Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

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

In steep terrain, steer clear of slabs of wind-stiffened and drifted snow 8 inches or more thick. These are more widespread at upper elevations. They will be more sensitive to a rider's weight where they sit on surface hoar or facets formed last week. Avoid committing to steep slopes above terrain traps. 

Avalanche problems

  • Wind Slabs

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

    On steep slopes with 8 inches or more of fresh or drifted snow, you can trigger slab avalanches large enough to bury or injure a person. You'll find these conditions more widespread as you climb higher in elevation and into terrain exposed to top-loading or cross-loading by westerly winds. Look for these slabs on the lee sides of ridges, downwind of saddles, and on the sides of chutes and gullies. 

    Slabs will  be more sensitive to your weight where they sit on surface hoar and/ or facets. This layer is most likely to have been preserved on mid-elevation, shaded slopes. Shooting cracks, whumpfing collapses, and recent natural avalanches are clear clues to this structure. In the absence of these clues, use inconsequential test slopes and hand pits to check for slabs sensitive to your weight. Avoid committing to steep slopes above terrain traps. 

    This layer of surface hoar formed during the recent dry spell and was buried by 12 cm of new snow on 12/7. It is very weak but the snow above has not yet formmed a slab in this location. 

    Photo: blase reardon

Avalanche Discussion

Accumulations from this weekend's storm was highly variable, leading to marked differences in the avalanche hazards across the region. The storm favored the northern parts of the forecast region, like the 10 Lakes are in the Whitefish Range and the Livingstone Range in Glacier National Park.  SNOTELS in these areas recorded 1.6 to 2.8 inches of SWE and 12 to 14 inches gain in snow height, which likely translates as 12 to 18 inches of storm snow. Meanwhile, the southern Whitefish Range and the Swan Range looked to have picked up only 3 to 4 inches of new snow. The Flathead Range and southern Glacier NP seem to be somewhere in the middle - 4-6 inches at mid elevations, and possibly double that up high. I suspect there are a few black holes in our weather station network that also did well - the Doris and Lost Johnny areas in the Swan, and Lake McDonald in Glacier NP. 

The takeaway is that paying attention to how much new snow is under your boards or machine will give you a good sense of the size and potential consequences of triggered slides. Generally, slopes with about 8 inches or more of new and/or drifted snow have the potential for avalanches large enough to bury or injure a person. 

The storm snow fell on surface hoar, facets, or suncrusts at mid-elevations, and a rain crust below about 5200 feet. It may be a little higher in the southern part of the region. The layers of surface hoar and/ or facets are my primary concern right now. The thin slabs sitting on these were not reactive in my tests Sunday, but further north, where the slabs are thicker and stiffened by wind, riders reported whumpfing collapses and natural avalanches. As the storm snow settles, I suspect slabs sitting on these weak layers will grow more unpredictable. 

Did you hear about the app? You can now access our region's forecasts, observations, and weather data directly from the Avy App (IOS; Android). Many features are accessible offline, so you can win debates about the avalanche problems right in the backcountry or submit an obs while driving home. 

Registration is open for this season's avalanche courses. Get some learning done!  There's an Avalanche Awareness class on Wednesday, December 11th, at 6:00 pm at the Stonefly Lounge in Coram.

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