Bulletin d’avalanche

Whitefish Range

Flathead Avalanche Center
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Danger d’avalanche

Tue
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Increasing winds will form fresh drifts that will become sensitive to the weight of a rider. Slabs of drifted snow will be larger and more dangerous the higher you climb. Buried weak layers 2 to 4+ feet produced avalanches on the same aspects where the wind is drifting snow today. You are less likely to trigger an avalanche on windward aspects, or on lower-angled slopes sheltered from the wind. 

The avalanche danger may rise to CONSIDERABLE overnight due to precipitation and wind. 

Problèmes avalancheux

  • Wind Slabs

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

    Increasing winds today will transport low-density snow into slabs that may become reactive to the weight of a person or machine. Slabs will thicken as you climb in elevation, and as the day progresses. Active wind transport will be your primary tool for identifying where this problem exists today. Choose terrain sheltered from the wind for a safer and higher-quality experience. Shooting cracks around your skis or machine are signs to steer away from steeper terrain.

    (3/4/2023)

    Slabs of drifted snow below ridgelines and crossloaded terrain features will grow throughout the day. 

  • Persistent Slabs

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

    Persistent Slab avalanche activity since mid-January occurred on aspects that face northwest-northeast-southeast. The last human-triggered avalanche was five days ago in the northern Whitefish Range. Snowpack tests continue to show mixed signs of instability. Wind loading and increasing temperatures today will add stress to these weak layers.

    A simple way to avoid this problem is to stick to low-angle slopes not connected to steeper ones where this problem exists. If you decide to step out into steeper terrain, choose slopes that do not terminate in a gully, stand of trees, or cliff band. This type of terrain will exacerbate the consequences of even a small slide. Avoiding common trigger points, such as convex rollovers, can reduce your chance of triggering. A cornice fall or triggered wind slab may step down to a much larger, more dangerous avalanche. 

    (3/12/2023)

    This avalanche was large (size 2 of 5). This means it was large enough to bury and injure a human. This failed on an east aspect at roughly 7600 feet 

    Photo: Cam

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    Freezing levels are forecasted to reach 6000 feet today, bringing the rain/snow line up to roughly 5000 to 5500 feet. Warming temperatures may wet the snow surface and cause it to slide. Precipitation this afternoon will start as rain before transitioning to snow at low elevations. Rain on dry snow is an ingredient for almost instant avalanching. Rollerballs or pinwheels are a precursor to a loose wet avalanche cycle. Choose a route that doesn’t expose you to traveling on steep slopes at lower elevations. Getting caught in a small wet slide above a gully or creek bed can have a devastating outcome. 

    (3/8/2023)

    roadside wetloose in Hay Cr.

    Photo: Guy

Avalanche Discussion

Increasing winds and warming temperatures are the primary drivers of changing avalanche conditions today. Observations from the past week primarily describe no wind and exceptional skiing and riding on low-density snow. Yesterday, the sun warmed southwest and west-facing aspects. These aspects now have a sun crust, which will reduce the amount of snow available to transport. I suspect that the strength of winds by the afternoon will reach areas where dry snow still exists, and transport it into fresh slabs. Slabs of drifted snow will thicken throughout the day. This will require constant attention to changing conditions. 

Warming temperatures and rain on snow will wet the snow surface and cause it to slide this afternoon. We suspect this will be confined to elevations below roughly 5000 feet, however, that line is not a guarantee and may inch up higher in elevation. Pay attention to the rain line, rollerballs, and snow falling out of trees as signs for where that line exists.

Both of these weather factors contribute to direct action avalanche problems, such as wet loose and wind slabs. These weather factors can also add stress to buried weak layers. Warming temperatures cause snowpack settlement and creep, while wind transport is an additional loading event. Although neither of these events today are significant, they are subtle changes worth acknowledging due to the challenges of managing a persistent slab avalanche problem. 

A snowmobiler-triggered avalanche in the northern Whitefish Range is a prime example of the tricky nature of persistent slabs. The triggered avalanche failed on weak snow from November. This layer has not produced an avalanche that we know of since late December. That is over two months ago! Luckily no one was caught in this avalanche. 

Fireside/Snowpack Wednesday, March 15. Join us for a virtual evening combining two favorite community ed workshops. FAC Forecaster Cam Johnson will kick things off with a State of the Snowpack presentation to help you digest the current conditions and plan your next trip. Then we'll shift to Fireside Chats for open dialogue about human and avalanche factors. A recreationist, forecaster, and avalanche educator all experienced a near miss in the first three weeks of February. Together, Kat, Cam, and Meg will explore the tough decisions, tricky snow, and lessons learned from their experiences. No registration is needed; click here for Zoom. The event will begin at 6 p.m. 

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