Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Mon
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
2 · Moderato
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In sintesi

We issued an Avalanche Warning in all zones of our advisory area. Dangerous avalanche conditions will continue during a period of prolonged warming. Avoid being on or under steep slopes above 5000 feet.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    Multiple large hard slab avalanches occurred Friday and Saturday in Glacier Park and the Flathead Range and likely failed on buried weak layers above an early February crust. Continued warm weather increases the chance that a 2-4 foot thick avalanche may be triggered or fail naturally in the Swan and Whitefish Range. 

    You're most likely to trigger a slide in steep, rocky terrain on shady slopes (photo at right/below). However, southeast aspects in some locations have weak layers and the same problematic structure capable of producing a destructive avalanche. Signs of instability, such as cracking or collapsing, may not be present before the slope you are on slides. Avoid avalanche terrain above 5000 feet. 

    (3/15/2024)

    This is the crown of a Very Large (size 3 of 5) Hard Slab that failed on a steep, unsupported southeast aspect on March 15th. 

    Photo: BNSF Avalanche Safety

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    1–2

    Today will be unseasonably warm, leading to another day of wet snow instabilities. Over the past three days, rollerballs and loose snow slides have occurred on sunny aspects. This problem has expanded to include the northern aspects, where previously dry snow is becoming wet. 

    Mushy snow that does not support your weight or machine is a sign you could trigger a loose slide. Although the debris is relatively small, these slides can pack a punch in chutes or gullies, where escape may be difficult. Be aware of the terrain above you and where an avalanche could carry you. 

    These small avalanches can trigger a much larger avalanche lower on the slope.

    Continued warm temperatures combined with a poor overnight freeze allow meltwater to drain to weak layers deeper in the snowpack, making wet slab avalanches possible. 

    (3/16/2024)

    Multiple wet loose slides failed during the March 15 warming on the westerly facing Swan Front.

    Photo: Mark Dundas

  • Cornices

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Impressive southwest winds built large cornices along alpine ridgelines in late February/early March. Intense warming this weekend may weaken these features. A failing cornice can send large blocks of snow trundling downhill, potentially triggering a destructive avalanche. 

    The nature of this problem is highly unpredictable and should warrant your respect. Give these whales a wide berth above and below a corniced ridgeline to reduce your risk. Don’t get surprised by a cornice breaking further back than expected.

    (3/7/2024)

    A cornice fall in Rubideau Bowl in the Apgar Range just north of Apgar Lookout. This feature substantially  increased in size during the intense wind event of February 28 and 29. 

    Photo: Mark Dundas

Avalanche Discussion

We are continuing an Avalanche Warning for the Swan Range, the Whitefish Range, the Flathead Range, and portions of Glacier National Park through 0600 tomorrow (Monday). If conditions dictate, we may extend this warning to Tuesday. As of Sunday mid-morning, temperatures at many mid- and upper-elevation stations had been above freezing for 48 hours, reaching 50F at two upper-elevation stations yesterday.

The abnormally warm temperatures triggered deep hard slab avalanches on shaded slopes and thinner slabs on sunny aspects. Friday, the BNSF Avalanche Safety Team reported 3 large slab avalanches observed from Highway 2. Cam observed a D2.5 hard slab avalanche on a northwest aspect of Great Northern, and outside our forecast region, Rob viewed several large slab avalanches on sunny aspects of the Swan Crest near Condon. Saturday, blase observed multiple slab avalanches in the eastern portion of the advisory area, while Rob and I noted slides in the mountains around Lake McDonald. WMR patrol closed all their avalanche terrain as the day warmed.

Our multi-day warming raises the risk of wet slab avalanches. Melting surface snow produces water that drains into the snowpack and may eventually reach the ground unless an impediment like a thick crust or weak layer is encountered. Here, the meltwater pools and increases the chance of releasing a wet slab avalanche. The meltwater production occurs faster on sunny slopes, where there are more crusts to interrupt the draining process. The general rule of thumb regarding this process is that three nights without a freeze are needed for meltwater production to drain into the snowpack. If forecasted temperatures materialize, we may begin seeing wet slab activity in the next day or two, although timing these is difficult. 

Planning an early exit and choosing low-angle terrain free of overhead hazards are ways to manage your risk in the face of uncertainty. If you observe avalanche activity, submit an observation to let us know what you are seeing.

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