trigger point.

Avalanche Activity:

A cycle of natural and human-triggered avalanche activity was observed in most areas in response to storm slab development above the Feb 12 weakness and/or wind-loading.

In the Northwest coastal region, extensive natural activity, including size 3.5 slabs which ran full path, occurred on Friday. Below treeline, rain led to slab and loose wet avalanches on all aspects, while storm slabs and cornices failed at alpine and treeline elevations. This cycle ended by Sunday. In the Northwest Inland region, two size 3 slabs failed naturally at ground.

In the Columbia Mountains, numerous natural and human-triggered avalanches were observed, failing within the storm snow or on the Feb 12 interface at the base of the storm snow. Many were size 1-3 and they ran on slopes of various aspects and elevations. Heavy loads like cornice fall and explosives triggered storm slabs, but mainly they didn’t step down deeper than the Feb 12 interface. An exception was found in the South Columbia, when a cornice fall triggered an avalanche on the Jan 23 layer, down 80 cm. A handful of remotely-triggered slabs indicated the touchy nature of the buried interface in some locations. This activity continued all week, but seemed to be slowing a little bit by the end of the week.

On the South Coast, warmth and sun triggered a natural cycle to size 2 on Friday and a few human involvements were reported (see incidents below). In the Kootenay Boundary, South Rockies and Lizard Range, only relatively minor avalanche activity was reported. This was limited to loose dry sluffing and isolated pockets of wind slab or storm slab.

Incidents:

A smattering of incidents was recorded on the South Coast this week: On Saturday 16 Feb, a skier accidentally triggered a size 1 slab near Pemberton. A skier also triggered a size 2 slab north of Whistler in a steep couloir feature. On 17 Feb, skiers triggered two separate slabs in one incident on Birkenhead Peak. One person was carried down the slope and lost some gear, but was unharmed. On 19 Feb, a skier accidentally triggered a slab on a west aspect on Rainbow Mountain.

On Monday Feb 18, a size 2 skier-triggered avalanche occurred near Invermere, which resulted in a fatality. The avalanche ran on a south/southeast aspect at 2700m and is thought to have failed at the February 11/12 interface.

More details on these incidents can be found here.

Weekly Review: February 7 - 13, 2013

Prepared by: Peter Marshall

Weather:

The general weather pattern was a large ridge of high pressure off the coast of BC driving weather north to the Gulf of Alaska. The position of the ridge allowed clouds and some moisture to push over the ridge and onto the north coast sliding down the east side of the ridge in a northwesterly flow pattern over BC. This meant that most regions saw at least some cloud with northern regions receiving higher amounts of snow. Moderate west-northwest winds were also a factor during the period.

There were at least a couple days where the ridge strengthened enough to allow for more sunshine. This was timed well with the BC long weekend no doubt making it a busy weekend in the mountains.

The Northwest and Yukon saw the most active weather pattern. The ridge of high pressure flattened enough at times to allow stronger systems to make landfall on the northwest corner of BC. This allowed for a significant storm to affect the White Pass region last weekend with very strong southwest winds and snowfall amounts of over 20 cm. The Northwest regions and parts of the North Rockies saw snow accumulations of up to 50 cm with a system that rolled through on the 12^th.

Snowpack:

In the south, 20-40 cm of snow fell early in the week. Periods of moderate or strong southwest-northwest winds also formed new wind slabs in lee terrain. This new snow buried another weak layer (Feb 3-4 interface) of surface hoar, facets, and/or a sun crust. This interface seemed to be most reactive in the Columbias and South Coast – Inland, where there was enough of an overlying slab.

The late January persistent weak layer (surface hoar, facets, and/or sun crust) has gained strength and proves to be much more difficult to trigger. It is now down 50-100 cm and is producing either hard or no results in compression tests. It seems to be most reactive in the eastern regions (Purcells, Rockies) where the overlying slab is less thick. There is not much talk about the early January surface hoar layer anymore. It is still found in isolated areas but seems fairly well bonded and generally unreactive. 

In the north, the snowpack concerns are more direct action. Fresh wind slabs and storm slabs have formed in the past few days. The late January melt-freeze crust is still mentioned as a concern in coastal areas near Bear Pass. This interface is down almost 100 cm.  

Avalanche Activity:

Ongoing natural avalanche activity occurred, decreasing in frequency throughout the period (except the North Coast and White Pass where an avalanche cycle occurred late in the period). Some events were in response to moderate snow loading early in the week, some were new wind slabs failing in steep alpine terrain, and a few were cornice triggered slab avalanches on northerly aspects. Avalanche activity also increased on solar aspects when the sun did shine. Most of the solar activity was loose sluffing, but there were reports of a few larger slab avalanches as well. Most regions are reporting large cornices and there was certainly an increase in cornice failures this week.

Skier or rider triggered avalanches also continued this week. There were 3 separate reports of size 2-2.5 skier triggered slab avalanches that resulted in full burials. These events occurred in the Columbia Mountains. Fortunately all events had positive outcomes. There were also a few reports of remotely triggered avalanches in the interior. Accidentally triggered avalanches were not limited strictly to the Columbias. There was at least one size 1.5-2 skier-triggered slab avalanche reported in almost every region this week.

Incidents:

A few new reports came into the incident database this week. The most notable incident came from Nova Scotia. One person was buried up to their chest after triggering a slab avalanche in the Wentworth Valley north of Truro. This event occurred during the large storm that hit the eastern seaboard on February 9. 

Weekly Review: January 31, 2013- February 6, 2013

Prepared by: Peter Marshall

Weather:

A ridge of high pressure brought sunny skies and mild temperatures to most regions during the first part of the week. The weather pattern changed on February 2^nd or 3^rd as the first in a series of frontal systems pushed in to the coast bringing light to moderate snowfall and a return to near seasonal temperatures.

The generally westerly flow meant that the bulk of the precipitation fell along the coast and the west side of the interior ranges, with much less reaching the Rockies. Coastal regions reported 30-50 cm of storm snow since February 3, while Interior regions generally saw 15-40 cm, and the Rockies received 10-20 cm.

Snowpack:

The new snow buried a sun crust on southerly aspects in most regions, and a new surface hoar layer reported on a variety of aspects primarily in the Interior regions. Strong southwesterly winds were associated with the frontal systems resulting in new wind slab formation and cornice growth on north through east aspects.

The January 23^rd interface, consisting of a sun crust, facets, and/or surface hoar, is now down 40-80 cm. The surface hoar seems to be most reactive at and below treeline in the interior.  

A deeper layer of buried surface hoar (most prevalent below 1600 m on shady aspects) or a sun crust (on solar aspects) buried sometime between Dec. 31 and Jan. 4, has been largely unreactive, but is still producing sudden shears in isolated areas, particularly where it is shallower.

Avalanche Activity:

Ongoing natural and human-triggered avalanches have occurred throughout the interior ranges this week. Avalanches were generally size 1-2.5 and most failed on the late January sun crust or surface hoar layer. A new storm cycle developed by February 5^th and most activity was reported to be failing on the early February interface.

On the coast, a fairly large avalanche cycle occurred in the Northwest with numerous natural avalanches up to size 3 being reported. Avalanche activity also increased on the South Coast by February 4^th in response to the moderate snowfall and strong southwesterly winds.

Incidents:

There were a few avalanches incidents reported this week. Most were skier or snowmobile triggered avalanches up to size 2 from a variety of regions. No one was reported to be buried or injured in any of these events. 

Weekly Review: Jan 24 2013- Jan 30 2013

Prepared by: James Floyer

Weather:

A heavy dump of snow fell at the beginning of the period in most areas, bringing 20 to 40cm new snow depending on location. Southern coastal and interior ranges received the heaviest amounts, with northern areas initially seeing lower amounts.

Following the initial intense storm, western Canada became dominated by a flow of air out of the northwest, bringing generally cool temperatures and a series of low to moderate intensity storms. These storms predominantly affected northern areas, although the Lizard range in the south also experienced enhanced accumulations of new snow through this period.

Far northern and eastern areas experienced a deep (below -30C near Tumbler Ridge) but short-lived arctic outbreak Monday and/or Tuesday.

Snowpack:

The new snow buried a well-developed surface hoar layer that had been growing on the surface on shady aspects and a sun crust on sun-exposed aspects. The generally cool temperatures helped to keep the new snow light. Slab development has been relatively slow. The exception is in exposed areas where moderate winds from a variety of directions (but predominantly northwest) have blown it into soft wind slabs.

