Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Sat
Alpin
4 · Fort
Limite forestière
4 · Fort
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

An AVALANCHE WARNING is in effect through 6AM Saturday. 

Heavy new snow and strong winds are creating very dangerous avalanche conditions. Slabs of new and drifted snow will be thick and easily triggered by the weight of a person or snowmachine. In isolated areas, avalanches may break in weak layers of old snow near the groud. Travel in and under avalanche terrain is not recommended.

Problèmes avalancheux

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Very Likely
    Taille
    1–3

    Up to an 1.5 inches of new water have fallen in the past 24 hours and another 0.5-1.0 inches are on the way for Friday. Winds have been moderate with strong gusts out of the southwest and will increase throughout the day. Temperatures are rising. Top-heavy storm slabs will be larger and more reactive with increasing elevation and on leeward slopes. New slabs are forming over weak, low-density snow and facets or stiff old wind slabs which means that they will bond poorly to the old surface.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1–3

    Layers of weak facets and crusts from mid-December sit 1-2 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 this storm. 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 more destructive.

    (1/5/2022)

    Layers of small facets did not propagate in large column tests, but may be more reactive with heavy new loading.

    Photo: Nelson

Avalanche Discussion

As of early Friday morning, Stahl Peak reports 0.9 inches of new snow water equivalent. Big Mountain has gotten 0.5 inches. Flattop Mountain in Glacier Park shows 1.1 inches of SWE. The Noisy Basin SNOTEL reads 1.5 inches of water. Tunnel Ridge reports 8 inches of new snow. Overnight and this morning, I’ve seen snowfall rates of over an inch per hour, which is a typical threshold for rapid loading. Rising temperatures mean the new snow is upside-down - dense snow above low-density snow. Southwest winds increased Thursday afternoon and held steady overnight. Another 0.5 to 1.0 inches of water, and 5 to 12 inches of snow are forecast by around 5pm Friday with increasing winds. Some freezing rain is possible. 

Thick slabs of new and drifted snow are developing over recent low-density snow and near-surface facets. Those weak layers sit atop old, hard wind slabs near and above treeline, and buried layers of sugary snow around crusts where the snowpack is shallowest: below about 5,500 feet and on typically windward slopes. So, we have rapid loading creating dense slabs. We have a weak interface and good sliding surfaces underneath the whole mess. 

Continued snowfall, rising temperatures, and increased winds are making natural avalanches likely and human triggered slides very likely. These avalanches may involve more than enough snow to bury a person even on small slopes. In terrain with larger avalanche paths, avalanches could run to valley floors. Backcountry travel (regardless of you chosen method of transportation) will be difficult. Avoiding steep slopes and the terrain below is the best recommendation I can give. Folks with snow piled up on their roofs should be wary of roof slides, especially if rain begins to fall midday. 

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