Storm slabs continue to develop at higher elevations and will be reactive to human-triggering on Wednesday. Conservative terrain choices remain critical.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain
Weather Forecast
The next storm pulse is expected to arrive Tuesday overnight and is expected to bring 5-20cm of new snow by Wednesday afternoon. Amounts are expected to be greatest along the coast so there is some uncertainty as to how much will make it inland. Alpine winds are forecast to be moderate from the south and freezing levels could reach over 2000m. A bit of a break is expected Wednesday afternoon or evening but the next storm pulse arrives Wednesday night. Another 10-20cm is expected by Thursday morning with freezing levels around 2000m and moderate alpine wind from the south. Unsettled conditions and light flurries are expected during the day on Thursday and on Friday. Freezing levels are forecast to fall below 1500m and alpine winds should generally be light.
Avalanche Summary
No new avalanches were reported on Monday. On Sunday, a natural size 3 wind slab avalanche was reported. The slab was 50cm thick and ran on an old reloaded bed surface in a shallow snowpack area. Ski cutting on Sunday produced size 1 soft slabs within the recent storm slabs. Lots of cracking was reported but limited propagation. On Saturday, a ski cut produced a size 1 slab avalanche in a leeward feature on a northeast aspect at 2200m. The slab was 10-20cm thick. Rain-soaked lower elevations are unlikely to see much in the way of avalanche activity on Wednesday, but storm slab avalanches are expected to be reactive to human-triggering at higher elevations with ongoing snowfall and wind continuing overnight Tuesday and Wednesday. Cornices are reported to large and fragile, and may fail under the weight of a person.
Snowpack Summary
30-60cm of new snow has accumulated in the last week with largest amounts to south of the region. This new snow overlies a thick melt-freeze crust which extends into the alpine. The snow is reported to be bonding well to the crust and recent avalanches have been failing within the recent storm snow, not on the crust interface. Moderate wind transport and cornice development have both been reported over the weekend and Monday night's storm will continue to develop wind slabs and cornices. In the deepest snowfall areas, a more widespread storm slab may become reactive with the additional loading overnight. Moist snow is being reported up to around 1800m on Sunday. Rain will continue to soak lower elevations overnight. 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.