Bulletin d’avalanche

Swan Range

Flathead Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Points saillants

Strong winds will transport new snow building fresh slabs. Triggering a large avalanche is likely—primarily in wind-affected terrain. To reduce your risk, avoid slopes with evidence of wind, such as fresh drifts or a stiffer snow surface that supports your skis or sled.

Problèmes avalancheux

  • Wind Slabs

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

    Impressively strong westerly winds will continue to transport Sunday's snowfall into fresh slabs downwind of ridgelines, saddles, and on sidewalls of cross-loaded gullies. These slabs will increase in size and thickness as you gain elevation. Drifts, dunes, and rippled snow surfaces confirm that the wind has been working. Today's blowing snow will be an obvious sign that slabs are forming.

    In isolated locations favored by snowfall, you may find thin storm slabs on slopes sheltered by wind. They can be identified by cracking at the surface in consolidated, dense snow. 

    Whether in sheltered or wind-affected terrain, cracking beneath your machine or boards indicates instability, directing you to lower-angle slopes free of overhead hazards. 

    Triggering a small surface slab may initiate a much larger avalanche on a buried weak layer described in our persistent slab problem below.

     

     

    (2/11/2023)

    A shooting crack in wind-drifted snow indicates you have found an unstable slab. This is a red flag directing you to sheltered or lower-angle terrain. The blue arrow denotes the end of the crack.

    Photo: Mark Dundas

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1.5–2.5

    Sunday's precipitation and wind transport have added weight to the snowpack, stressing weak layers buried 1 to 3 feet below the surface. These layers are found above the very hard, early February ice and crust layer. You can find this weak snow on shady, middle and upper-elevation slopes and isolated sunny slopes. 

    Recent mild temperatures have initiated the strengthening of the weak layer, but this process is notoriously slow. You may not experience any signs of instability like collapsing or cracking before triggering a slide. 

    Choose terrain free of convexities to increase your safety margin. Steer around thin areas, such as those scoured or peppered with rocks, in favor of a deeper, more consistent snowpack. Cracking beneath your boards or machine indicates an instability directing you to lower-angle terrain. 

Avalanche Discussion

We issued an Avalanche Warning for the Flathead Range/Glacier National Park zone on Sunday night, with the start of a powerful storm favoring this area. This zone will remain at high avalanche danger, as we expect natural avalanches to be likely and human-triggered avalanches to be very likely. Traveling in avalanche terrain today, specifically on leeward slopes at upper elevations, is not recommended. 

Strong winds are the driving force for avalanches in all areas of the forecast region. We’ve listed Wind Slabs as our primary problem. Ridgetop winds are cranking in the Flathead, with Tunnel Ridge recording 104 mph gusts at 4 am. Other regional stations include 78 mph gusts at Hornet and 66 mph at Aeneas Ridge and Snowslip. 

Warm temperatures associated with the start of this storm have brought impressive SWE totals. Flattop recorded 1.9 inches of water in less than 24 hours, while Stryker recorded 1 inch. The Swan Range missed out in this storm, only picking up 0.3 inches.

We expanded the distribution of the lurking Persistent Slab problem to include middle elevations where we’ve observed the facet/crust structure. Impressive wind drifting with the current storm is loading slopes harboring this layer.  This additional weight could be the tipping point for slabs to fail. 

Traveling today in the mountains will require monitoring winds and snow. If you are able to keep your hat on, let us know what you see by submitting an observation.

Régions couvertes