A deeper layer of buried surface hoar (most prevalent below 1600 m on shady aspects) or a sun crust (on solar aspects) buried sometime between Dec. 31 and Jan. 4, has been largely unreactive, but is still producing sudden shears in isolated areas, particularly where it is shallower.

Avalanche Activity:

An avalanche cycle was noted at the beginning of the week, with many loose snow and soft slab avalanches noted, particularly in the Columbia Mountains.  In virtually all cases, only the recent new snow was involved; avalanches did not step down to lower layers. Typically sizes were small, mostly in the size 1 to 1.5 range, with some size 2s and 2.5s reported.

Subsequent avalanche activity has been more sporadic, although each pulse of precipitation brought a handful of generally small natural avalanches running in steep terrain. Pockets of wind slab and steep convex rolls could be ski-cut readily through the period on specific terrain features.

Incidents:

There were no avalanche incidents reported during this period.

Weekly Review: Jan 17 2013- Jan 23 2013

Prepared by: Cam Campbell

Weather:

A high pressure system kept the past week dry for most regions. Temperatures were inverted with above freezing alpine temperatures every day. Valley bottoms were generally below freezing with valley fog, at least in the mornings. Clear skies at higher elevations meant strong solar radiation and overnight radiative cooling.

Snowpack:

Large surface hoar growth and surface faceting continued in sheltered and shady areas, while sun-exposed slopes were subject to melt-freeze cycles. Open unprotected areas are highly wind-effected with scoured areas, hard wind slab, sastrugi, and delicate cornices.

A widespread persistent weakness of facets (treeline and alpine), suncrust (steep solar aspects), and/or surface hoar (shady treeline) buried sometime between Dec. 31 and Jan. 4, has been largely unreactive, but is still producing sudden shears in isolated areas particularly where it is shallower.

Avalanche Activity:

Avalanche activity during the period was predominantly natural wet loose avalanches on steep sun-exposed slopes in the afternoon, and human-triggered dry loose sluffs involving faceted snow on steep shady slopes. There was isolated persistent and deep persistent slab avalanche activity particularly in the Purcells and Rockies with both natural and human triggers. Glide avalanche activity was also prevalent during the latter half of the period.

Incidents:

There was one reported involvement involving backcountry skiers/boarders in Alberta. Both members of the party were caught in the slab avalanche and one went for a 50-100m ride but remained on the surface. Neither sustained injuries.

Weekly Review: Jan 10 2013- Jan 16 2013

Prepared by: Cam Campbell

Weather:

A high pressure system kept the past week relatively dry for all regions. The week ended with four days inverted temperatures with very strong gradients. Despite strong to extreme northerly winds, alpine temperatures rose well above freezing in most regions.

Snowpack:

Surface faceting and surface hoar growth was prevalent at all elevations and on all aspects before strong to extreme winds, above freezing temperatures, and freezing rain in some regions, caused wind slab formation and melt-freeze metamorphism to be the dominant surface snowpack process at treeline and alpine elevations.

A widespread persistent weakness of near-surface facets (treeline and alpine), suncrust (steep solar aspects), and surface hoar (shady treeline) buried sometime between Dec. 31 and Jan. 4, started showing signs of strengthening, but became more active again with warming during the latter half of the period.

Avalanche Activity:

The beginning of the period was the tail end of an avalanche cycle with the transition from natural triggers to artificial triggers. Avalanche activity tapered off after that until significant warming resulted in a regime shift to wet loose activity as well as increased persistent slab activity.

Incidents:

There were no high profile incidents during the period. Some notable incidents include two Size 2 skier accidental slab avalanches with involvements in the South Coast Inland region; one in the Duffy Lake area resulted in injuries.

Weekly Review: Jan 02 2013- Jan 9 2013

Prepared by: ilya storm

Weather:

Last week’s weather block broke down this week with two pacific storms passing through the western mountains. The storm track generally benefitted the South Coast & Columbia Mountains (North & South Columbia, Kootenay-Boundary). Lesser amounts fell in the Northwest Coastal and Cariboos. Northwest Inland, Lizard, and South Rockies were relatively dry. Temperatures were generally seasonable with a short lived warm-front passing through Tuesday Jan 08 which brought rain to low elevations in the south coast and southern Columbia. Strong winds were recorded with Tuesday’s storm.

Snowpack

Approximate storm snow accumulations are150 cm South Coast, 40 cm North Coast, 70 cm Kootenay-Boundary, 100 cm South Columbia, 40 cm Cariboos, 40 cm Purcells, 40 cm Lizard, 20 cm South Rockies. On Wednesday morning (January 09) Coquihalla Pass had recorded 85 cm in 24 hours. All highways to the coast, and many in the interior were closed for avalanche control.

Generally this snow rests on a variety of surfaces including near-surface facets (treeline and alpine) suncrust (steep solar aspects), and surface hoar (shady treeline). Surface hoar in the Northwest Coastal was noteworthy for its “touchiness”. Generally the storm snow itself is reasonably “right side up”, but bonds with the interface below remains limited at this time.

Avalanche Activity

Widespread, direct action natural avalanche cycle is expected Wednesday (January 09). Areas under the storm track this week saw avalanche activity changing character from loose dry (sluffs) to storm- and wind-slabs. Sizes were 2.5 to 3 in many areas by Tuesday.

Incidents:

Comox SAR responded to an incident in Strathcona Park late on Tuesday Jan 08. SAR techs from national defence supported the rescue with a Buffalo Aircraft and Cormorant Helicopter night extrication. No fatalities, one person injured.

Weekly Review: Dec 28 2012- Jan 1 2013

Prepared by: Penny Goddard

Weather

In general it has been a week of stable weather. A ridge of high pressure was the dominant feature, deflecting any approaching frontal systems from reaching the interior and keeping winds mainly light. Most regions have been under cloudy skies, with the odd break-through of sunshine.

A couple of fronts which were confined to the North Coast on 28 Dec and 30/31 Dec brought snow and westerly winds but didn’t pack a lot of punch. Up to 10 cm of snow fell gently in interior regions. Temperature inversions began to form on New Year’s Day as warm air was trapped aloft, leading to unseasonably warm temperatures at alpine elevations by Jan 2.

Snowpack

In the North Coast region, about 20 cm of snow accumulated over the week. Snowfall was accompanied by periods of moderate to strong SW wind, creating localised wind slabs.

In the rest of the province, benign weather promoted a gradual settlement of the snowpack. Occasionally rising winds left behind some thin wind slabs.

Surface faceting developed almost everywhere. Surface hoar growth is reported to be widespread on the South Coast and in the Purcells, and isolated to sheltered treeline areas in other regions. The extent of surface hoar formation is likely to vary from place to place, and it will be a layer to watch once buried with future snowfall. Some areas developed a sun crust on steep solar aspects and/or a temperature crust at low-mid elevations.

Persistent weak layers* identified in the snowpack underwent little change. The mid and upper snowpack layers gradually settled to a point where triggering a persistent weak layer became very unlikely, apart from in isolated thin, rocky terrain in shallow snowpack areas.

Avalanche activity

Interior

Generally there was very little avalanche activity apart from isolated loose dry avalanches, small wind slabs or solar-triggered loose wet avalanches. A few larger (size 2-2.5) loose snow avalanches were triggered by explosives or cornice fall on 31 Dec. These entrained surface facets and ran far.

South Coast

Several small loose dry or soft slab avalanches were reported. Cornice chunks were failing naturally but mostly not causing slab failures on the slope below. A very heavy vehicle (winch-cat) triggered a size 3 slab on a basal weakness from a shallow trigger point on 28 Dec. Solar-related avalanche activity was observed on 29 and 30 Dec.

Northwest regions

Avalanche reports were limited to occasional small slabs or loose dry avalanches in steep terrain. A natural size 2 failed in the Inland region.

Yukon

Very little was reported, except two small avalanches on an east aspect.

Incidents

No incidents were reported during the period.

* Persistent weak layers include the November crust/facet layer, found across the entire province near the base of the snowpack; the late November surface hoar, which is particularly persistent in the Kootenay Boundary region; and a December crust, mainly isolated to the South Rockies and Lizard Range. Surface hoar was also buried under light snow on Boxing Day in the Lizard Range and on Dec 27 in the NW Coastal region, but it remains to be seen whether this develops into a persistent weakness.

Please note: this discussion does not include the Rockies Mountain Parks.

