Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
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Limite del bosco
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Sotto il limite
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In sintesi

Riders can trigger a large, destructive avalanche on a buried weak layer, especially on slopes with a thin snowpack or exposed rocks. Today's warming and sunshine may initiate loose snow slides as the surface softens. Rollerballs and mushy surface snow indicate you've worn out your welcome and need to move to a shadier aspect or stick to lower-angle terrain free of overhead hazards. 

Problemi valanghivi

  • Persistent Slabs

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

    In late February, multiple large avalanches occurred in the Flathead Range and southern Glacier Park on a layer of weak, faceted snow 2-4 feet below the surface. This weak layer is best preserved and most problematic on shady slopes; however, some sunny aspects contain a mix of crusts and faceted layers capable of producing a destructive slide. This problem is magnified in the mountains between Essex and Marias Pass, where the snowpack is thinner and weaker. Today's warming temperatures may make the slab above the weak snow more reactive to triggering. 

    You're most likely to trigger one of these beasts in steep, rocky, wind-swept terrain. Signs of instability, such as cracking or collapsing, may not be present before the slope you are on avalanches. Choosing lower-angle terrain without convexities is the easiest way to manage this problem. 

    (3/8/2024)

    Trigging large hard slab avalanches like these on buried weak layers remains possible. These slides are on a southeast aspect at mid-elevations of Mt Penrose.

    Photo: Mark Dundas

  • Loose Wet

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

    Today's intense March sun and warming temperatures will increase the threat of wet loose avalanches, especially on steep, rocky, wind-sheltered terrain. These slides could entrain substantial snow and travel long distances on the underlying crust. Rollerballs, pinwheels, and snow falling from trees indicate warming and are precursors to loose snow slides.    

    Though these slides will generally be small, tangling with one in long-running gullies, chutes, or large faces may make escape difficult. If you travel on or below steep sunny slopes, stay ahead of the sun and plan your exit strategy to avoid these areas in the afternoon. The shady side of the compass still holds dry, loose snow, which will keep you away from this problem and offer a higher-quality product. 

     

    (3/24/2022)

    Today's warm temperatures and solar input may initiate a wet loose cycle capable of triggering slides large enough to bury, injure, or kill a person.

    Photo: Mark Dundas

Avalanche Discussion

Yesterday (Friday), cool morning temperatures, occasional high clouds, and light winds mostly kept the wet loose problem at bay. This morning, temperatures are 10-15 degrees warmer than yesterday, which may expedite the warming of the surface snow. The unknown is the amount of cloud cover that will make or break a wet loose cycle today. Generally, 6-12 inches of surface snow rests on a slick rain crust below around 5800 feet, with similar amounts above resting on old, harder snow. Yesterday's warming and solar input likely left a thin surface crust at low and mid-elevation sunny aspects. Wet loose activity will be delayed until the crust breaks down. An approaching cold front this afternoon will bring cloud cover and minimize this problem. 

Clear skies continue to allow observers unimpeded mountain views following recent snowfall and blustery winds. In the Swan Range, there has not been a documented avalanche on the early February buried weak layer, and we have trimmed the distribution of the Persistent Slab problem by removing the middle elevation band and dropped the likelihood of triggering at the upper elevations to unlikely. In the Whitefish Range, slides failing on buried weak layers are clustered in a small area on the southeast side of this zone at mid-elevations. These are located on the south and southeast aspects, and we have not had a chance to verify what layer they are failing on, as the crowns seem thinner than slides in the Flathead Range. The epicenter of large to very large avalanche activity is the Flathead Range, interior Glacier Park, and southern Glacier Park. 

Observations yesterday (Friday) noted no instabilities but found quality snow in sheltered locations in the Swan Range. An observer on Snowshed Mountain remotely triggered a cornice while kicking a separate section of cornice, but otherwise found no instabilities. A party in Rescue Creek found two faceted layers in the upper half of the pack, but they had no propagation in large column tests. Blase and I took a short walk up Rescue after our team meeting and enjoyed viewing impressive crowns and piles of debris from the February 28 and 29 loading events (photo, photo, photo). A party in central Glacier Park noted a whopper of a slide that failed on the early February crust (photo, photo). 

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