Fridays storm is driving the danger ratings. If your local riding area receives more than 30 cm of new snow expect the danger to be HIGH at upper elevations.
Confidence
Moderate - Forecast snowfall amounts are uncertain on Friday
Weather Forecast
An organized low pressure system continues to intensify over the Pacific Ocean bringing strong winds and significant precipitation by Friday and through the weekend. A ridge of high pressure will set up Sunday with an arctic front forming along the coast. Residual moisture will exist Sunday and Monday so you can expect unsettled conditions through Tuesday before the Arctic front sets in with a typical clear, dry and cooling trend. Friday: Freezing Levels 1200 m, alpine temperatures -2.0, snow amounts 10-25 cm, strong SW winds.Saturday: Freezing levels 1100 m, -6.0, snow amounts 5-10 cm, moderate-strong SW winds.Sunday: Freezing levels dropping to valley bottom, alpine temperatures high of -9.0, isolated flurries, and moderate West winds.
Avalanche Summary
Reports on Wednesday suggest that recent wind slabs remain reactive to rider triggers in isolated lee terrain. This next system will bring significant wind and snow to the region and natural avalanche activity will be on the rise for Friday and Saturday.
Snowpack Summary
New storm slabs will blanket the region, continuing to bury a recent surface hoar layer. Areas with this layer and where buried wind blasted surfaces exist may be more reactive. Watch for shallow wind affected areas that may not gain strength as quickly as deeper well settled snowpack areas. Snow depths increased rapidly last week bringing treeline snow depths to about 200 cm. The mid- November crust is buried anywhere from 20-200 cm depending on aspect and elevation. So far this crust has been unreactive, however; this new load may be the breaking point especially in shallower snowpack areas.
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.
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.