Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Tue
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Out of date — this forecast expired 2 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Terrain-specific evaluation of avalanche likelihood and consequences remains key at upper elevations. On some shaded and leeward slopes, slabs remain sensitive to the weight of a person or snowmachine. Be especially leery of mid-slope convexities in terrain sheltered from the wind and sun. Snowfall forecast for Tuesday and Wednesday may bring more widespread hazards.

Avalanche problems

  • Persistent Slabs

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

    While we've had few recent reports of instabilty, I'd still be leery of two kinds of terrain today. One is relatively easy to identify; the second is more subtle and sneakier. Triggered slides above terrain traps can have outsized consequences.

    The first is upper-elevation leeward slopes that harbor some slabs of stiff, dense snow that remain sensitive to triggering by a person or snowmachine. The second involves slopes sheltered from the sun and wind where a buried layer of weak surface hoar and facets lurks. It's a distinct grey stripe 8 to 16 inches below the surface in terrain above the rain-snow line from the Dec. 7th storm. Layers like this tend to be spatially variable in strength, and thus unpredictable. People often trigger slabs on these layers mid-slope, at convexities or roll-overs where a slope steepens. These changes in slope angle aren't always obvious from above. Both these hazards are isolated at mid-elevations.

    Choose terrain today that minimizes the chances and consequences of a surprise today. Stick to slopes that are unlikely to harbor the 12/7 surface hoar and facets, like south and southwest facing slopes. Avoid slopes where a dense layer of wind-stiffened snow rests on the this layer. Choose terrain with a planar or concave profile and runouts clear of gullies, trees, rocks, or cliffs.

    (12/14/2024)

    The distinct grey stripe in the snowpit wall is the layer of surface hoar and facets buried 12/7.

    Photo: David Steele

Avalanche Discussion

This morning, I'm paying attention to two competing trends. The first are the observations that include few or no signs of instability (Swan Range; southern Whitefish Range; northern Whitefish Range). The second is the incremental loading on the layer of surface hoar and facets buried 12/7. 

The last reported slab avalanches in the region look to have occurred on Dec. 7 or 8 (here, here, here and here), when a storm rapidly buried a layer of surface hoar and facets. Since then, we've had only occasional reports ( 3 of 42 obs) of collapses. Most people report large column tests that don't propagate, or only do so with moderate or hard force. Good to go, right? 

Maybe not. That's where the second trend comes in. Flurries and weak storms are gradually building a thicker slab above the 12/7 layer, and as this layer consolidates, it becomes stiffer and more likely to propagate a collapse. It doesn't appear we've crossed that threshold yet. But a few inches of snow Sunday, and at upper elevations, some wind-blown snow, is pushing some slopes closer to it. 

The takeaway is to continue to evaluate the specific terrain you're considering riding. Dig down and look for a distinct grey stripe. Propagating or collapsing snowpack tests on this layer are a sign you've found a slope where the slab is overloading the 12/7 layer. So are shooting cracks and whumpfing collapses. If you don't get those, pick lines that avoid mid slope convexities and aren't above terrain traps, in case conditions are different lower on the slope than your evaluation. 

Downloaded the app yet? 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 doublecheck danger ratings and travel advice on the drive or in the backcountry. And submit an obs while driving home. 

Are you looking for avalanche education opportunities? Check out our calendar for upcoming events. Tonight, Monday, December 16, there is an Avalanche Awareness Class at Bias Brewing.

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