Lingering instabilities will provide tricky conditions in the alpine, especially with the effects of solar radiation. Use a conservative approach to travel and avoid wind-loaded features.
Confidence
Fair - Due to the number of field observations
Weather Forecast
On Tuesday, a weak front will move onto the coast bringing generally light precipitation to Interior regions later Tuesday night. Ridgetop winds will blow light-moderate from the E-SE and freezing levels will range from 1500-1800 m. Wednesday will see mainly cloudy skies with light convective precipitation which will continue through midday. Ridgetop winds will be light from the NW with freezing levels near 1600 m. On Thursday, a strong front will approach the North Coast and move inland bringing precipitation amounts to the Interior regions anywhere from 2-10 mm. Ridgetop winds will be strong from the SW and freezing levels will hover around 1500 m.
Avalanche Summary
On Sunday, reports of numerous size 1-3 natural slab avalanches and skier remote avalanches were observed. Many of these larger events started as wind and storm slab avalanches from upper elevation start zones and entrained moist snow at lower elevations, running to valley bottoms. These avalanches have occurred on a variety of aspects with northerly aspects being most reactive. These have initiated from 2000-2300 m in elevation. Most of these failures have been on a weak storm shear. I suspect that the deeper releases are occurring on the mid-February layer down 30-70cm.
Snowpack Summary
At higher elevations, new snow sits over a plethora of surfaces including moist snow, crusts, wind affected surfaces, and/or old wind slabs which may still be reactive especially to rider triggers. Rain has soaked the upper snowpack to around treeline elevation. In the alpine, strong winds during the storm have redistributed the new snow into wind slabs in leeward terrain features. Prior to the storm, 30-50cm of snow was sitting over a weak facet/crust layer that was buried in mid-February. This layer has been quite reactive recently and may increase the likelihood of triggering a storm slab. The late-January crust/surface hoar layer (over 1m deep) and the mid-January surface hoar (around 1.5m deep) have been dormant for several weeks.
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.
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind drifts of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.