Reactive storm slabs will build on Thursday with new snow, strong wind, and warmer temperatures.
Be mindful of the lingering deep persistent slab problem. Caution on shallow snowpack areas and thin to thick steep rocky zones.
5-20 cm of snow accompanied by strong winds and warmer temperatures by Thursday afternoon may build reactive storm slabs. They will likely be touchier on leeward slopes with deeper, wind-loaded conditions, especially where they sit on surface hoar or crusts.
Loose-dry sloughing may exist on steeper terrain features.
A deep persistent slab problem formed by a crust down 140-200 cm has created a low likelihood, high consequence scenario that is difficult to forecast. This old and deeply buried weak layer is one of the most challenging problems to predict and for people to safely manage in the backcountry.
Although it hasn't shown recent activity, this problem is most likely to be triggered from thin or variable snowpack depth on wind-affected features, ridge crests, and near rocky outcroppings. The same feature or path can slide repeatedly when the persistent weak layer is reloaded with recent snow.
Thursday Night: Increasing cloud cover later in the evening with some new snow forecast up to 5 cm. Ridgetop wind light from the southwest and alpine temperatures near -5.
Thursday: Snow 5-15 cm accompanied by moderate ridgetop wind from the southwest. Alpine temperatures warming to near -2 and freezing levels 1300 m.
Friday: Mostly cloudy with sunny periods. Ridgetop winds moderate from the northwest. Alpine temperatures near -5 and freezing levels 1000 m.
Saturday: Cloudy with sunny periods. Alpine temperatures near -2 with light northwest winds. Freezing levels near 1100 m.
No new reports by Wednesday afternoon.
During the warm previous storm, mid-last week, a large widespread avalanche cycle occurred with most avalanches releasing within the storm snow and some on Jan 11 surface hoar layer. Strong solar radiation and warm temperatures were also at play after the storm, producing numerous avalanches at all elevations and scouring avalanche paths to the ground in places.
We have not heard reports of persistent slab avalanches for the last 10 days when we saw numerous sizes 2 to 3 avalanches releasing on the early December facet/crust layer. Although the activity on this layer has tapered off, it still remains a concern as it is extremely difficult to forecast.
The new snow forecast for Thursday may have a poor bond to the plethora of snow surfaces. These older surfaces consist of wind-scoured on westerly slopes to soft snow and new surface hoar development. Around 1900 m and below, a crust is now capping the upper 30 cm of the snowpack which has settled significantly with the past mild temperatures. The crust varies from thin/breakable to thick/supportive, according to the aspects and elevation. Below this, feathery surface hoar and additional melt-freeze crusts exist. Lower elevations have gone through a melt-freeze cycle with past freezing levels up to 1500m.
The early December crust/facet layer has been responsible for sporadic but very large, persistent slab avalanches over the past month. The crust is now buried 120-200 cm deep except in thin, wind-affected areas near ridgetops where most of the recent avalanches have been triggered.