General

In the early part of this season so far, the dominant style of avalanche activity seen in most parts of Western Canada can be described as “direct action”. Direct action means directly linked to loading from new snow. We have seen plenty of avalanches, but the timing and nature of the avalanches have been very closely tied to storm events. Typically, direct action avalanche conditions are more predictable, result in generally smaller and less destructive avalanches (compared with persistent slab or deep persistent slab avalanches), and tend to result in fewer surprise events.

Early November Crust

There was an early season rain crust that formed at the beginning of November in all regions. In most regions, the snowpack depth was quite shallow when this rain crust was laid down. As a result, it is accommodated among the underlying ground roughness in most places. However, it has caused some problems where the crust formed on smooth, rocky or planar slopes, or on glacial terrain. In the Northwest regions, there was more snow on the ground when the early crust formed. Subsequent cold temperatures in these regions promoted facet growth next to the crust. As a result, it has been more reactive in both the Northwest Coastal and Northwest Inland regions.

Late November Surface Hoar

A surface hoar layer was laid down in many regions near the end of November. During the intense storms that followed, there was a widespread avalanche cycle that failed on this layer, particularly in the interior range. By the end of the first week in December, activity on this layer had all but tapered off.

December Storms

Storm snow continued to fall in pretty much a constant stream in most areas from the beginning of December until approximately December 23^rd. In some areas, notably the North Shore Mountains, the Lizard Range and Kootenay Pass, there were very intense snowfall events, although the cold temperatures and low density snow prevented the upper slab stiffening in many instances. People noted “power sluffing” and bottomless riding/skiing conditions. Towards the end of this period the temperatures warmed up slightly, especially in the southern regions. This helped form a more cohesive upper slab in many areas. Storm slabs started to increase in size, releasing avalanches up to size 3 in some instances; wind slabs also became more of an issue with increased ridgetop winds.

Dec 23^rd to Dec 27^th

A couple of days before Christmas, a high pressure ridge had developed over much of western Canada and the snow tap had essentially turned off. While storm and wind slab problems lingered in some areas (with noteable releases near Fernie and Golden), the general trend across the board has been an decrease in avalanche potential.

At the time of writing (27^th December), concerns for deeper releases on lower crust/facet layers remain heightened in the Northwest Interior region and Northwest Coastal region. Lingering concerns for lower layers remain in specific terrain (shallow, steep, rocky) in the Lizard Range, Purcells, and the mountains around Rossland and Invermere.

Notable Avalanche Incidents:

There was a fatality near Stewart BC on 23^rd October 2012.

There were two high profile skier-triggered avalanches in the past week. One occurred near Fernie, where a small section of wind slab stepped down to release a size 3 avalanche on a deeper weak layer (likely the early season crust). The elevation was around 1900 m and the aspect south-southeast. The second occurred near Golden, where a size 3 avalanche released initially in hard wind slab, but stepped down to the lower crust/facet layer. The elevation was around 2400 m and the aspect west. Nobody was caught in either of these two avalanches.

The weekly summary will commence on Thursday 13 December. These will continue to come out either on a Wednesday or Thursday throughout the season until approximately mid-April.

Weekly Review: April 12-18, 2012

Prepared by: James Floyer

Weather

Last Thursday and Friday (13^th and14^th) a low pressure system centred initially over Montanna moved up to lie over central Alberta. This resulted in easterly winds over most of the interior and brought moisture, particularly to the eastern upslope regions. Most regions saw at least 10 cm new snow, while the Cariboos and north Rockies received 30 to 50 cm. Winds were typically light. A high pressure ridge developed in the wake of this storm, bringing clear weather to coastal regions and cloudy weather to the interior. By Sunday, most of the residual moisture had burned off and there was widespread sunshine. In the first half of the week, a couple of weak Pacific frontal systems brought moderate precipitation to immediate coastal areas, but only light amounts to the interior.

Here’s the satellite loop for the past week—notice the initial storm centered over Alberta followed by a clearing trend, before weak Pacific storm systems approach from the southwest.

Snowpack

Interior areas saw some accumulation on Thursday/Friday and even into Saturday in some areas. Despite this, settlement of the snowpack continued apace, and almost everywhere snowpack depths were lower at the end of the week compared to the beginning of the week. Most areas now have between 10 and 30 cm new snow sitting above a melt-freeze crust, amounts in the Cariboos and north Rockies were more like 30-50 cm. The new snow has remained dry on north aspects in the alpine. Light winds have restricted wind slab formation to immediate lee areas in exposed alpine areas. Isothermal conditions are widespread at treeline and below treeline.

Coastal areas saw typically light accumulations on Monday through Wednesday (16^th – 18^th), with the North Shore receiving the highest amounts of around 20 cm on Tuesday night and Wednesday morning. This new snow overlies a crust, which is widespread.

Avalanche activity

Interior

On Thursday 12^th, the tail end of a solar-induced cycle was still occurring in the eastern Selkirks and the Purcells. Several wet slabs were reported from these areas up to size 3. On Friday 13^th, a natural avalanche cycle was reported from the Cariboos (and likely affected the north Rockies as well) in response to storm snow. There was some suspected activity on the mid-February surface hoar layer during this cycle. From the weekend to the end of the period (Wednesday), avalanche activity diminished. Loose snow avalanches occurred on southern aspects; the size of these was typically small, although bigger in the Cariboos where snowfall amounts were higher. Glide activity was reported on south-facing slopes at lower elevations.

South Coast

Only limited avalanche activity was reported from this area.

Northwest regions

Early in the period, solar activity produced wet slab and glide avalanches to size 3. Less activity was reported from the second half of the period.

Yukon

Loose wet avalanches to size 2 and wet slab avalanches on steep, mid-low elevation south-facing terrain were reported.

Incidents avalanches

No incidents were reported during the period.

Weekly Review: April 4-11, 2012

Prepared by: James Floyer

Weather

Storms affected the southern interior near the beginning of this weekly period (Wed 4^th and Thurs 5^th) with 25-35 cm new snow falling in the Selkirks and Monashees south of Revelstoke and similar amounts in the South Rockies. During the same period, the South Coast saw around 10 cm and the northwest regions only trace amounts. Since then, a zone of high pressure has brought warm, dry conditions with high temperatures and lots of sunshine. Initially, night-time temperatures went well into the freezing zone; towards the end of the period, little or no overnight freeze occurred. Temperatures on the Coast and especially the north coast have been a little more moderate than inland; however, even these regions have seen good amounts of sunshine.

Here’s the satellite loop for the past week—notice how much of the province remains clear and moisture free.

Snowpack

Lots of settlement has occurred in the snowpack during the past week in all regions and the snowpack is becoming isothermal in many areas. Storm snow, particularly in the Selkirks and Monashees, from early on in the period transitioned from being touchy and giving “sudden” results in snowpack tests to being stubborn and giving “resistant” results. The primary layer of concern for many operators remains the Mar 26^th (25^th-27^th) melt-freeze crust. This layer is widespread on southern aspects on the South Coast as well as the interior. On the Coast it appears to have healed relatively well, while in the interior, it has remained a concern through the period. It is typically buried 50-80 cm below the surface, but up to 1 m in high snow areas. Some concern for lower weak layers (particularly the mid-February surface hoar layer) remains. The focus is whether isothermal snowpack conditions will wake up deeper interfaces as melt-water percolates through the snowpack. Some evidence for this has been noted in the Selkirks and Purcells; however, it seems as though most of the deeper releases have been going all the way to ground

Avalanche activity

Interior

Early in the period, avalanche activity in the interior was centred on the new snow instabilities and was occurring on all aspects at treeline and in the alpine. Frequent releases up to size 3 were noted on the March 26^th weak layer, either directly on it, or stepping down to it. A number of remote-triggered avalanches were also reported on this layer from the Selkirks, Monashees and the Cariboos. One noteable avalanche in the Selkirks was triggered from 500 m away, but remote triggers of 10-50 m were more typical.

By Saturday 7^th, remote triggering had pretty much ended and the focus shifted to warming-related instabilities and solar activity. Large amounts of loose snow and slab activity was reported from solar aspects from 7^th to 11^th; at the time of writing, this activity was still ongoing. Some of the slab releases have been large—several size 3.5 avalanches have been recorded. Many (but not all) of the larger releases have been attributed to glide activity, which has been noteable in intensity. Some activity has been occurring on other aspects, as prolonged warming has started to affect other aspects.

South Coast

Activity has been more limited in the south coast regions, with the majority of reported activity being loose snow avalanches up to size 2. It’s therefore a little ironic that two of the three incidents that we’ve heard about occurred in this area (see below). Some wind slab related avalanches were reported from the Whistler backcountry over the Easter weekend.

