Regions
Northwest Coastal.
Confidence
Fair - Timing or intensity of solar radiation is uncertain
Weather Forecast
Synopsis: Northwest flow gives way to a frontal system that barely brushes the region on Tuesday night/Wednesday.Monday: Flurries, with some breaks in the cloud giving daytime warming. Afternoon freezing level around 500 m. Winds light northwesterly.Tuesday: Light snow starting late in the day. Freezing level 500 m. Southerly ridgetop winds increasing to around 50 km/h.Wednesday: 5-10 cm new snow. Freezing level 500 m. Winds light southeasterly.
Avalanche Summary
On Saturday, natural slab avalanches were reported up to size 3 on all aspects in the alpine. Solar aspects were particularly reactive and daytime warming triggered slides on the surface hoar layer from March 9th buried approximately 45 cm. Cornice releases were also numerous and triggered slab avalanches on the slopes below. On Friday, a skier triggered a size 2 avalanche on the March 9th surface hoar layer with a crown depth of 45 cm. Several other skier-triggered avalanches occurred on north and west aspects on the same weak layer.
Snowpack Summary
25-50 cm of recent storm snow rests on a variety of old snow surfaces, including crusts, previous wind slabs and surface hoar. Warmer temperatures and recent winds have now set this new snow into a reactive slab, with wind slabs building in exposed lee areas. The surface hoar (buried March 9th) is reported to be well-developed and fairly widespread, at treeline and alpine elevations. Recent snowpack tests have produced sudden failures with moderate loading force at this interface. The mid snowpack is generally well settled and strong. Cornices are large and untrustworthy, especially when the sun is out.
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.
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.