Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Thu
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The best and safest riding today will be on slopes with a refrozen snow surface that aren't below sunny start zones or looming cornices. Plan your travel so you can reach this terrain early and exit early, before warming temperatures and solar radiation increase the danger. Stay well back from corniced edges if you're traveling on ridgelines. 

Problèmes avalancheux

  • Wet Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3.5

    Today's primary avalanche hazard remains large to very large natural wet slab avalanches. The debris from these slides can run the full length of avalanche paths, particularly where it's channeled in gullies. Stay out of the way of these monsters. Pick routes that are outside the tracks of open slopes at upper elevations that are receiving or have recieved sustained sun. That's particularly important late in the day.   

    (3/28/2022)

    Part of the crown of a very large natural avalanche on Stanton Mountain, above Lake McDonald. The thick part of the crown is at about 7450 feet elevation. The slide released Sunday, 3/27, on a cross-loaded, southerly slope and likely involved meltwater weakening a mid-pack persistent weak layer.

    Photo: Cam Johnson

  • Cornices

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Large cornices are steadily weakening in the sustained warm weather. Large sections can break off, sending car-sized chunks of very hard debris tumbling down the steep slopes below and clobbering anyone in its path. Avoid this painful and ignominious fate by staying out from under overhanging cornices. If you're traveling on a ridge, keep in mind that cornices can break surprisingly far back.

    (3/28/2022)

    Warm temperatures have weakened cornices. This one in John F. Stevens Canyon failed Sunday, 3/27. The debris ran over 1000 vertical feet and entrained wet snow as it descended.

    Photo: Adam Clark/BNSF

Avalanche Discussion

Expect a refrozen snow surface and mostly supportable travel conditions this morning, thanks primarily to radiational cooling. Skies were mostly clear last night, and weak inversions are in place this morning. Most stations are showing below-freezing temperatures, though these were slower to arrive at mid and upper elevations stations. The surface refreeze should be shallower at those elevations. 

Surface melt-freeze crusts will break down this morning as air temperatures rebound above freezing to about 7000 feet. I don't expect today's melt to exceed Tuesday's, thanks to the competing influences of increasing clouds, increasing winds, and a weak cold front.  If anything, the snow surface may remain slightly cooler on sunny/ windward aspects yet warmer on shady aspects. Yes, Virginia, there is a greenhouse effect. 

The primary avalanche hazards today remains large to very large wet slabs and large cornice falls. The water factory at the snow surface only worked two of three shifts Tuesday, with only two shifts scheduled today. But above-freezing temperatures and strong solar input for the daylight and early evening hours are still enough to produce plenty of meltwater at the snow surface. This water is still percolating deep into the snowpack, so the possibility of large or very large natural wet slab avalanches continues. The debris from these slides can run surprisingly far, endangering you even if you're several thousand feet below the start zones. So far, these have occurred on windward, sunny slopes where the snowpack is thinner. They've released between 7000 and 7500 feet. It's possible meltwater is percolating deep enough to reach persistent weak layers on leeward, shaded aspects where the snowpack is generally thicker. That's why we've added easterly slopes to the distribution of the Wet Slab avalanche problem.

The bottom line for today is to start early, leave while the party is good, and have an exit route that doesn't cross under steep, sunny start zones near about 7000 feet. You'll know the party is winding down when the snow surface softens enough that you or your machine are sinking about six inches into the snow. This can happen sooner on steeper slopes, so look up from shaded valleys and consider whether start zones or cornices above you are softening and weakening. 

Need to catch up on conditions? Check out the text and video summaries of the past week's development in our Conditions Blog below the Bottom Line or here. Megan, the FAC March intern, wrote this week's summary before heading back to grad school in Oregon.

Join us for our last education event of the season. Topics Class! The Scoop on Spring Touring, Wednesday, April 6th, 6:30 to 7:30. No registration is necessary for this in-person event.

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