Confidence
Fair - Timing, track, or intensity of incoming weather is uncertain
Weather Forecast
Monday: A cold front brings moderate snow overnight Sunday. This is forecast to continue on Monday until the front departs in the afternoon. Moderate to strong westerly winds, switching to north-westerly and decreasing. Freezing level falling to valley floor.Tuesday: A brief ridge of high pressure should bring a break in precipitation and possibly clear skies and sunshine, although northern areas will probably cloud over by afternoon. Light winds and cool temperatures.Wednesday: Light snow. Increasing westerly winds.
Avalanche Summary
A natural avalanche cycle is under way, however poor weather is limiting observations. Natural, skier-remote and accidental avalanches were observed on Sunday to size 2. Avalanches reported throughout the week illustrate the unpredictable nature of persistent weak layers in the upper snowpack. On Thursday, a skier triggered a size 2.5 slab very easily and very close to recent ski tracks, which failed on buried surface hoar. On Wednesday, a cornice fall triggered a size 2 slab on a north aspect, which failed on the Feb 16. weakness. On Tuesday, numerous size 2-3 avalanches ran naturally, or with a remote trigger, on a variety of slopes. Most failed on upper snowpack persistent weaknesses. These layers are touchy and are likely to fail under the new storm load, creating large or widely-propagating avalanches.
Snowpack Summary
40-80cm snow has fallen, with snowfall forecast to continue until Monday. Strong winds and fluctuating temperatures are creating storm slabs and wind slabs on a widespread basis. Persistent weak layers buried in February are a key concern at all elevations and could be easily triggered by storm slabs, wind slabs or sluffs, or with a light additional load (like a sled or skier). Avalanches may be unexpectedly large, given the amount of fresh snow. Large cornices also loom as potential triggers for deep avalanches.
Problems
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-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.
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.
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.