Regions
Banff Yoho Kootenay.
The drought is over . . . and so is the Low danger! Snowfall amounts are building by about 10-15 cm per day this week, burying a dangerous layer of surface hoar and then accumulating a slab above it. Time to change our habits from the last few weeks.
Weather Forecast
At last the weather pattern has changed and we are into a NW flow, which has warmed up the air and finally brought some new snow. Expect another 5-10 cm on Wednesday, then another 10-15 for Thursday. Temperatures will be from -10 to -13 with strong wind gusts. Perfect for burying the weak layers . . .
Snowpack Summary
5-15 cm of new snow overlies a series of weak layers (surface hoar, facets and suncrust) formed from the past three weeks of cold, dry weather. As of today, this new snow had been blown into thin, soft slabs in specific areas above treeline. With additional snow forecast over the next few days, expect the soft slabs to increase in depth and size.
Avalanche Summary
Several soft windslabs 10-15 cm deep have been ski cut in windloaded areas above treeline. Avalanche activity is expected to increase on Wednesday and Thursday with the additional new snow.
Confidence
Forecast snowfall amounts are uncertain on Wednesday
Problems
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Deep Persistent Slabs
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.