Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Highlights

Fresh snow has created new avalanche hazards at upper elevations. You can trigger slabs of new and drifted snow 8 inches or more thick on steep, upper elevation slopes, particularly at the tops of chutes and on the lee sides of exposed ridges and summits. At mid-elevations, look for similar slabs that are the exceptions to the generally safe conditions. Upper-elevation terrain also harbors weak layers close enough to the surface to be collapsed by the weight of a person or snowmachine. You can find great riding on wind-sheltered slopes where the pitch stays the same throughout or gets less steep.    

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    The combination of 3 to 10 inches of fresh snow and moderate winds has deposited soft slabs on a variety of weak and slick old snow surfaces. Expect these to be sensitive to a person's weight today, and dangerous on steep slopes where they're 8 inches or so thick. The slabs should announce themeselves with shooting cracks and whumpfing collapses. They'll be most common in upper-elevation terrain prone to collecting drifted snow, and isolated at mid elevations. Before committing to steep terrain, look downslope and assess the consequences of getting caught. If you see gullies, trees, or cliffs, move on to slopes with runouts free of obstacles and terrain traps. 

    (1/28/2021) Small, human triggered slab avalanche. North aspect at about 7000 feet. Glacier National Park.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2

    As you approach 7000 feet elevation, the 1/13 crust thins and peters out, so look for the isolated slopes that harbor weak layers buried 1 to 3 feet deep. You can distinguish this terrain by slope shape and snow surface. Specifically, check for convex profiles (less steep to more steep) and signs that winds have left variable snow depths, like drifts alongside eroded patches or rocky outcrops. On these slopes, weak layers that have been dormant the past two weeks may have reactivated under the incremental loading of the past few days. In a snowpit, these layers appear as a gray stripe of surface hoar, or thin, soft layers of sugary snow around thin crusts. You can avoid this guessing game by sticking to slopes less than about 35 degrees that have a constant pitch throughout, or that get less steep lower on the slope. 

    (1/25/2021) Slopes that are not covered by the 1/13 crust, that did not slide during the last avalanche cycle, are still suspect for the persistent slab problem.

Avalanche Discussion

Three to 10 inches of low-density snow over the past two days. It's no snow-pocalypse, but we'll take it. The fresh snow is enough to create new avalanche hazards in some parts of the region, while potentially reactivating older concerns. Wind speeds and accumulations have varied across the region, so the changes are localized. In general, though, slopes where you find 8 inches or more of recent or drifted snow will be the most dangerous.  Shooting cracks and whumpfing collapses are clear signs of this danger. 

Thursday, observers reported triggering small slabs of drifted snow in John F. Stevens Canyon and soft, breakable cornices in the southern Whitefish Range. I expect that triggering similar slabs will be the most common hazard today. Look for soft but dense slabs of drifted snow about a foot thick near ridges at mid and upper elevations. They'll be most common and largest on northerly and easterly slopes near the crest of the Swan and Flathead Ranges, and the peaks around Lake McDonald. They'll be most dangerous where they can carry you into or over terrain that increases the chances of deep burials or trauma - gullies, trees, or cliffs. 

Because the fresh snow has fallen on a buffet of weak or slick interfaces, the new snow hazards won't be limited to areas prone to collecting drifted snow. The weak snow surfaces include facets and surface hoar that developed in the recent cold weather, as well as wind-polished crusts. As the fresh snow consolidates above these layers, slabs will likely become reactive to the weight of a person or snowmachine. Most will not produce enough debris to bury or injure a person. The exceptions will be about 8 inches or more thick. These can break wide enough to catch a person, and the debris may run fast and far on underlying crusts. Thursday, the signs of this danger were obvious - audible collapses and cracks shooting up to 25 feet away. On very steep slopes where the snow hasn't yet consolidated into a slab, you may be able to trigger sluffs of loose, cohesionless snow that can pose a danger above terrain traps. 

For two weeks, we've been highlighting slopes above about 7000 feet where the 1/13 storm didn't deposit a hard crust. In this terrain, a person could still trigger slabs that break 1 to 2 feet deep on weak layers of buried surface hoar or facets near crusts. With incremental loading by recent snow over the past few days, those concerns linger on steep slopes with likely trigger points. Those are spots where the weak layers tend to be most developed, or are closer to the surface and more easily collapsed by the weight of a person or snowmachine.  Visualize slopes with convex profiles (steeper further downslope) or that have variable snowdepths due to underlying talus and rock outcrops, or because of erosion by cross-slope or upslope winds. Look for terrain like this and then find alternatives, so you can avoid exposing yourself and your party to the potential of triggering large and dangerous slabs. 

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