Regions
Northwest Inland.
Incoming precipitation with mod SW winds will drive the wind slab problem. Choose conservative terrain at treeline and above.
Confidence
Fair - Due to the number and quality of field observations
Weather Forecast
An upper level trough will move into the area from the Pacific tonight and into Tuesday bringing light precipitation, with 5 to 10 cm of new snow at upper elevations. A bit of clearing on Wednesday, then more light precip. on Thursday and Friday. Winds from the S-SW, moderate to strong at upper elevations throughout the forecast period. Freezing levels should remain around 500 m but may spike to 1800m on Wednesday, then return to around 500 m on Friday.
Avalanche Summary
No reports of avalanche activity yesterday.
Snowpack Summary
Recent winds have created wind slabs on lee aspects in exposed terrain. Below the recent storm snow is a rain crust at lower elevations and a temperature crust formed on the surface above treeline in some locations from the recent temperature inversion. In the mid-pack a surface hoar layer has been reported, although it appears to be spotty in distribution. Near the base of the snowpack is a November crust-facet combination.
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.