New snow is expected to form reactive storm slabs. In the north of the region, storm snow may test the strength of deeply buried weak layers, and step-down avalanches may be possible. Stick to simple terrain and minimize overhead exposure to welcome in the New Year.
A problematic weak layer is buried 30-70 cm deep in the snowpack in the north of the region. New snow and wind will add a critical load to this fundamentally unstable snowpack structure. This problem is largely absent in the south of the region.
New snow is expected to become increasingly easy to trigger as its depth increases. Slab problems will become increasingly widespread, with wind-loaded areas seeing more slab formation. Ongoing snowfall is covering a new layer of surface hoar observed in the north of the region.
Tuesday night: Cloudy, 20-30 cm of snow, strong southwest winds, alpine temperatures around -1 C with freezing levels around 1400 m.
Wednesday: Cloudy, 5-15 cm of snow, strong southwest winds, alpine high temperatures near -2 C with freezing levels around 1300 m.
Thursday: Mostly cloudy, isolated flurries with 2-4 cm of snow possible, light variable winds, alpine high temperatures around -7 C with freezing level dropping below 500 m.
Friday: Mostly cloudy, 15-30 cm of snow, strong southwest winds, alpine high temperatures near 0 C with freezing levels rising to 1600 m.
No recent avalanche activity has been reported, but observations are limited for this region. A natural avalanche cycle is expected on Wednesday.
In the widespread avalanche cycle a week ago, avalanches were reported to be running to valley-bottom in the north of the region and failing on deeply buried weak layers. See this MIN report of an avalanche involvement from last week for an example of this avalanche problem.
The possibility for large human-triggered persistent slab avalanches remains a concern in the northern part of the region (e.g., Duffey, Hurley, etc.), especially as reactive storm slabs create the potential for avalanches to step-down to these layers.
Overnight snowfall is forecast to bring storm snow totals to 20-30 cm for most of the region. This will create a widespread storm slab problem at all elevations that will need to be managed conservatively. At higher elevations, strong southwest winds are expected to exacerbate the reactivity of the new snow in drifted areas. The new snow is falling on another recent layer of surface hoar that has been showing increasing reactivity as snow accumulates.
The snowpack below varies significantly between the northern and southern parts of the region. In the north (e.g. Duffey, Hurley), the upper snowpack consists of around 30-70 cm of snow that overlies a weak layer from late November composed of sugary faceted grains and hard melt-freeze crust. This presents a persistent slab problem that may remain problematic for weeks to months with the potential to produce large and destructive avalanches. This persistent weak layer is largely absent in the south of the region.
