Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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In sintesi

Slopes sheltered from the wind and sun offer the safest and best riding today. At upper elevations, you can trigger slabs of wind-drifted or wind-stiffened snow 1 to 2 feet deep on steep slopes, mostly near ridgelines. These are isolated at mid elevations. 

Problemi valanghivi

  • Wind Slabs

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

    Think twice  - maybe even thrice - if you find slabs of wind-drifted or wind-stiffened snow on slopes steeper than about 35 degrees. These slabs will feel denser and firmer than snow on sheltered slopes, which means your skis, board or snowmachine may not sink into the snow more than a few inches. They may also feel hollow and crack around you. They'll be most common near upper elevation ridgelines, on slopes that are typically windward or scoured. These slabs can be dangerous if they carry you into a terrain trap - anything that can make for a deeper burial or trauma. 

    (3/8/2022)

    Shooting cracks are a sign that wind-stiffened snow is sensitive to your weight

    Photo: Zach Armstrong

Avalanche Discussion

"Surprise, surprise, surprise!" It was pouring rain a week ago, yet Tuesday people reported 1 to 2 feet of recent snow above the melt-freeze crust left by last week's rain and warm temperatures. The new snow hasn't been accompanied by many signs of instability - a small slab of drifted snow east of Whitefish Mountain Resort Tuesday, and some sluffing in the Swan Range above Hungry Horse Reservoir. Guy and I found a few thin layers of facets in the central Whitefish Range, but these did not propagate in tests or produce any collapses. 

Guy and I also found pockets of dense, drifted snow 1 to 2 feet deep near the ridge we climbed. I expect similar conditions on and near other ridges exposed to northerly and easterly winds, mostly at upper elevations. The highest stations in the region - Sperry and Aeneas Ridge - have recorded sustained easterly winds for the past 36 hours, while stations below 7000 feet have mostly reported light easterly and northerly winds. So expect the Wind Slab avalanche hazard to be more widespread and more serious as you gain elevation. You may find exceptions on mid-elevation faces on the south and west sides of the Whitefish, Swan, and Flathead ranges. The cold, dense air from east of the Divide tends to sink down these slopes, drifting snow at mid elevations. That phenomenon is sometimes obvious at Whitefish Mountain Resort in these conditions, where the front side snow is more wind affected than the backside.

The bottom line is to look for steep slopes where winds have drifted or stiffened Monday night's snow into slabs. These may be obvious - hard snow that doesn't allow you to sink in more than an inch or too. Or they may be more subtle - snow that's just slightly denser and stiffer than snow on sheltered slopes. This snow may feel upside down - denser at the surface than below. These slabs tend to take longer to stabilize in cold temperatures, so look for them even if the winds aren't actively blowing snow. Settlement may have also allowed isolated slabs to form on steep, sun exposed slopes.

Below about 6000 feet, you may find only 5 to 10 inches of recent snow above a very hard - and deeply runneled crust. This snow may sluff as you sidehill or turn on these slopes. Most of these sluffs will be harmless. The exceptions will be sluffs that drag you into treewells or other terrain traps. 

The Friends of The Flathead Avalanche Center and FVCC are hosting the last Intro to Avalanches next weekend. And on March 22, join us online for a Community Chat where we review recent conditions and debrief a near miss. 

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