Northwest regions

Solar activity has produced loose snow activity, which has been generally isolated to solar aspects. Glide activity up to size 3 has also been reported.

Yukon

The focus of the avalanche activity has been at the Haines Pass area, with large slab avalanches reported to have been triggered by heavy loads such as cornice failures. Elsewhere, loose snow avalanches have been observed up to size 2.

Incidents/noteable avalanches

• On April 5^th a size 2.5 slab avalanche was remotely triggered from 500 m away in the Selkirks on the late March crust.
• On April 6^th, a skier triggered a size 2 avalanche on a northeast facing, alpine slope in the Duffy Lake area. The sliding surface was a crust, likely buried at the end of March.
• On April 8^th, a skier triggered a size 2 avalanche on a west aspect at the bottom of the Overlord Glacier in the Whistler/Blackcomb backcountry. The avalanche was a wind slab release on a moderate pitch with a convex slope.

Weekly Review: March 29 – April 4, 2012
Prepared by: Peter Marshall

Weather

Click for this week’s satellite loop.

On the South Coast, the first part of the week saw moderate to heavy snowfall in most areas. Storm snow totals from March 27-30 were around 100-150cm. The weekend (Mar 31/ April 1) was unsettled with light snowfall and periods of sunshine. The next system affecting the South Coast was forecast to arrive on April 2 but the bulk of the precipitation and winds did not hit until later that night.

Interior: Generally 10-20cm of snow each day, with a couple heavier pulses hitting the southeast corner of the province (Kootenays and South Rockies). Many days saw periods of sunshine, particularly on April 1-2, but higher elevations and northerly aspects remained cool and dry.

Northwest: Only light amounts of snow over the week. Strong southerly winds were more of a factor. There was also considerably more sunshine which resulted in more spring-like conditions.  

Snowpack

South Coast: The main feature of the week involved a substantial amount of storm snow overlying a smooth sun crust (with spotty surface hoar reported in some areas). The new snow did not initially bond well to the crust. There were also several storm snow instabilities. Wind slabs were also an issue and cornices continued to grow throughout the period.

Interior: Similar to the coast, the primary feature was the new snow sitting on a sun crust. By the end of the week some areas were reporting this to be facets sitting on a crust. On average this layer is 60-100cm deep and produced moderate “pops or drops” results in snowpack tests. The mid Feb surface hoar layer is still lurking but was generally inactive (with the exception of a couple cases in the Purcells).

Northwest: This region was primarily concerned with wind slabs at higher elevations and spring melt-freeze conditions on solar aspects and at lower elevations. In general the snowpack is deep and well settled (less so inland).

Avalanche Activity

A fairly widespread and large avalanche cycle occurred on the South Coast between March 29 and 31. Several avalanches up to Size 4 were reported with some taking out a substantial amount of forest. Most of these events are believed to have released on the March 27 crust layer.

Similar activity followed in the Interior with a few larger avalanches occurring on March 29/30 and a more widespread cycle being reported on April 1^st. There were numerous reports of natural avalanches up to Size 3.5 in most of the interior ranges. There were also many remotely triggered avalanches failing on the March 27^th layer. Some were triggered by up to 300m away. The majority of avalanches failed on the March 27 interface, with isolated events involving the mid Feb surface hoar or stepping to ground.

Several larger natural avalanches (Size 4) were observed or reported in the Lizard Range and South Rockies on March 30 and 31. A fairly widespread natural avalanche cycle also occurred in the Central Rockies on April 1^st, with numerous avalanches up to Size 3.5 being reported.

In the Northwest avalanche activity was generally limited to loose wet snow avalanches from solar warming and small wind slabs in the alpine. There were also a few larger avalanches from helicopter bombing along the highway corridors, and several large glide slab releases at lower elevations.

Incidents

There were several notable near-misses this week with people taking rides in large avalanches or triggering avalanches from a distance. These are the events we heard of (I’m sure there were others):

• On March 30^th a Size 3 avalanche in the Monashees resulted in two full burials. Fortunately both were rescued with only minor injuries. The trigger for this slide is uncertain.
• On March 31^st there was a Size 3 remotely triggered avalanche in Rogers Pass.
• On April 1^st a Size 3 skier-triggered avalanche outside the Panorama Ski Area boundary involved two people – one sustained serious injuries.
• A group of three skies were involved in a Size 2.5-3 avalanche in Jasper Park on April 1^st. One ended up going for a 200m ride but fortunately ended up uninjured and on the surface.
• A boarder took a 300m ride in a Size 2.5 avalanche in Kananaskis Country on April 1^st. The rider ending up on surface with no serious injuries.

All incident reports can be viewed here.

Weekly Review: March 21-28, 2012
Prepared by: Peter Marshall

Weather

Click for this week’s satellite loop.

On the 21^st a storm delivered heavy snowfall to the southern part of the province, primarily in Coquihalla and Kootenay Boundary. This system gave way to a more unstable airmass and convective precipitation for the next couple days. Many areas saw brief periods of sunshine but no real prolonged solar radiation.

Northern areas saw unsettled conditions for the first part of the period with sunny breaks and no significant accumulations. There were strong diurnal temperature swings with mild temperatures during the day and cold temperatures overnight.

On Friday the 23^rd a ridge of high pressure started to build in bringing sunshine but generally cool temperatures to most of the province. The exception was in the southeastern part of the province (East Kootenays and South Rockies) where a system pushing up from Washington gave periods of snow and cloudy skies. This system cleared out by Sunday and left clear skies and probably the warmest temperatures of the year throughout most the province. Alpine temperatures reached well above zero in most areas.

The next frontal system began to affect the South Coast by Monday the 26^th bringing cooler temperatures, more cloud, and light precipitation. This marked the beginning of a wetter and more active weather pattern for the entire province for the next week.  

Snowpack

Coastal regions saw significant settlement of the snowpack with milder spring-like weather for most of the week. Most areas reported that all aspects went through a melt-freeze cycle with the exception of steeper north aspects in the alpine. By Monday there were no notable weaknesses reported in the top 2m.

In the Interior, there was also a fairly widespread spring melt-freeze cycle with the snow surface becoming moist during the day (except N aspects above 1600m) and good overnight recovery as temperatures dropped. The mid-Feb surface hoar layer is now down 100-240cm. It generally produces no results or resistant shears in snowpack tests and has become increasingly hard to trigger. Basal facets are still mentioned as a concern for very large triggers in the eastern Purcells and Kootenays.

Almost all regions reported a new surface hoar and/or surface facet layer being buried by Monday. It is likely that this interface was buried as a sun crust on solar aspects in some areas. This layer could become reactive later this week with expected precipitation.

Avalanche Activity

On Tuesday there was an impressive avalanche cycle (both natural and explosive triggered) in the Coquihalla Pass with several avalanches up to Size 4 taking out extensive timber. Avalanche activity also increased in the Interior, particularly in the Kootenay Boundary region in response to heavy snowfall. At least one observer in the Kootenays reported several very large avalanche events (primarily west aspects) that ran well beyond historical run outs.  

Wednesday through Friday avalanche activity slowed down with generally small wind slabs being reported and the occasional large avalanche involving deeper persistent weaknesses. The Purcells and East Kootenay seemed to be the hot spot with at least one avalanche up to Size 3.5 being reported almost every day. However, these larger events decreased in frequency throughout the period.

Very large cornice failures were reported in all regions throughout the period. One very experienced observer reported an event in the Monashees to be one of the largest cornice failures he has ever seen.

Sunny and mild weather on the weekend resulted in numerous loose wet avalanches and isolated slab avalanches up to Size 3, primarily from south facing terrain in the afternoon or cornice triggered on northern aspects. This activity began to decrease on Sunday or Monday as clouds pushed in and temperatures slowly started to cool.

Incidents

A Special Public Avalanche Warning was issued once again for many regions for March 23-26. The reason for issuing this warning was based on the fact that we were still seeing occasional large and unpredictable avalanches resulting in near-misses, coupled with the forecast for strong solar radiation and mild temperatures on the weekend.

On Tuesday, two people were killed in an avalanche in the Bonnington Range near Blewett, BC. This incident involved 5 people who were heli-skiing. The avalanche released above the group at the bottom of a run. It resulted in 4 of the 5 members being buried. A rescue was completed by other guides and heli-ski groups in the area.

On Friday a Size 3 remotely triggered avalanche was reported in Rogers Pass. This avalanche was triggered by skiers from a ridge top and ran down a north facing slope taking out the previous ski tracks and some timber. Fortunately no one was involved in this slide.

