Regions
Northwest Coastal.
Confidence
Fair - Forecast snowfall amounts are uncertain
Weather Forecast
Synopsis: A frontal system will hit the coast early on Tuesday spreading moderate snow and increasing winds. This is followed closely by a second system late Wednesday. Tuesday: Light to moderate snowfall – 10-15 cm. The freezing level is around 600 m and winds increase to very strong from the south-southeast. Wednesday: Unsettled with light snow during the day and heavier snow in the evening (~15 cm). The freezing level is steady near 600-700 m and winds remain strong from the south-southwest. Thursday: Moderate to locally heavy snow. The freezing level climbs closer to 1000 m.
Avalanche Summary
Better weather on Sunday allowed for more observations of a fairly widespread natural avalanche cycle in higher alpine terrain. Slabs were reported to be size 2-3 and were primarily from wind loaded features. On Sunday, the storm and wind slabs were still reactive to ski cuts to size 1.5 and explosive control to size 3. Loose moist avalanches were also observed on steep southerly aspects when the sun came out.
Snowpack Summary
Mild temperatures have promoted rapid settlement of the recent storm snow. Strong southwest winds also created deep and weak wind slabs in exposed lee terrain and cross-loaded gully features. This new snow fell on a crust, old wind slab, and/or surface hoar depending on aspect and elevation. One recent snowpack test near Shames resulted in a hard "pops" shear on the buried surface hoar, down around 60 cm at the time. Most areas are reporting that the snow has bonded well to the underlying melt-freeze crust. Cornices that now loom over many features may become weak with continued growth and/or daytime warming. The mid and lower snowpack layers are generally well settled and strong. The exception seems to be the Bear Pass area where basal facets are still reported.
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.