Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Winds from multiple directions have formed thin slabs in both usual and unusual locations. Cracking in the snow surface directs you to seek out softer snow in sheltered locations. The chance of triggering a large, destructive avalanche on a buried weak layer is declining, though it remains a concern. Choose planar slopes with runouts free of surface obstacles to minimize your chance of injury.

Problemi valanghivi

  • Wind Slabs

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

    Shifting winds have drifted snow on various aspects, forming thin slabs.  Slightly older features from Friday's easterly wind event will be found in unusual locations. Recent prevailing winds have been light and only affecting the most exposed terrain. 

    A  rippled surface, dunes, and pillow-like features are signs of wind transport. Another clue is denser snow, where your boards or machine do not sink into the surface snow as deeply as they do on surrounding slopes. Cracking beneath you indicates you have located a sensitive slab. Slopes sheltered from the wind will hold softer and safer snow.

    (11/26/2024)

    Westerly winds blowing snow into the Shed 7 West Avalanche Path, John F. Stevens Canyon.

     

     

    Photo: Adam Clark

  • Persistent Slabs

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

    Although unlikely, you can still trigger a slide on various deeply buried weak layers. Observations confirm weak faceted snow resting on a stout crust from mid-December as the culprit in recent large avalanches. The weak snow from early December has not been as active lately, but it is still an issue where you find a grey stripe in the snowpit wall.  

    These layers are buried deep in the snowpack and subsequently hard to affect. Slopes where a rider could trigger a large, destructive avalanche are those where the snowpack is thinnest and the layers closer to the surface. These include convexities or rollovers, wind-scoured terrain, and slopes peppered with rocks. Choosing descents with runouts free of surface obstacles and terrain traps will aid in case something goes wrong. Defaulting to lower-angle terrain makes sense if you are unsure if these layers are present.

    (1/4/2025)

    The crown of a slab avalanche that failed during the post-Christmas storm cycle. This avalanche failed on a faceted crust 2+ feet below the surface.

    Photo: Mark Dundas

Avalanche Discussion

Friday's cold air intrusion accompanied by north through east winds was generally weak and short-lived, ending early Saturday morning.  These events often affect the Essex to Marias Pass corridor more than other parts of the region, partly due to the broad low profile of the pass that allows cold air to move west. The BNSF avalanche team noted wind slabs at all elevations Saturday, but they were stubborn to ski cutting. They did note one natural D2 wind slab from the recent winds. Observers in the Essex area did not find similar slabs (here and here).

The winds shifted back to the prevailing south through west direction Saturday morning. However, speeds at upper-elevation stations remained generally below the threshold for transporting snow. Saturday, observers noted a rime crust and freezing rain crust capping the low-density surface snow in the Flathead Range. These crusts vary in elevation and thickness and limit the amount of snow available for transport by wind. 

The Persistent Slab remains on the problem list, albeit unlikely. Recent observations of large natural slides failing on buried weak layers in the Swan Range (Image) and the Flathead Range (Image)  confirm triggering a slide on a buried weak layer is a concern.

Avalanche Awareness Class: Monday, January 6, at Great Northern Cycle and Ski at 6:00 p.m. FREE!

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