All incident reports can be viewed here.

Weekly Review: March 14 – 21, 2012
Prepared by: ilya storm

Weather
Click for this week’s satellite loop.

March 14 & 15 (Wed & Thurs) BC was under a SW flow with most precipitation focussed on southern areas: primarily coastal regions and upslope areas in the interior (western slopes). The wind disappeared March 16 through 19 (Friday through Monday), at least in terms of organized flow. During this period there was a mix of sun or clouds (depending on what location on what day), warm daytime temperatures with overnight cooling, and convective precipitation. Convective precip means patchy localized distribution, often falling in intense but short-lived squalls, and localized gusty winds. In the interior that often meant 20 cm of snow every 24 hours with daily freezing level rising to around 1600 m (near treeline).

Snowpack
It may be equinox today; however, the snowpack shows little evidence of new season – it’s still, by and large, a wintery snowpack across most regions.

South Coast saw up to 200 cm of storm snow over the week with strong winds brought into play storm slabs, wind slabs, cornices and even the mid-February deep persistant weak layer.

The story was very similar in the Interior except that storm snow amounts were slightly smaller, say 80 to 150 cm of storm snow.

In the North West storms were less intense with only light to moderate accumulations of storm snow. Several (3) potential weak layers are reported in the area including mid- and early-March surface hoar layers within the top metre or so of the pack and the early February layer. 

Avalanche Activity
South Coast saw a major avalanche cycle with natural, accidentally triggered, and remotely triggered avalanches throughout the week, but activity is starting to settle down during the past few days. Avalanche activity was large (up to size 3.5).

In the Northwest the avalanche activity was more muted by comparison; yes there were avalanches up to size 3 but with nowhere near the frequency or ease of triggering as the south coast or interior areas experienced.

Interior ranges behaved much like the South Coast, earlier in the week there was a major avalanche cycle with natural avalanches to size 4, 4.5, possibly even a size 5! Mature forests fell! These involved either the mid-February deep persistent weak layer or basal weakness. This activity is becoming much less frequent as the slab becomes more difficult to trigger.

As avalanche activity starts to diminish, the important question to answer is whether the problem was shocked and is now healing, or if we’re going into a period of low likelihood but high consequence lingering problems. Current thinking at the CAC is the lingering ongoing scenario; adopting this theory is the prudent approach.

Incidents
The CAC issue its third Special Public Avalanche Warning in four weeks for March 16 – 19. One input parameter for these warnings is the expectation of heightened risk for incidents. The problem was primarily the unusual avalanche sizes, especially when compared to the only moderate storm snow amounts.

A few interesting avalanches were reported including a cornice failure/size 3 slab where one person deployed their airbag, a sled triggered size 3 that propagated 1300m, and a low angle logging cutblock size 3. In other words accidentally triggered avalanches were large! Additionally, even with the frequency of natural avlanches diminishes, the frequency (and size) of accidentally triggered slides remains elevated. Incident reports are available here.

Weekly Review: March 7 - 13, 2012
Prepared by: Karl Klassen

Weather
Click for this week’s satellite loop

The period started with fairly dry, calm weather in most regions. The exception was the Northwest where storms were already active early on. Over the week, weather moved south and inland and by the 10^th windy, warm, and snowy weather was prevalent in most regions, although the south and more so the SE corner remained somewhat drier. Warm temperatures, strong winds, and moderate to locally heavy snowfalls prevailed through the weekend. On Monday the 12^th, the south coast was hit by a significant system that was much less intense in the north and inland. Tuesday afternoon and evening saw a clearing, cooling trend with reduced winds.

During the most recent storm cycle, most central and southern interior regions experienced rain at lower elevations as freezing levels rose to 1600-1800m or so during the last few days. The northern interior and north and south coastal areas saw somewhat lower freezing levels and even the North Shore ski hills had not seen a significant rain event in the latter part of the period.

Snowpack
Persistent weak layers (in some cases now probably better described as deep persistent) continued to plague many interior regions. These were most problematic in the eastern and southern areas, although they were on the radar in all the Columbias as well as the Cariboos and inland coastal areas, especially in the south. These layers include the early and mid-February surface hoar/crust/facet interfaces which now lie up to 200cm below the surface at treeline in many areas. The greatest PWL concerns were at and below treeline—in the alpine loading and avalanche activity seem to have settled these layers in or wiped them out for the most part.

In the interior at alpine elevations and in coastal areas at all elevations, the snowpack consisted mostly of non-persistent layers related to new snow and wind. These generally settled and bonded relatively quickly. Isolated semi-persistent storm layers some of which dated back to early March were noted early in the period but seemed to be pretty much gone by the end of the week.

There were a few mentions of surface hoar and suncrusts that formed during short breaks in the weather but I don’t think any of these will amount to much; certainly they are not widespread or well developed.

Rain soaked snow at lower elevations will almost certainly have frozen up and formed crusts of varying thickness with the cooling and clearing trend that occurred Tuesday. Upper elevation of the crust will vary according to where freezing levels were and how high rain fell. I suspect no higher than about 1000m  in coastal and northern areas and probably to 1500 or 1600m in the southern and eastern interior.

Avalanche Activity
After the major avalanche cycle of the week before, the last week looked quiet by comparison. It’s relative though—by anyone’s standards the amount of action was significant even though it looked like a big drop in activity compared to what happened in early March. On the coasts at all elevations and in the interior alpine, avalanches tended to be storm and wind slabs that cycled with weather inputs.

At treeline and below in the interior, there was a mix of storm/wind related events and persistent/deep persistent slabs. Of the latter, most failed on early- to mid-February interfaces where remote and sympathetically triggered slides were in the mix. In numerous cases persistent slabs at and below treeline were triggered by debris from avalanches that ran from above or were step-downs from avalanches that started as storm or wind slabs.

Propagations were impressive both in the storm snow layers as well as the PWLs with some very large slides including a number of size 4s, at least a couple of 4.5s, and many 3s and 3.5s.

Incidents
While it doesn’t show in the incident database, I noted a pattern of escalating numbers of increasingly serious incidents. This included stories surviving a 30 minute burial and injury accidents involving experienced people. This pattern culminated with three fatal accidents in a five day period from March 6 to 11. While there is no hard data to indicate which layers or what kind of avalanche problems caused these incidents and accidents, what can be said is many were at relatively low elevations. This makes sense as the treeline and below treeline problems are complex and tricky coupled with violent weather and poor visibility in the alpine that likely drove users to lower elevations, right into the jaws of the tiger.

Graphical Overview
I can't insert graphics into this blog. See this report with graphics here.

We are unable to complete a weekly summary due to high workloads and staffing issues. One of our forecasters is stuck in the field due to high avalanche hazard--ironic?

Weekly Review: February 23 – 28, 2012
Prepared by: ilya storm

Weather
Click for this week’s satellite loop.
The week was almost a repeat of several past weeks. Weak systems continued to move through BC carried along in a SW to NW flow. The exception was the storm that tracked through Friday & Saturday (Feb 23 & 24) which managed to actually break down the persistent eastern-pacific high and hit BC with a bit of a punch. Snowfall amounts were generally less than or around 25 cm on any day, more on westerly upslope sides of the ranges. The Lizard Range was the big winner with almost 100 cm in one day, but it was so blower that it was only around 25 mm of water equivalent. Temperatures were mostly seasonal; with day time warming when the sun was out!

Snowpack

We were still in a period where widespread weather patterns mean widespread common snowpack conditions. Even though the province is big; there really are lots of similarities. The exceptions are the coastal areas the Sea to Sky (South Coast) and possibly the North West Coastal where the problems are more spotty.

I’ll confine my discussion to the more persistent problems, like surface hoar, and not discuss short-lived wind and storm slabs. The upper 100 to 150 cm of the snowpack is complex; has been causing lots of people grief (close calls & accidents), and will likely continue to hide surprises for the foreseeable future. Feb 08 is the lower interface and includes surface hoar on facets, surface hoar on suncrusts, facets on suncrust, etc., depending on aspect and elevation. A late January (Jan 29) smoooooth hard crust, where it exists, is below. Undoubtedly there are interfaces above the Feb 08^th nightmare layer; for example in the Columbia Mountains near Revelstoke layer of preserved stellars is down 50 or 60 cm and looks & acts just like surface hoar.

Below the Jan 29 crust, most places report a uniform strong, well-bonded snowpack. The exceptions are areas with variable, thin snowpacks where basal facets / depth hoar remain a concern. These are mostly areas like the east side of the Purcells, east side of the South Rockies, possibly the east side of the Selkirks. This kind of a problem can occur locally in any range – for example windswept morainal terrain.

