Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

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

The likelihood and consequences of triggering avalanches increase today as you climb higher in elevation. Look for - and avoid - freshly formed slabs near ridges and other terrain that catch wind-blown snow. Stick to planar slopes with runouts free of things that can increase your chances of injury or burial in the event of a misjudgment or surprise.

Avalanche problems

  • Wind Slabs

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

    Look for - and dodge around - dense slabs of drifted snow. You'll find these on slopes that are usually windward, because winds have been blowing from the northeast. Most will be small and tough to trigger, though they can have surprisingly big consequences on steep slopes above terrain traps.

    Cracking around your boards or machines indicates you've found a spot where the wind-blown snow isn't well bonded to softer, lower-density snow below. Look up and down the full length of your chosen line for uneven, pillowy snow surfaces that can show whether a slope has been top-loaded or cross loaded. Pick lines that aren't above cliffs, rocks, gullies, and trees, so you're less likely to be buried or injured if you're surprised.

    (12/17/2023)

    Cracking around your boards or machine are signs you've found a slab. Test them on inconsequential slopes before commiting to larger slopes. 

    Photo: David Kerner

  • Persistent Slabs

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

    Planar slopes with runouts clear of terrain traps will be your safest bet today. In this terrain, you are less likely to trigger slabs 18 to 24 inches thick that release on thin layers of weak snow above the very hard, early-February ice and crust layer. These layers are most developed on shady, upper elevation slopes; they're isolated on sunny and mid elevation slopes. 

    Pick slopes with a roughly constant angle all the way up or down, rather than those with breaks where the slope abruptly steepens. Surprises and misjudgments will result in less serious consquences on slopes that are free of gullies, rocks, and trees. Regardless of the aspect or elevation, whumpfing collapses and shooting cracks are clear signs of unstable structures. 

Avalanche Discussion

An artic intrusion has brought winds out of the north, northeast, and east over the past 36 hours. While speeds were light Wednesday, they picked up overnight, with gusts exceeding 50 mph at several stations. These gusty winds may have transported recently-fallen snow, forming drifts and slabs on atypical aspects. These will be larger and more widespread at upper elevations and will likely grow today as the gusty winds continue. I expect wind speeds to diminish by tomorrow morning, with light winds forecast for Friday.

We're several days out from the most recent snowfall, and observations indicate that avalanches involving new snow are unlikely. We have no reports of triggered slides breaking on density changes in the recent snow since Tuesday (Flathead Range, here). With up to 4 inches of settlement in the new snow, sluffs of loose snow are unlikely and isolated. So we've dropped the Storm Snow avalanche problem that's been our primary hazard. 

Our concerns now turn to the potential for triggering slabs up to two feet thick that break on thin layers of old, weak snow. These layers might be composed of small rounding facets, decomposing surface hoar, or both, depending on location. These layers are above the knife-hard ice or melt-freeze crust left by the late January warming and rain. It was buried the first few days in February - the 1st in the Swan Range. the 3rd in most other locations. At some sites, you'll find these layers right above the that ice/ crust complex. In others, it's two to 6 inches higher, at the snow surface buried a week later. 

While this structure is widespread, observations aren't indicating the recently-formed slab is senstive to the weight of a person. We've received only scattered reports of whumpfing collapses (southern Whitefish Range, here and here) and almost no reports of propagation in extended column tests. The caveat, though is that most obs since the storm are from mid elevations. I suspect the weak layers are better developed at upper elevations, particularly on slopes scoured by a previous bout of northeast winds (Feb. 3-4). 

A stepping out mindset is appropriate today. For my partner and I Wednesday, that meant riding slopes steeper than 30 degrees but avoiding terrain with likely trigger points or higher potential consequences. We were prepared to step back to more conservative terrain if we found signs of instability or a snow structure that was likely to propagate a collapse. We were rewarded with great riding under blue skies and no surprises. 

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