Storm slabs are expected to remain reactive through the weekend, especially on solar slopes if the sun comes out or on wind loaded slopes in the alpine.
Confidence
Moderate - Track of incoming weather systems is uncertain on Sunday
Weather Forecast
The last in the series of storms fizzles out Saturday morning. Unsettled conditions will continue through Saturday bringing mostly cloudy skies with some clearing later in the day. Alpine temperatures near -6 with freezing levels falling to 1000m. Ridgetop winds will blow light from the southwest. Sunday morning could see strong outflow winds along coastal inlets with the next Pacific frontal system rolling in by the evening. This could bring anywhere from 5-15 cm, accompanied by strong southerly winds and freezing levels near 1100m. Monday will remain unsettled with cloudy skies and flurries.
Avalanche Summary
On Thursday, a skier remotely triggered a size 1.5 wind slab from a gully feature on an easterly aspect at 2300m. Explosives control showed numerous size 2 storm slab releases from 1900-2300m. Storm slab avalanches are expected to be remain reactive to human-triggering with ongoing snowfall and wind, especially if the sun pokes out on Saturday. Cornices are reported to large and fragile, and may fail under the weight of a person.
Snowpack Summary
Recent storm accumulations are 50-80cm over the past week. This snow overlies a thick rain crust which extends into the alpine. The snow is reported to be bonding well to the crust and all the recent avalanches have been failing within the recent storm snow, not on the crust interface. At treeline and below, other crusts may exist in the upper snowpack due to the recently fluctuating freezing levels. As the freezing level continues to drop, a crust is expected to form at lower elevations. Ongoing southeast through southwest winds have been loading leeward features in the alpine and large cornice development has been reported over the last few days. The weak surface hoar layer from early January can be found down over a meter and is still reactive in isolated snowpack tests but triggering an avalanche on this layer has become unlikely.
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.