Two areas which appear to be different are the Cariboos (where we think the problems are not as widespread, there’s more variation in the region), and the Purcells (where the layers haven’t received as much new snow so have yet to be as fully stressed).

Avalanche Activity

A significant natural avalanche cycle was experienced in most areas as the Feb 08 layers woke up. All aspects and elevations were involved, up to size 3.5 typically reported. It was not a historical “full-path” avalanche cycle and the terrain did not completely “clean out” – many slopes did not slide. Since the weekend reports of “remote-triggering” are increasing, often from low angle terrain and this trend continues. Intentionally (controlled) and accidentally triggered slides continue.

Incidents

Many many close calls from accidentally triggered avalanches were reported to the centre. Some appear on our incident reporting system available here. The overall pattern is a typical one for persistent weak layer (PWL) avalanche cycles: some critical threshold is reached and a natural avalanche cycle begins, eventually the snowpack adjusts and regains strength. Therefore natural activity declines, but controlled avalanches increase. People test slopes intentionally (for example with explosives, or ride small control slopes) and are able to tip the balance back to “instability”. With a bit more time the problem continues to subside, clues are less obvious, people start to push out into more terrain, it appears conditions have improved, and sometimes they get surprised – accidentally triggered slides / close-calls / accidents. 

Weekly Review: February 16 - 22, 2012
Prepared by: Karl Klassen

Weather
Click for this week's satellite loop.

The week was characterized by a steady stream of small systems in a flow that shifted from NW to W. These systems generally produced more precipitation in the coastal and southern regions than inland and farther north. Because the flow was relatively weak, west facing upslope areas got more precipitation than easterly downslopes even when the distance between them was small. In some cases, one valley reported 60cm in 36 hours while just a few kilometres away amounts were 10-15cms at best.

Winds were generally light until Wednesday the 22^nd when a wind event in many areas moved significant amounts of snow especially at upper elevations.

Temperatures were mostly seasonal or slightly above but no significant warm spells occurred.

Mostly overcast conditions and cool temperatures when skies did clear minimized the impact of solar radiation.

Snowpack
The Feb 8 surface remains a player: this consists of a mixed bag including surface hoar on facets, surface hoar on suncrust, facets, suncrust, and probably more. In some areas, the January 29^th surface was smooth and firm and where it exists, this is what underlies the whole February 8^th complex.

Incremental loading from the series of small systems gradually buried the Feb 8^th surfaces to the point where there is now anywhere from 20ish to 60ish cms of accumulated storm snow on them. In some places one or more layers of surface hoar (Feb 12/Feb 16) formed during short-lived clear spells between pulses. These surface hoar layers are sandwiched between layers of new snow.

On Wednesday, windloading became a player in many areas as the firs decent wind event in what seems like weeks rolled through the province.

I could write a novel about the upper metre of the snowpack right now. Suffice to say it’s what I’d call a very complex and unpredictable sandwich with some potentially nightmarish elements.

Below about a metre down (Jan 29^th), the snowpack is well settled, uniform, and well bonded in most places except those areas still struggling with  deep, basal weak layers of facets and/or depth hoar in snowpacks that are shallow and variable in depth. Places that immediately come to mind are the east-central Selkirks, the border regions, the South Rockies, and the Purcells. While probably better than they were, these basal weaknesses are still troubling layers that bear watching if/when significant weather changes occur or as more load is added or if the potential for large triggers (cornice, icefall, stepdown) exists.

Avalanche Activity
There was a very gradual but clearly discernible increase in the size, distribution, and magnitude of avalanches. This started with just a few human triggered avalanches when a bit of wind touched certain areas early in the period. By Saturday, we were seeing more avalanches involving mostly the upper (Feb 12/16) layers, also mostly human triggered with a few naturals. By Monday sympathetics and remotes started showing up, along with larger events, although still mostly human triggered. By Tuesday, most people were commenting on it being a little unnerving how little activity there was considering the storm snow accumulations and the layers involved.

On Wednesday, the tide turned and a widespread cycle of human and natural avalanches was occurring in many regions. Data is still spotty as I write this but people are pulling the plug and coming in early, headed for the groomers. Skier remote triggered avalanches from 70m away on 20 degree slopes were reported as was a cycle of windslab avalanches in the alpine up to size 3.5. It’s still a bit early to tell for sure, but it looks like it’s going off in the Monashees, Selkikrks, and Purcells; possibly in the Cariboos; and in my opinion, probably in many other areas also.

Incidents
Reports jumped on Feb 18 and between the 18^th and 21^st there were 10 new incidents added to the database. On the 22^nd, several more came in from the previous day and they continue to arrive as I write this. The number and nature of the incidents closely mirrors the incremental loading pattern and the avalanche activity reports. It’s a classic pattern of gradually increasing hazard that’s almost imperceptible at the outset unless you are watching from outside and have access to the big picture.

We saw the developing pattern early and started talking about it on the weekend.

Graphical Overview (as of 16:00, Feb 22, 2012)
I can’t include graphics in this blog so you can check here for the full text with graphics.

Weekly Review: February 9-14, 2012

Prepared by: James Floyer and Tom Riley

Weather

One word sums it up: drought. There has been little to no precipitation in any of the forecast regions over the last week. The lack of precipitation was caused by a broad high pressure system stubbornly centred over Montana. The Jet stream diverged around this blocking system and set up a split flow upper air pattern over western Canada. This left behind a stagnant zone where incoming Pacific frontal systems would essentially stall and disappear, like ghosts, as soon as they merely touched the coast. Check out this time sequence (it may take a while to load). http://www.atmos.washington.edu/~ovens/loops/wxloop.cgi?sat_sfc_archive+start+2012020803+end+2012021203+interval+3h

Minor incursions of moisture in the latter half of the period occurred overnight between Thursday (9^th) and Saturday (11^th). Interior regions saw 1-5 cm new snow—just enough to lightly bury the very large (30 mm+) surface hoar that was widespread at elevations above 1000 m. Coastal regions have seen slightly more, with 10-15 cm new snow now lying on the surface in both Northwest coastal and Sea-to-Sky regions. Overnight on 13^th, the South Rockies and Lizard range received up to 15 cm of low density snow with no wind.

Snowpack

Overall, the near surface layers are varied and complex. Surface hoar growth has been spectacular and widespread, occurring at all elevations and many aspects. It was initially enhanced by the presence of valley fog from the previous week, and preserved by the lack of wind and cool temperatures this week. On 10^th or 11^th Feb, temperatures spiked for a brief period. Some people in inland areas reported seeing rime or a thin freezing drizzle “glaze” over the surface hoar at specific locations. The main crystal structure appeared to remain intact. The large surface hoar layer now lies buried somewhere between 1 and 15 cm, depending on the region. Some of the “snow holes” are reporting 20cm. A sun crust formed on steep south and southwest aspects at all elevations. Surface hoar has formed above the crust, but may have been destroyed by the strong solar heating. Some facets may have developed below this crust. Poor skiing quality has kept traffic low on these aspects. When most eyes are focusing on the obvious surface hoar problem, it may be a good idea to have a few eyes paying attention to developments regarding this crust layer.

Avalanche activity

Regions in the south east, particularly the east Kootenays, The Purcells north of Kimberley, The South Rockies, the Lizard Range, and possibly the Cariboos inherited a low probability, high consequence, deep slab problem, following activity associated with intense warming at the start of February. Natural activity on deeper layers this week has been isolated to the occasional cornice release that has pulled out a slab on lower layers. Explosive testing has been able to trigger large avalanches on deeper layers in the Purcells and the Lizard range. Some of these regions continued to show moderate alpine danger ratings (as opposed to low) to account for the possibility of large avalanches occurring on deeper weak layers.

Mostly, however, the story this week has been the good stability and aggressive riding that people have been able to do. Many of the forecast regions have been completely painted green (low danger). There have been a few isolated problems: cornice falls, localized wind slab, glide crack releases, loose wet snow avalanches. There was one report of a size 2.5 windslab avalanche on a southwest aspect in the alpine that released near the end of this period. The avalanche occurred after about 15 cm of dry new snow was re-distributed by a few hours of moderate strength winds. This was an isolated occurrence, but it happened when the stability was consistently reported to be G/VG/VG and the danger level was rated as LOW at all elevations. I think this is worth noting and watching, because it may not take very much snow and wind to change the current conditions.

Incidents

 No incidents have been reported during this period.

