Bulletin d’avalanche

Swan Range

Flathead Avalanche Center
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Danger d’avalanche

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

Due to abnormally warm temperatures and rain, dangerous avalanche conditions have developed. Moist and wet surface snow may slide naturally or be easily triggered by a rider. The chance of triggering a large, destructive avalanche on a buried weak layer has increased. Today is a good day to practice safe travel techniques by sticking to low-angle slopes free of overhead hazards.

Problèmes avalancheux

  • Loose Wet

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

    Warm temperatures and rain-on-snow at all elevations will increase the chance of wet snow avalanches. Slides could entrain substantial snow, travel long distances, and be destructive. Pay attention to the snow’s surface becoming wet and watch for rollerballs and snow falling off tree branches.

    If your machine or boards sink into mushy snow, seek out lower-angle terrain. Limit the time you spend above terrain traps and in long-running gullies where dense debris may make it difficult to escape. Be aware of steep, rocky terrain above you, as wet debris can initiate slides that can push you to places you do not want to go.

    (3/24/2022)

    Recent warm temperatures and solar input initiated slides large enough to bury, injure, or kill a person.

    Photo: Mark Dundas

  • Persistent Slabs

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

    Natural avalanches, audible collapses, and propagation in snowpit tests have been reported within the last week, as a result of weak layers buried 1-4 feet below the surface. This weak snow structure is found at all elevations and aspects but is most troublesome at mid and upper elevations on slopes facing east through south to west.

    Ongoing warming temperatures and rain may increase the likelihood of natural and human-triggered slides, possibly resulting in large and destructive avalanches. Loose wet slides, especially from upper elevations, could initiate a larger, more dangerous, persistent slab. Signs of instability, such as collapsing or cracking, may not be apparent before the slope you are on slides.

    Conservative terrain selection remains the most sure-fire way to deal with this unpredictable problem. Choose low-angle slopes free from overhead hazards to minimize uncertainty. If you desire steeper terrain, dig into the snowpack looking for faceted, “sugary” snow near a crust 1 to 4 feet below the surface.

    (1/26/2024)

    A 12-18 inch thick crown triggered by a skier in facets resting on a crust. This slide may be up to one week old.

    Photo: Mark Dundas

Avalanche Discussion

Residual moisture from an Atmospheric River impacting western Washington and Oregon moved into the Flathead yesterday. Rising temperatures and increasing rain occurred as the day progressed. Noisy Basin in the Swan Range picked up 1 inch of water; most, if not all, fell in the form of rain. Stahl Peak in the northern Whitefish Range recorded one-half inch of water, also primarily as rain. Other locations appear to have received far less precipitation. Temperatures climbed overnight, and as of 0600, all stations are above freezing except for WMR and Link, which are both at 31F. Temperatures are forecast to climb throughout the day, with snow levels reaching 7500-8000 feet. Precipitation should be generally light and tapering off later today.

Rain on snow and warming temperatures will keep the avalanche danger elevated throughout the day. Yesterday (Saturday), I assume a wet loose avalanche cycle occurred at mid and possibly upper elevations favored by rainfall. The natural cycle should continue today and climb in elevation. Hour by hour, it will become easier for a rider to trigger a wet slide in moist or wet surface snow. 

Rain adds weight to the snowpack without adding strength. This added weight stresses buried weak layers, which have repeatedly shown this past week that they are still a concern. Multiple parties noted propagating pit results, natural avalanches (Example 1, Example 2), and collapsing. Debris from today's wet loose slides may be enough weight (stress) to tip the balance and trigger a large destructive avalanche.

Riders enjoyed dry snow and fun conditions above 6000 feet yesterday before the rain and warm temperatures affected the snow surface (Example1, Example 2). With dry weather and warm temperatures forecast for the next few days, riding dry snow will have to wait until the return of winter, perhaps later this week or next weekend.

 

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