Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Thu
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Steer away from slopes recently loaded by snow and wind. Cracking in the new snow is a sign that slabs are ripe for triggering. Buried weak layers two to four feet deep are lurking under cohesive snow on various aspects and elevations. Sticking to lower-angled slopes will help to minimize your risk.

Problemi valanghivi

  • Wind Slabs

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

    Southwesterly winds have been strong enough to drift snow into thin slabs near and below ridgelines at mid and upper elevations on NW-N-E-SE aspects. Though shallow, these slabs could be reactive as they will have formed over a variety of weak surfaces. Signs of Wind Slabs include textured surfaces or hollow-sounding snow under your board or snowmachine.

    Look to identify and avoid terrain features where wind slabs may form. Examples include steep slopes (>35 degrees) with overhanging cornices, deposition zones with convex rollovers, and smooth wind pillows. These slabs may be small but have the potential to knock you off your feet and send you downslope into more consequential terrain. The worst-case scenario would be for a small wind slab to step down to a Very Large and dangerous Persistent Slab. 

    (3/7/2024)

    Southwest winds drift loose surface snow onto leeward aspects on Nyack Mountain. These winds have built an extensive cornice line despite a season of below-average snowfall. 

    Photo: Mark Dundas

  • Persistent Slabs

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

    Signs of instability are inconsistent, creating a problem that may be difficult to manage. You may or may not see recent avalanches during your travels, or cracking/collapsing before triggering a slope. In addition, it could be the 1st, 2nd, or 10th person on the slope that triggers an avalanche that fails on the buried weak layer. 

    The February 3 crust/facet combo is buried 2 to 4 feet below the snow surface. Another facet/crust exist about 1 foot below the surface and was the culprit for a few human-triggered avalanches last week. 

    The safest way to manage this Persistent Slab problem is to choose slopes with lower slope angles and ones that aren't in the runout of large avalanche paths. 

    (3/7/2024)

    The crown of a hard slab avalanche that failed during the intense wind loading event of February 28 or 29. This crown spans over 1000 feet wide and wraps around several terrain features. 

    Photo: Mark Dundas

Avalanche Discussion

Yesterday, (Tuesday) freezing levels reached roughly 6000 feet. Cooler temps overnight should keep the wet snow problem off the list for one more day. Recent wind speeds were strong enough to transport new and recent snow across the forecast area.  

You may not see signs of instability or recent avalanches but still trigger a potentially destructive avalanche. Conditions become more complex as you climb in elevation. If you choose to travel on NW-N-E-SE aspects, consider the overlapping problems at mid and upper elevations. These are the places where conservative terrain choices: (ex. <35 degrees, deep snowpack, planar slopes) will help you make it home to ride another day.

Digging a snowpit or probing the snow to identify weak layers below you are helpful, but realize this is only one data point on a slope, and your results can be different only a few feet away. How these buried layers will react to the weight of a rider or machine is difficult to predict. Yesterday, (Tuesday) Guy and I visited the site of a human-triggered avalanche in the Southern Whitefish Range (obs). The rider triggered a large avalanche (D2.5) on the February 3 crust. Judging by a set of tracks near the crown, it appeared that the rider hit a thin part of the slab, which likely triggered the avalanche. Fortunately, no one was caught or carried in this avalanche.  Identifying thin spots on a slope is nearly impossible at times. 

As we move into a period of drier and warmer temperatures over the next couple of days, the potential for wet snow avalanches will increase at all elevations. Shallow wet loose avalanches may run quickly and farther than you expect on top of recently formed crusts. Consider adjusting travel times as activity will increase in the afternoon hours. 

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