Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

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

An Avalanche Warning is in effect. New snow and blowing snow are creating dangerous avalanche conditions. The avalanche danger will increase later in the day and will increase the higher you climb. Natural and human-triggered slides can be large enough to bury, injure, and kill someone. Avoid traveling on and beneath steep slopes. 

Problèmes avalancheux

  • Wind Slabs

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

    Winds are drifting snow into thicker slabs on the downwind side of ridgelines, beneath saddles, and cross-loading the sidewalls of gullies further downslope. Expect slabs to be large enough to bury and injure a person. Yesterday, two experienced riders were caught and carried in slabs of drifted snow. Drifts will be thicker and more dangerous today. A rippled surface and dune-shaped pillows are visual clues of a wind slab. Cracking beneath your skis or machine signifies that slabs are ripe for human triggering. Safer options exist on slopes protected from the wind. 

    Copious amounts of new snow later today and tonight will form slabs large enough to bury a person in all aspects. Reduce your chances of getting caught in a large slide by sticking to slopes less than 30 degrees and free of overhead hazards. There is excellent and safe riding on slopes protected from the wind with lower slope angles. 

    (2/19/2023)

    Crown and debris from an unintionally triggered avalanche.  This slab caught and carried this rider. 

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    A weak layer of snow near a crust exists 1 to 3 feet below the surface. If triggered, slabs can propagate wide distances, wrap around terrain features, and be large enough to bury, injure, and kill a person. A triggered avalanche in the new snow can step down, causing a larger and more destructive avalanche. The only way to manage this problem today is through effective terrain management - avoid traveling on and below steep slopes.

    (2/10/2023)

    Buried weak layers continue to produce avalanches long after their burial. Recent loading may cause these layers to fail. 

Avalanche Discussion

We have issued an Avalanche Warning for the Swan and Flathead/Glacier National Park forecast regions and an Avalanche Watch for the Whitefish Range. Expect increased precipitation rates throughout the day, causing the avalanche danger to rise. Natural and human-triggered avalanches may begin to run long distances and entrain an impressive amount of snow as it travels downslope. Give steep slopes a wide margin for error today. 

The next wave of moisture enters the forecast region this afternoon. The timing of this storm is challenging to predict, causing us to rate the danger to the higher end of our 24-hour forecast period. Today's avalanche danger rating depends on precipitation for the Flathead and Whitefish Ranges. If snowfall does not reach forecasted amounts, expect the avalanche danger to be one step lower in those ranges. We have higher confidence in the Swan Range due to current snow totals.

Storm totals vary throughout the forecast area. Noisy Basin in the Swan Range is taking the lead with 3 inches of snow water equivalent (SWE), with 2 inches falling yesterday (Sunday). The Flathead Range and Whitefish Range are coming in second and third with just shy of 1 inch of SWE. Snow totals average 1 to 2 feet of snow above roughly 5000 feet. 

Two noteworthy observations from yesterday (Sunday) were of unintentionally triggered avalanches. I was knocked off my feet by a small slab in relatively benign terrain, and another experienced rider got carried roughly 50 feet. Both of these were in slabs of wind-drifted snow. One major takeaway from these incidents was that both groups didn't experience signs of instability before these avalanches, and both slabs were on slopes with relatively minimal wind drifting. Both avalanches were in relatively benign terrain with slope angles between 32-38 degrees in steepness. 

Buried weak layers exist throughout the forecast area. Steady loading from snow and wind may cause these layers to fail, leading to a large and destructive avalanche. Due to the uncertainty around persistent slabs, the safest bet during a loading period is to offer slopes that harbor these weak layers a wide margin for error. We can do that by avoiding slopes steeper than 30 degrees. 

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