As we head into a couple days of friendlier weather, resist the allure of bigger terrain and be especially wary of recently wind loaded features. Recent storm snow sits on a thick layer of weak, sugary facets and bonding at this interface may take some time.
A persistent weak layer buried 80 to 160 cm deep is being tested by significant new snow loads. We are awaiting reports of avalanche activity during the storm, specifically whether they indicate step- downs to this layer, to determine whether or not it continues to be a problem.
40-80 cm of recent storm sits overtop a thick layer of weak, sugary facets. Due to this weak interface, storm slabs may be surprisingly sensitive to triggering. At upper elevations, the recent snow has likely been blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features.
MONDAY NIGHT: 5-10 cm new snow, northwest wind easing strong to moderate, freezing level dropping from 1500 m to valley bottom.
TUESDAY: Scattered flurries with sunny periods, light northwest wind, freezing level 1000 m.
WEDNESDAY: Mix of sun and cloud, light westerly wind, freezing level 1000 m.
THURSDAY: 10-15 cm of new snow, moderate southwest wind, freezing level 1000 m.
The majority of observations that came in during the storm on the weekend were skier controlled storm slabs size 1-2, with a few natural size 2-3. Explosive control work in neighboring Glacier National Park on Monday produced very large avalanches, size 3-4. We suspect that reports of more widespread natural activity will start to come in as the weather clears and visibility improves.
Last week, older winds slabs in cross and reverse-loaded terrain features surprised a number of skiers, triggering avalanches up to size 2.5. This MIN report describes one such event on a southeast aspect in the alpine that stepped-down to deeper persistent weak layers to produce a very large avalanche that ran full path and destroyed mature timber.
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.
Recent snowfall amounts of 40-80 cm have been highest in the north and lowest in the south of the region. The recent snow has likely been blown around by strong to extreme westerly winds, forming thick, reactive slabs in lee features at upper elevations. At lower elevations, the new snow is likely settling and gaining slab property in the mild temperatures.
The recent snow sits on old, facetted and/or extensively wind affected snow that sat on the surface during the extended cold, windy drought period. There is uncertainty around how long it will take for this interface to bond.
We've now got 80 to 160 cm of snow above the late January persistent weak layer which is composed of surface hoar in openings in the trees and/ or a crust on steep solar aspects. This layer is most prevalent at treeline and has produced very large low probability/high consequence avalanches this month. Preliminary reports of avalanche activity during this storm have not included any step downs to this layer, indicating that it may have healed.
