Bollettino valanghe

Swan Range

Flathead Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

North and east winds have formed dunes, cornices, and slabs in unusual locations. Cracks shooting out from your skis or machine indicate an unstable slab. Triggering a large destructive avalanche on buried weak layers remains a decreasing possibility of high consequence.

Problemi valanghivi

  • Wind Slabs

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

    Recent low-density snowfall on shaded aspects has remained dry and available for transport. Since Thursday evening, winds from the north and east have been drifting this snow onto atypical aspects. Look for fresh wind features in unusual locations. As you gain elevation, slabs may increase in thickness and become more widespread. These may form on crusts, which makes them easier to trigger and potentially propagate wider than expected.

    Fresh cornices, dunes, rippled snow surfaces, and areas with stiffer snow are telltale signs of wind transport. Cracking beneath your boards or machine indicates an unstable slab. 

    (3/16/2024)

    This image is an excellent example of the cross-loading of two gulley features from prevailing southwest winds. In this image, the wind generally blows from right to left.

    Photo: Mark Dundas

  • Persistent Slabs

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

    The recent heat wave helped strengthen buried weak layers, but triggering a 2-4 foot-thick avalanche remains possible. You can reduce risk by avoiding steep, rocky, heavily wind-affected terrain on shady slopes, where the weak layers remain preserved. 

    The persistent slab is a complex problem to manage. You may not experience snowpack collapsing or other obvious signs of instability before the slope you are on avalanches. The easiest way to manage this problem is to choose lower-angle terrain without convexities. Locations harboring a supportable near-surface crust will be safer as your weight may not impact the buried weak layers. 

    (3/15/2024)

    This image is the crown of a very large hard slab that failed on a steep, unsupported southeast slope on March 15th. Triggering a hard slab avalanche is most likely to occur in rocky, steep, complex terrain like this. 

    Photo: BNSF Avalanche Safety

Avalanche Discussion

Cold air dropping south from Alberta has left us with below-average temperatures and low-density snowfall. This morning, readings were in the single digits at mid-elevations in John F Stevens Canyon and all upper-elevation stations elsewhere. Settled new snow totals are generally 4-6 inches with less than 10% water content. Winds increased yesterday and overnight and are north through east.

In unusual locations, expect to find wind features such as dunes, cornices, and slabs. North and east winds do the darndest things, and predicting where you will find wind effects is not always straightforward. Narrow drainages in the Middle Fork generally escape these winds, as a group in Marion experienced yesterday. Other locations feel the full effect of these winds, as an experienced observer noted in the side-country outside of WMR, where his skin track filled in within one hour. Jeff noted wind transport in the northern Whitefish Range that shut off as winds quieted in the afternoon. I traveled to the Continental Divide to feel the full effects of the cold front and noted recently formed features at mid and upper elevations. Soft slabs were abundant but were not stiff enough to propagate.  

Due to continued wind, we expect fresh slabs to be stiffer and more reactive than yesterday. This problem should be straightforward to manage, as these wind features should be easy to identify and avoid.

The chance of triggering a large avalanche on a buried weak layer continues to diminish. With further observations, we hope to soon remove this problem from parts of our advisory area and trim its distribution from other areas. 

Help avalanche researchers! The Colorado Avalanche Information Center, the Avalanche Research Program at Simon Fraser University, and the U.S. National Science Foundation National Center for Atmospheric Research are researching how skiers, snowmobilers, snowshoers, ice climbers, and anyone else who travels in the backcountry interpret avalanche forecast information. This research includes a survey to understand better how helpful avalanche forecast information is for trip planning. 

Completing this survey will take approximately 20 minutes. Thank you!

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