Wind slabs in exposed areas, a dodgy interface just waiting for storm slabs to settle over it, power sluffing where slabs haven't formed, and a lurking deep persistent slab problem. It's a lot to think about! Seems like seeking out the soft, sheltered snow and managing its predictable sluffing might be the best strategy for Wednesday.
In higher snowfall parts of the region like the Monashees, deep accumulations of new snow that haven't formed into a slab are likely to create large sluffs with rider traffic in steep areas.
With 20-80 cm of low density new snow (greatest in the Monashees, least in the Selkirks) from the weekend overlying weak grains like faceted snow and small surface hoar, we are seeing quite a few operators in the region keeping this problem in their assessments even days after the storm. It's all about assessing for slab properties, which are most likely to be found in wind-affected areas.
A weak layer of facets exists near the base of the snowpack. The likelihood of human triggering is low given the layer's deep burial depth, but storm slab avalanches in motion have the potential to step down. Suspect terrain for human triggering includes steep, thin, shallow, and rocky terrain, which is often found near ridgelines.
It's been a busy time for avalanche activity in the region with almost all operators reporting some variation of the theme of a natural and skier-triggered avalanche cycle as 40-70 cm of new snow this weekend settled into touchy storm slabs in some areas and produced powerful dry loose releases with rider traffic in areas where slabs hadn't quite formed. Improved travel conditions on Monday allowed for more observations of the aftermath of the cycle, generally observed to have produced avalanches to size 2.5 (large!) and for the most part confined to the depth of new snow. A few operators noted the cycle being less widespread than expected.
Clear skies last week allowed for observations of many deep persistent slab avalanches as well as explosive control missions, which produced large to very large (size 2 to 3.5) avalanches. Another one was triggered in Glacier National Park on Monday. These avalanches have been occurring on all aspects and generally between 2200 m and 2800 m in elevation. Although many of these avalanches have occurred in unrideable terrain, they are evidence that the weak basal snowpack is still triggerable. Riders should continue to avoid thin snowpack areas, which are often found near ridgelines.
Looking forward, storm slabs from the weekend may remain sensitive to human triggering a bit longer than usual, owing to the weak faceted snow they overlie. Otherwise we expect surface instabilities to gradually become more focused toward wind loaded areas.
Stormy conditions over the weekend brought up to about 40-80 cm of new snow to the Monashees, closer to 20-50 cm in the Selkirks, along with strong southwest wind. The new snow buried an interface from the recent cold period which includes faceted (sugary) surface snow, small surface hoar in sheltered areas, and widespread heavy wind effect and wind slabs in exposed terrain from recent northeast winds. A significant number of operators in the region have been hesitant to drop storm slab problems from their assessments, likely owing to this problematic interface.
Around 100 to 130 cm of snow may now rest on a small layer of surface hoar crystals that was buried mid-February. This layer is most likely found around treeline elevations in areas sheltered from the wind. This layer generally appears to be dormant now but could still be capable of producing avalanches in isolated areas. The remainder of the mid-snowpack is generally strong.
A layer of large and weak facets that formed in November is deeply buried, found near the base of the snowpack. The likelihood of human triggering this layer is currently low given its depth. However, alarming reports like this one from Glacier National Park continue to trickle in, confirming it is still best to avoid steep, thin, rocky slopes near ridges at alpine and upper treeline elevations.
Tuesday
Brief clearing before clouding over again. Light northwest winds shifting southwest and increasing.
Wednesday
Cloudy with isolated flurries, increasing in late afternoon and overnight. Light west winds shifting southwest and increasing. Treeline high temperatures around -9.
Thursday
Cloudy with continuing snowfall bringing 10-25 cm of new snow, including overnight amounts, greatest in the north of the region. Moderate southwest winds, potentially strong in the alpine. Treeline high temperatures around -7.
Friday
Cloudy with continuing flurries bringing 5-10 cm of new snow, including overnight amounts. Light to moderate southwest winds. Treeline high temperatures around -8.
More details can be found in the Mountain Weather Forecast.