Weekly Review: February 2-8, 2012
Prepared by: Penny Goddard

Weather
A storm, which had brought snow and wind for about the previous five days, was replaced by a strong ridge of high pressure on Thursday 2^nd /Friday 3^rd Feb. As the ridge became established, a warm southerly airflow was directed over the province, causing unseasonably warm temperatures. Combined with direct sunshine, this produced a dramatic weather change, akin to a sudden transition to spring.

Alpine temperatures spiked on Friday to a few degrees above 0C in many locations. Warm alpine temperatures continued through the weekend, with valley inversions and only slow, minor cooling in the early part of the week. The ridge continued to dominate the weather for the rest of the period.

The North Coast had some continued precipitation on Friday (which fell as rain below treeline) before the ridge established itself, while no significant precipitation was recorded in the rest of the province throughout the week.

Outflow winds were observed in coastal valleys and occasionally in the interior, with otherwise mostly light southerly winds. The last of the valley inversion cloud in the Columbias burned off by Wednesday, leaving clear skies everywhere.

Snowpack
The snowpack underwent dramatic change beginning with sudden warming on Friday (Thursday on the South Coast, Saturday on the North Coast). Solar aspects developed moist or wet surfaces and storm snow quickly turned into a very reactive slab, even on some north aspects. After a long hiatus from the spotlight, persistent weak layers (Jan 20 facets, Jan 13 crust/facet/surface hoar and basal facets/ground) were reported as the failure layers for a handful of large avalanches.

A melt-freeze cycle began in parts of the province on Friday, forming a crust to ridge top on sunny aspects. No overnight freeze recovery occurred in the alpine throughout the weekend in parts of the Columbia mountains. By about Monday night, almost everywhere had a melt-freeze crust on all aspects below treeline and a sun crust on southerly aspects to ridge top. North aspects in the alpine remained dry and became progressively more facetted, with 10cm or so of near surface faceting.

Surface hoar growth was rapid, leaving gargantuan crystals at low elevations and 10mm crystals well into the alpine. Overnight surface hoar growth was probably destroyed on sun-exposed slopes on a daily basis, but I suspect that where surface hoar overlies the melt-freeze crust at treeline and below, it has the potential to become a highly reactive and problematic layer once buried.

The persistent weak layers which woke up with rapid warming stopped performing after about two days. Are these healed now? My feeling is that although it’ll be aspect and elevation-specific, in general the answer is no. Although it was a relatively prolonged period of unusually warm weather for the time of year, it’s not actually spring. The snowpack did not become isothermal, apart from in isolated very low elevation spots. I suspect it was more a change in slab properties than a change in the weak layers themselves that led to diminishing reactivity. I am not going to write the Jan 20, Jan 13 or basal persistent weak layers off my list of possible concerns next time a mechanism for deep releases appears on the horizon. On steep solar aspects, they are probably pretty well healed. My biggest concern will be north-facing alpine features.

Avalanche Activity
On Thursday, the first signs of a significant avalanche cycle began to show in the Purcells and southern Selkirks. At least four large avalanches released naturally on facets at ground. Two vehicle-triggered slabs in the southern Selkirks were also reported, one failing at ground and one on a persistent weak layer.

On Friday, the cycle became more widespread as warmth and sunshine spread to the rest of the province. Numerous slabs of size 2-4 were triggered naturally, by cornice fall, and by machinery and people, both directly and remotely. Many failed within, or at the base of, recent storm snow. Some failed on old snowpack layers and a large handful failed on basal facets. Many were reported as being deep releases with long runouts, but observers did not always get a clear look at the failure plane. Widespread loose solar-triggered avalanches also occurred from Thursday to Sunday.

Focussing specifically on the avalanches which failed at ground or on basal ice, these were limited to the interior mountain ranges, with most occurring in southern areas (Kootenay Boundary, Purcells and Lizard Range).  Further north, a smattering of reports came from operators immediately north, south and east of Revelstoke. The furthest north that a release on basal facets was reported was an isolated event near Blue River.

In the North and South Coast regions, large avalanches were also reported, the deepest of which failed on the Jan 20 facet layer.

On Saturday, natural avalanche activity on persistent weak layers dropped right off. Two human-triggered avalanches failed on basal facets and led to burial (one near Kimberley, one near Kaslo) and explosives control continued to produce results at ground in the Purcells and Lizard Range.

Activity started to slow down and become more predictable by Sunday, with the odd glide release thrown in. Continuing cornice fall and moist solar-related avalanches were observed on a daily basis, generally diminishing in size and frequency. The only exception was the Northwest Coast, where natural avalanches continued to fail on the Jan 20 persistent weak layer with cornice fall or step-down triggers.

In summary: The pattern of deep releases was mainly focussed in the south-eastern part of the province and spiked quickly with rapid temperature rise and sunshine on Thursday and Friday. Stability also improved rapidly. Almost all of the deep avalanche activity started above 2300m. The bulk was on sun-exposed slopes. North aspects were also failing at ground, but required an additional trigger like a cornice fall, skier or explosives.

Incidents
Several incidents were reported. On Thursday a snowmobiler triggered a slab near Nelson which took him and his snowmobile for a ride through trees. He stayed on the surface with his airbag deployed, while the sled was buried. The slab was triggered from below and ran to ground.

On Friday a skier was killed in an avalanche while skiing with a mechanised guiding operation on Meadow Mountain near Kaslo. The size 3.5 slab was triggered by the third skier on the run, on a north-east aspect at 2430m. It failed on facets at ground.

On Saturday a skier was buried 1.5m deep in Hellroaring Creek, near Kimberley. He triggered the avalanche from a thin spot in a cross loaded gully. The failure plane is suspected to be the Jan 13^th surface hoar. In places it stepped down to basal facets. He survived with injuries.

Also on Saturday, skiers triggered a size 3 slab on a north-west aspect at 2500m near Kaslo, which buried and killed their dog. It failed on depth hoar at ground and was triggered from a thin spot on a lee slope.

All incident reports can be viewed here.

Weekly Review: January 19 - 25, 2012
Prepared by: Karl Klassen

Weather
The week started with the end phase of the first artic outbreak of the season as a warm, wet, windy storm ushered in a return to a zonal (westerly) flow containing a series of systems.

The initial storm that started pushing out the polar airmass was strongest on the South Coast and southern interior mountains. The North Coast also got a decent hit but inland areas in the north and northern Columbias were less affected. This storm was accompanied by above freezing layers (AFLs) and inversions (warmer air at upper elevations), which resulted in some high elevation rain and low elevation freezing rain in the South Coast and Cascades as well as very southern  interior. Eventually the airmass reverted back to a more normal temperature profile and a cooling trend following passage of the cold front late Saturday/ Sunday.

The next system of the zonal flow arrived Monday/Tuesday and once again raising freezing levels, provided a slug of precipitation, and was accompanied by a decent wind event in most areas. Again, the coastal mountains and southern interior were more in the bulls-eye although central and northern areas saw a decent dose of snow. As is common in a zonal flow, this second storm was short-lived and was followed by a cooling and clearing trend Wednesday.

Snowpack
Persistent weak layers (PWLs) remain an issue in the Purcells (mid-December interface and basal facet layers in shallow areas) and the Kootenay-Boundary where the mid-December layer is less deeply buried and has had less overlying weight on it than in other areas. While dormant in the K-B, I think this layer can`t be written off here yet. It needs a bit more weight and time to see if it will die out completely or stay as a low-probability/high-consequence problem for a while.

In other areas of the province, the mid-December and basal weaknesses are essentially no longer an issue. That said, it remains on the radar in select locations where it was a significant problem earlier in the winter—the central Selkirks and northern Monashees in particular come to mind, where folks are saying they’re not seeing any reactivity but are still thinking about it in certain terrain (shallow or variable snowpack areas especially where combined with steep, convex, and unsupported features).

The January 19/20 surface that got covered up by the first warm storm consists of  cold, dry, loose settled storm snow, perhaps with some facetted grains mixed in. I personal think the faceting is not as well developed as many people thought because, while the temperature gradients in the surface layers were significant, faceting is slower at colder temperatures and the gradients just weren’t there for very long in most cases. This is a fairly thick layer and all things being equal, it’s my experience that thick layers tend not to be as reactive and don’t persist as long as thinner ones. The first storm definitely produced instability on this layer as warm, dense, and often windloaded snow was laid over the colder, lighter stuff below but things righted themselves, settled, and bonded up fairly quickly in most places. By the time the second storm arrived, the lack of reactivity at this level was a bit of a surprise to many.

