Watch for signs of new wind slab formation on Monday. Alpine winds are forecast to become strong in advance of the storm which is expected to arrive Monday night.
Confidence
Moderate - Intensity of incoming weather systems is uncertain
Weather Forecast
The ridge of high pressure will be displaced by a low pressure system late Monday. A mix of sun and cloud is expected Monday morning and cloudy conditions with light flurries are expected for the afternoon. Alpine wind should be light in the morning and increase to strong during the afternoon. Treeline temperatures are expected to be around -12C. Around 10 cm of snow is expected on Monday night with strong alpine wind from the southwest. Another 5-10 cm of snow is forecast for Tuesday with moderate to strong alpine wind from the southwest and treeline temperatures around -10C. A mix of sun and cloud is currently forecast for Wednesday.
Avalanche Summary
On Saturday, a couple natural size 1 storm slab avalanches were reported from northeast and east aspects at 2250 m elevation. These were 15 cm thick slabs. Solar triggered size 1 avalanches were also reported from steep southeast facing terrain in the alpine. No new avalanches were reported on Friday. On Monday, it is possible that new wind slabs may develop in the afternoon as the wind increases. Old stubborn wind slabs may still be lingering from the strong wind that ended Friday morning and could be hiding under 10-20 cm of low density snow from Saturday.
Snowpack Summary
40-60 cm of snow has accumulated over the past week and overlies the mid-December interface which consists of a variety of old snow surfaces including wind affected snow, faceted (sugary) crystals, surface hoar crystals, and/or a thin sun crust on steep solar aspects. Recent tests suggest the new snow is well bonded to the interface in most areas. However, where a spotty layer of surface hoar has been preserved, the interface is still reactive in snowpack tests. In wind affected terrain, the storm snow had been redistributed into reactive wind slabs. These wind slabs may now be buried under 10-20 cm of low density snow which fell without much wind on Saturday. The thick mid-November crust layer typically sits 1-2 m down in the snowpack. Recent test results on this layer suggest it is generally well bonded to the adjacent snow with the possible exception of shallow snowpack areas. This layer is considered dormant at this time but remains an isolated concern that we will likely be tracking through much of the season.
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.