Regions
Northwest Inland.
Stormy conditions on Tuesday are expected to form new wind slabs at higher elevations. Use a conservative approach in wind affected terrain and watch for changing conditions throughout the day as the storm progresses.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Stormy conditions are expected Monday night through Tuesday night. 5-20cm is expected between Monday night and Tuesday afternoon with strong southwest winds in the alpine and freezing levels around 1000m. Another 5-10cm is expected Tuesday overnight before a weak ridge of high pressure dries things out on Wednesday. A mix of sun and cloud is expected for Wednesday with freezing levels around 500m and light alpine wind. There is uncertainty for Thursday with one model showing a weak storm pulse and another showing dry conditions persisting.
Avalanche Summary
On Sunday, a ski cut produced a size 1 storm slab avalanche on an east aspect at 1400m elevation. The slab was 25cm thick. Numerous natural avalanches were also heard from steep rocky terrain but not observed. On Saturday, a natural size 1.5 wind slab avalanche was reported on a west aspect at 2200m elevation which was 20cm thick. No new avalanches were reported on Friday. On Thursday in the far north of the region, a skier accidently triggered a size 2 persistent slab avalanche on a northwest aspect at 1650m and explosives triggered a size 2.5 persistent slab on a northeast aspect at 1400m. These both failed on the early January surface hoar layer down 100-150cm. New wind slabs are expected to form on Tuesday and old wind slabs may still be reactive to human-triggering. In the far north, deeply buried persistent weak layers may still be reactive to large triggers such as a small avalanche stepping down or a cornice failure.
Snowpack Summary
15-35cm of snow has accumulated over the past week and overlies a melt freeze crust which extends up to around 2000m elevation. This recent snow is reported to be bonding well to the crust but some avalanches have released on it. Moist snow was reported below 1100m on Sunday and wet snow below 800m. Recent wind has redistributed the surface snow at higher elevations forming wind slabs in leeward features. The early January surface hoar/facet layer is down around 1m and remains a concern for commercial operations. This layer seems the most reactive in the north of the region but may still be a concern in the south as well. Shallow snowpack areas in the east and north of the region may have a weak base layer of facetted snow on or just above the ground.
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.
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.