As the week closed out, storm and windslab instabilities on the January 19/20 layer looks like it’s dying out quite quickly and concerns are shifting to new wind/storm instabilities or deeper layers such as the mid-January surface (a crust in many places sometimes with surface hoar or faceting). I understand the concern about potential stepdown from storm or windslabs to Jan 13^th or other post-Christmas deeper layers or perhaps even stress concentrating there as upper layers settle and bond, but in my opinion it’s a long shot that some of these might reactivate—and if they do, I think stepdown is the most likely scenario.

Avalanche Activity
Avalanche activity consisted almost entirely windslab and storm snow problems for the majority of the period, especially in the latter stages. I’m calling avalanches on the January 19/20 layer storm or windslab problems—I personally don’t think that layer will persist and even if it does, the first round on this layer has been entirely related to the storm and windloading.

Even in places with PWL concerns, non-persistent problems dominated the picture. That said, as is to be expected, there were a few large, deep avalanches in the far south that sound like they might be the mid-December layer at play.

Incidents
A notable incident involving a full burial occurred in the Northwest Coastal region. You can view a report in the incident database.

A number of close calls involving partial burials and surprises were reported in various locations as the first warm, wet, windy storm loaded new snow on top of the January 19/20 layer.

Graphical Overview
Graphics are difficult to post in this blog. You can read the entire original summary with graphics here.

Weekly Review: January 12 – 18, 2012

Prepared by ilya storm

Weather: The past week was characterized by a cold arctic outbreak and significant localized winter storms, primarily in the southern sections of our regions. Coastal areas saw shifting winds, including the reverse loading that comes with outflow, NW winds at higher elevations, and west or southwest winds as the onshore flow started up towards the end of the period. Interior areas seemed to get less wind, but it too came from less common north and east directions at times. Cold dry snow arrived over the weekend (Jan 14 & 15) with about 40 cm reported from most areas of the coast although the Coquihalla area received more than 100 cm. In the interior around 30 to 70 cm was reported, and around 30 to 50 cm in the South Rockies although more than 100 cm of storm snow is reported in the Lizard Range (it’s that cold, dry, & light).

Snowpack: Persistent weak layer problems – namely the mid-December surface hoar layer – seems confined to the Purcells and South Rockies.

This week’s storm snow created a new interface – the Friday the 13^th layer. The weather pattern was widespread over most of BC and therefore this weaklayer is also widespread; however, how it reacts & develops (or disappears) will take time to figure out ,and undoubtedly will evolve differently in different areas. The basic problem is that the storm snow fell on a crust, surface hoar, or surface facets.

The present snow surface is faceting (weakening) with the current cold temperatures. In many areas the upper snowpack remains unconsolidated and loose (i.e. loose snow avalanches rather than slabs); we expect this to quickly change with warmer temperatures or wind – both are expected with the next incoming storm.

Avalanche Activity: Where significant accumulations of new snow occurred widespread loose snow avalanches (sluffing) in the cold loose snow was reported. The snow simply didn’t develop into a soft, cohesive slab. It seemed to take some time for wind and / or natural settlement to set up this low density snow.

Weekly Review: January 4 - 11, 2012
Prepared by: Karl Klassen

Weather
The period started with the tail end of a series of storms embedded in a zonal or near zonal (strong westerly) flow with strong winds, relatively warm temperatures, and light to moderate precipitation events with greater accumulations in the northwest and south coastal areas and lesser amounts inland. This was followed by a couple of days where the winds abated, temperatures cooled off and precipitation eased.

The period closed with a warming trend, renewed precipitation, and winds. As in the week before, the Northwest region took the brunt of the weather and saw the greatest accumulations of new snow, strongest winds, and most pronounced swings from warm to cool to warm again. The South Coast mountains were close behind while inland, weather was most significant in northern areas and generally less intense in the south and east.

Significant local variation was observed within this general pattern; often locations in fairly close proximity reported quite different conditions—mostly in terms of snowfall amounts; everyone seemed to get the wind and most had warmer than normal temperatures for much or most of the period.

Snowpack
Persistent weak layers slipped down the list of concerns in most areas over the last week, the notable exception being the Purcells.

New snow and wind-related instabilities dominated in the Northwest and northern interior. The near and post-Christmas surface hoar layers and crusts seemed to be bonding reasonably well and wind/storm related instability was the main concern. The South Coast was similar—the persistent weak layers (PWLs) there are now quite deeply buried and well bridged, and the latest storm cycle look like a more normal coastal system with short-term instabilities settling out and bonding relatively quickly.

As far as the interior mountains go, there’s a definite difference north to south. The mid-December interface was essentially unreactive the entire period in the Cariboos in spite of decent loading events and not insignificant weather stresses such as temperature swings. A significant decrease and, in many cases, a cessation of reactivity on the mid-December interface was also noted farther south later in the period. Most folks in the northern and central Monashees and Selkirks still have this layer on the radar although wind and storm related instabilities were generally higher on the list of concerns by yesterday. There are some surface hoar and crust layers in the upper part of the snowpack but so far they’re acting more like short-medium term storm instabilities than PWLs.

The southern Monashees/Selkirks (Kootenay-Boundary region) and the South Rockies look similar to the central Columbias except there’s been less new snow so the mid-December and later layers are not as deeply buried nor as well bridged as farther north. To me, this suggests that even though reactivity is very low, PWLs here remain more trigger-able here than farther north—they are more easily affected by human or natural loading and have not had as much  weight from overlying snow to promote settling and bonding.

I think the mid-December layer can now be considered fully dormant in the northern and western regions, although more like just napping in the Kootenay-Boundary and South Rockies. Without writing it off completely, I personally think this layer may be essentially done in some areas where they’ve been problematic (I’m thinking the Cariboos, the western side of the central Monashees, and the western South Coast mountains) but I sure wouldn’t want to be wrong about that so have to hedge a bit by saying let’s see what happens in the next weather cycle.

Now, to the Purcells. Here things came to a head in the last seven days. Deep basal weaknesses and the mid-December interface got loaded by new snow and wind. These layers are deep enough and have enough of a slab over top to produce significant propagation, yet remain shallow enough to be easily triggered. The recent post by Ruby at Kicking Horse and the one from the pro patrol at Panorama showing a very graphic ECT test result say and show it better than anything I could tell you here. Yikes. I’d say this region gets the award for trickiest snowpack in the Province right now.

Avalanche Activity
Natural and human triggered windslab and storm slab activity was common in most regions as stormy weather and strong wind events came and went. These conditions were most pronounced in the Northwest, Northern interior, and South Coast mountains. In some cases, storm- and wind-slabs probably ran on one or the other of the embedded Christmas and post-Christmas surface hoar or crust layers but it remains to be seen if those layers will persist after the initial loading cycle they’ve just undergone. In the Northwest, some avalanches were very large (size 4/4+). Farther south and in the interior size 3s and 3.5s were not uncommon with lots of size 2s. Triggers were more naturals early in the week then lots of explosive triggers as avalanche control operations got the weather to hit big targets.

By the end of the period, it appeared natural activity on the PWLs, especially the mid-December layer, had essentially stopped outside of the Purcells and most people were thinking a large trigger and/or specific terrain would  be required to get more action on this layer.

I think we’re looking at a low probability-high consequence scenario for the mid-December layers with the lowest probability in the north and likelihood increasing as you go south and east. Big triggers (cornice falls), step-down avalanches, major weather changes, areas with variable depth snowpacks, and specific terrain features (e.g. steep, convex, unsupported) are the likely times and places that might wake this thing up again. It might be a good time to review some of the risk management advice in the PWL papers from 2007-08 and 2009-10 in the CAC library. Not that I’m suggesting we’re in for a winter like those again, just that a low-probability/high-consequence cycle requires a certain approach, especially early on when uncertainty is high.

In the Purcells, we saw deep (maybe a better word is old) persistent slab avalanches on the basal facet/crust layers from October and persistent slabs on the mid-December layer combined with the expected storm snow and windslab events. Up to size 3 for deeper ones.

Incidents
A fatal accident Golden on January 6^th was attributed to the mid-December interface—it’s uncertain whether it was facets or surface hoar that formed the failure layer.

Several close calls were reported from various regions of the province, notably a full burial and successful companion rescue in the Flathead region of the South Rockies.

The town of Stewart was dusted by a size 4 on January 8^th that took 2 – 3 hectares of mature timber out of the track and knocked the lines off 30 power poles near town.

Graphical Overview
It's difficult to place graphics in this blog so you can view the original document with graphics here.