Conditions are expected to be tricky on Tuesday following the storm Monday night and conservative decision making is very important. Use extra caution on steep south facing slopes in afternoon if the sun is out in full force.
Confidence
Moderate - Timing, track, or intensity of incoming weather system is uncertain on Monday
Weather Forecast
20-30cm of new snowfall is expected at higher elevations from the storm Monday overnight. Alpine winds are expected to be strong from the SW and freezing levels could reach as high as 2000m during the storm. A ridge of high pressure should replace the storm early Tuesday morning. A mix of sun and cloud is expected for Tuesday with freezing levels around 1300m and light alpine wind. The next storm pulse is currently forecast to arrive Tuesday overnight and another 5-15cm of snowfall is forecast for Wednesday. Freezing levels are expected to reach around 1700m and alpine winds are forecast to be moderate from the southeast. Unsettled conditions and light snowfall are expected for Thursday.
Avalanche Summary
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. On Friday, ski cuts produced size 1 loose wet avalanches at treeline in the recent storm snow. Rain-soaked lower elevations are unlikely to see much in the way of avalanche activity on Tuesday, but storm slab avalanches are expected to be reactive to human-triggering at higher elevations with ongoing snowfall and wind continuing overnight Monday. Wind loaded features should be the biggest concern but a more widespread storm slab avalanche problem may develop in the deepest snowfall areas. 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. 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
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.
Loose Wet
Loose Wet avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.