Regions
Northwest Inland.
Strong to Extreme winds accompanied by 5-10 cm will likely build reactive wind slabs. Deeper slabs may exist on leeward slopes.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
Wednesday night-Thursday: snow amounts 5-10 cm with light winds gusting extreme from the SW. Alpine temperatures near -3 and freezing levels 1000 m. Friday: Periods of snow 2-7 cm. Ridgetop winds light gusting to 65 km/hr from the SW. Alpine high of -8 and freezing levels falling to 500 m. Saturday: Mostly cloudy with sunny periods. Ridgetop winds generally light. Alpine temperatures high of -10 and freezing levels at valley bottom.
Avalanche Summary
On Tuesday, no new avalanche activity was reported. With forecast snow and strong winds, reactive wind slabs are likely over the next several days.
Snowpack Summary
New snow 5- 15 cm accompanied by strong- extreme SW winds is burying a plethora of old snow surfaces including isolated wind slabs and wind press, sun crusts, and surface hoar. The new snow may have a poor bond to these interfaces. At treeline elevations the average snowpack depths are 150-190 cm and a well consolidated mid-pack of approximately up to 100 cm sits above weak basal facets (sugary snow) near the ground. Recent snowpack testing has shown sudden results down 100 cm within the facetted snow. Although possibly a low probability for triggering the persistent slab exists, the consequence would be high as it has the potential to produce very large destructive avalanches especially in shallower snowpack areas.
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.