Regions
Northwest Inland.
Light snowfall and ongoing southwest winds are expected to continue to from fresh wind slabs at and above treeline this weekend.
Confidence
Moderate - Forecast snowfall amounts are uncertain
Weather Forecast
The Northwest Inland should see a steady progression of relatively weak frontal bands through the weekend. FRIDAY: Freezing level around 1200 m, trace of snow, light to moderate southwest winds. SATURDAY: Freezing level around 1300 m, 1 to 5 cm of snow, moderate to strong south/southwest winds. SUNDAY: Freezing level holding at 1200 m, 1 to 5 cm of snow, light variable winds.
Avalanche Summary
On Wednesday storm slabs to size 1 were reported on north/northeast facing terrain at treeline, running on the old melt freeze crust. Small loose wet avalanches below treeline were also reported. In recent days a natural cornice fall triggered a size 3 slab avalanche in the southwest corner of the region. The avalanche occurred on an east aspect in high alpine terrain and stepped down to a few persistent weak layers from January and December. This avalanche demonstrates how a cornice fall can trigger layers that may otherwise be difficult to trigger.
Snowpack Summary
In the last 48 hours 5 to 20 cm of new snow has fallen with moderate to strong southerly winds. At treeline and in the alpine small wind slabs are expected to have formed. These wind slabs overlie a variety of surfaces which include a hard crust on solar aspects above 1300 m, moist or refrozen snow on all aspects below 1300 m, and settled wind slabs in exposed high elevation terrain. A layer of surface hoar from early March can be found down 50 to 70 cm but only seems to be a problem for the northern half of the region. Professional operators are still tracking two deep weak layers from early-January and early-February which can be found down about 1 meter or more. These layers are generally dormant but could wake up with substantial warming or a heavy trigger (like a cornice fall). Basal facets exist in most areas and have recently been reactive in the north of the region
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.
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.