Regions
Northwest Coastal.
The ongoing storm continues and is driving the Danger Ratings. The snowpack does not typically handle rapid change well and deeply buried persistent weak layers have become overloaded.
Weather Forecast
Another frontal system is targeted for the North Coast Wednesday/ Thursday bringing moderate-heavy precipitation and freezing levels hovering around 800-1200 m.Tuesday night: Snow amounts 5-10 cm. Ridgetop winds strong from the SW. Freezing levels falling to 600 m. Wednesday: Snow amounts 15-25 cm. Ridgetop winds strong from the SW. Freezing levels rising to 800 m. Thursday: Snow accumulations up to 20-25 cm. Ridgetop winds light from the SW gusting strong. Freezing levels near 1000 m.Friday: Unsettled conditions follow the frontal system bringing light-moderate precipitation, light SW winds. An approaching upper ridge will gradually start to dry things out later in the day.
Avalanche Summary
An ongoing, widespread natural avalanche cycle up to size 4 occurred on Monday and will likely continue through the forecast period. Deep persistent weak layers are overloaded in some places, initiating very-large avalanches. Travel in avalanche terrain is not recommended.
Snowpack Summary
The recent storm has brought 50 cm or more too coastal regions (60-100 mm) forming thick storm slabs. Consistent strong southerly winds have transported the new snow building thick wind slabs on leeward features and cornices have grown large. At lower elevations the snowpack has become saturated with rain adding to the already spring-like isothermal conditions.Two persistent weak interfaces exist deeper in the snowpack. The early March layer can be found down 75 -140 cm. The early February layer is down 150 - 240 cm. These layers have become overloaded in some places initiating some very-large 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.
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.
Wet Slabs
Wet Slab avalanches are the release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slabs can be very unpredictable and destructive.