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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Highlights

At upper elevations, you can trigger slabs that break in the new snow, or deeper in the snowpack, on weak layers that have survived the eary December deluge. Expect the likelihood and size of potential slides to increase as you climb higher in elevation.  The Goldilocks terrain today - not too much new snow, no old weak layers - holds less than 8 inches or so of fresh snow and lie between 6000 and 7000 feet.

Avalanche problems

  • Persistent Slabs

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

    If you climb to near 7000 feet or so, be alert for signs of instability such as shooting cracks and whumpfing collapses. Terrain above this elevation may harbor two persistent weak layers that survived last week's rain more or less intact. You can  still trigger avalanches that release 2 to 3 feet below the snow surface. On isolated slopes, weak layers near the ground may remain sensitive to a rider's weight.

    These layers may break in upredictable ways, making careful routefinding essential to reduce your exposure. Digging into the snow can help you identify whether these layers are present. Look for a soft, grey stripe two to three feet below the surface and a layer of soft, loose grains above a hard crust near the ground.

    (12/8/2023)

    Be wary of slopes WITHOUT this rain/ melt-freeze crust. In this profile at 6300 feet, rain has destroyed persistent weak layers. 

    Photo: md

  • Storm Slabs

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

    On isolated slopes, you can trigger avalanches that break up to a foot deep and involve new and recent snow. Be watchful for slopes over 35 degrees in steepness where 8 inches or more of new and recent snow has accumulated. This snow may feel upside-down - denser at the surface than below.

    Active tests can help you determine the sensitivity of these surface slabs. Sidehill across short, steep slopes and look for cracking or small slabs. Step above a skintrack or stomp of switchback corners and look for the same. If you commit to steep slopes, choose those with runouts free of trees and gullies, to reduce the consequences of a surprise. 

    (12/2/2023)

    Cracks like these are clear signs the soft slabs near the snow surface are sensitive to a rider's weight and can be dangerous on larger slopes. 

Avalanche Discussion

If the 12/1 surface hoar layer were a fugitive, we've narrowed its possible hideouts to terrain above 6500 feet in the Swan Range and even higher- above 7000 feet - in the Whitefish and Flathead Ranges. Observations below those elevations indicate that rain and heat pretty much destroyed the surface hoar (Mt. Brown, Grant Ridge, Scalplock Mountain). Higher, the weak snow endured, but is capped by a stout crust (central Whitefish Range) or still propagating in tests (Noisy Basin). Where that crust thins , a rider's weight can still collapse this weak layer, or another soft layer near the ground and above a knife-hard crust.

While I suspect this structure only exists above 7500 feet, if not 8000 feet, I'd start looking for it above about 7000 feet in the Whitefish and Flathead zones. Only the highest peaks in the Whitefish Range exceed that elevation. In the Flathead Range and Glacier National Park, there's far more terrain above that threshold. Because a rider can trigger persistent weak layers from below or from adjacent slopes, I'd be avoiding the runouts of start zones well above me as I approached ridges or summits. In the Swan Range, our limited observations suggest that the threshold is closer to 6500 feet, because the range received less rain.  

What about the snow that's fallen after the rain? The Swan Range has been favored by recent storms, with 1.5 inches of SWE recorded at Noisy Basin since Sunday morning, and over 3 inches since the rain ended. It's also likely upside down - denser at the surface than below - thanks to rising temperatures Sunday. That snow is settling into cohesive slabs that can be sensitive to a rider's weight on steep slopes at mid and upper elevations. A thin rain or rime crust that formed Saturday night may break up these slabs. 

You're less likely to find soft slabs that break wide and deep enough to bury or injure a person in the Whitefish and Flathead zones. Stations in those zones are reporting only 0.2-0.6 inches of SWE since Saturday, or about 3 to 8 inches of new snow. The 12/10 rain crust was also found in the southern Whitefish Zone Sunday, though it didn't affect the riding much. The Storm Slab hazard is most widespread at upper elevations in these zones. 

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