Recent snow and strong winds continue to rapidly load fragile old surfaces creating dangerous avalanche conditions. Slabs of new and drifted snow will be thick and sensitive to the weight of a person or snowmachine. In isolated areas, avalanches may step down to weak layers of old snow. Avoid slopes steeper than about 30 degrees, convexties, and runouts of large avalanche paths.
1.9 inches of new water have fallen in Noisy Basin over 48 hours. Strong winds will continue to rapidly load fragile interfaces of weak, low-density snow and facets or stiff old wind slabs through Saturday. Top-heavy storm slabs will be larger and more reactive with increasing elevation and on leeward slopes. Avoid open slopes steeper than about 30 degrees, the runouts of avalanche paths, and convexities.
Layers of weak facets and crusts from mid-December sit 2-4 feet below the surface where the snowpack is shallowest: near and below treeline and on scoured and southerly slopes. Depth hoar and facets rest near the ground and above an early December crust deep in the snowpack. These layers have not been loaded rapidly until now. Observers Friday reported loud collapses below treeline, which may be the result of failure in one of these layers. Dense new slabs may overload these weak structures, and slides involving the storm snow may step down to these layers. In either case, the slides would be larger and more destructive.
(1/5/2022)
We found a shallower, weaker snowpack on south- and west-facing mid-elevation slopes
Photo: Zoellner
Between 11 and 20 inches of new snow and 1.5 to 3.1 inches of snow water equivalent have fallen over the past 36 hours. Snowfall tapered off in the Swan Range on Friday. Weather stations in the Northern Whitefish Range and near Marias Pass reflect continued snow showers as of early Saturday morning.
Temperatures rose throughout Friday, creating upside-down, top-heavy storm slabs. Strong winds drifted new snow into thicker and more reactive slabs in steep, open terrain. Temperatures have fallen overnight. Sustained winds are driving new slab formation.
Before the onset of heavy snowfall Thursday afternoon, the snow surface consisted of very-low-density snow and near-surface facets resting atop old hard wind slabs in open terrain. Riders reported natural avalanches and shooting cracks on Friday. BNSF Avalanche Safety reported a large natural avalanche that ran as recently as midnight. So, new and drifted snow is overloading weak interfaces.
Concerning layers of faceted snow and crusts sit 2 to 4 feet below the surface and near the ground above a crust buried in early December. On steep slopes with a shallower snowpack (so, near and below treeline, or on rocky, windward, and southerly slopes), it’s more likely that avalanches can step down, becoming larger and more dangerous. Riders in the Whitefish range reported loud, whumpfing collapses at lower elevations Friday, which are likely the result of failure in one of these bruised weak layers.
