Continued snowfall, mild temperatures and strong winds are a recipe for HIGH avalanche danger. Minimize your exposure to avalanche terrain Monday.
A persistent weak layer buried 70 to 100 cm deep has produced recent large avalanches and is expected to become increasingly reactive this weekend. This layer has been most sensitive around treeline, but may also extend into alpine terrain and/or down into the trees. Avalanches failing on this interface will be large and destructive.
30-100 cm of recent storm snow continues to accumulate and settle into a slab in the mild temperatures. This storm slab sits overtop a thick layer of weak, sugary facets. At upper elevations, the recent snow will be easily blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features.
SUNDAY NIGHT: 10-25 cm of new snow, freezing level 1500 m, strong to extreme westerly wind at ridgetop.
MONDAY: 5-15 cm of new snow, freezing level 1300 m, strong westerly wind easing slightly.
TUESDAY: 5-15 cm of new snow overnight then clearing, freezing level 1000 m, light northwest wind.
WEDNESDAY: Sunny, freezing level 1000 m, light northwest wind.
Preliminary reports include a natural cycle observed in neighboring Glacier National Park on Saturday and Sunday, with storm slab avalanches up to size 3. Otherwise, as of Sunday afternoon, recent avalanche activity within the storm snow has been limited to a few natural and skier triggered loose dry and soft storm slab up to size 1.5.
Last week, older winds slabs in cross and reverse-loaded terrain features surprised a number of skiers, triggering avalanches up to size 2.5.
Persistent slab avalanche activity has dwindled since the beginning of February but a conservative approach is critical as this storm begins to add weight to it. This old MIN report from Joss Mountain is a helpful example of the lower elevation slopes where the surface hoar has been preserved and may reactivate this weekend. This old MIN report from the Gorge is a helpful example of the lower elevation slopes where the surface hoar has been preserved and may see a resurgence of reactivity in the storm.
10-20 cm of new snow is forecast to fall overnight. Recent snowfall amounts have been erratic through the region, varying widely, between 20-80 cm even within small sub regions. At upper elevations, all of this recent snow will be easily blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features. At lower elevations, this recent low density snow is likely settling and gaining slab property in the warm temperatures.
Below sits the old, facetted and/or extensively wind affected snow that sat on the surface during the extended cold, windy drought period. There is uncertainty around the strength of this interface with the new snow, which will surely be tested by the ongoing loading.
We've now got 70 to 100 cm of snow above the late January persistent weak layer which is composed of surface hoar. This layer is most prevalent at treeline and in "treeline-like" features such as cutblocks. On steep south-facing terrain, this layer may consist of facets on a sun crust, which has produced very large low probability/high consequence avalanches.
