Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Near high-elevation ridges and summits, you can trigger slabs of drifted snow that can bury or injury you. Avoiding steep terrain where these conditions exist is the most straightforward way of avoiding trouble. Stick to slopes less than about 35 degrees where the surface snow remains loose and unconsolidated. 

Problemi valanghivi

  • Wind Slabs

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

    Look for slabs of wind-blown snow at the tops of chutes, on steep slopes on the lee sides of saddles and passes, and below cornices on the lee sides of ridges. You can trigger these freshly-formed slabs on steep slopes, and they can be dangerous where they're more than about a foot thick or above terrain traps, like rock bands, talus, gullies, and trees. Winds today will be strong enough to continue drifting snow, so avoid traveling or climbing below ridges where you see blowing snow. 

    (12/5/2021)

    Westerly winds scoured windward aspects and formed cornices on the ridgeline's leeward (easterly) side.

    Photo: Dundas

Avalanche Discussion

Avalanche hazards today and Wednesday will primarily involve surface and near-surface layers of the snowpack. This past weekend's storm left a foot or more of unconsolidated, low-density snow at mid and upper elevations across the region. That loose snow made for good riding Sunday, with some small loose dry sluffs in steep terrain. It's also easily drifted by moderate winds, which arrived Monday and will continue through Wednesday. Riders reported thin or harmless slabs of wind-blown snow Sunday and Monday near ridgelines. Monday's winds averaged around 20 mph at the highest stations (Aeneas Ridge and Sperry) - ideal speeds for transporting snow. I expect that above about 7000 feet, you can find and trigger some slabs of drifted snow on easterly aspects. These slabs mostly sit on slick, hard crusts or low-density snow deposited this weekend, so they can be triggered by a person's weight. With northwest flow, expect these to be more common in the southeast quadrant of the compass than typical. 

Winds will increase as the flow shifts southeast late today, and it appears they'll to mix down to lower elevations. The light snow forecast overnight and Wednesday will provide fresh fuel for the wind slab machine, so I expect the Wind Slap avalanche problem will extend to mid elevations tomorrow. Unless freezing drizzle at the onset of the storm caps the snow surface with an ice layer. Ugh. Not already.

The snowpack below recently-formed wind slabs and this weekend's snow consists of hard, consolidated snow, ice layers, and a several thin weak layers. The limited observations so far indicate these weak layers aren't reactive the weight of a person or snowmobile. Two caveats, however. The first is that observations so far this season are limited. The second is that conditions are likely different at the highest elevations - above about 7500 or 8000 feet, where much of this fall's near-record precipitation fell as snow, weak layers of facet snow are better developed, and and any rain crusts are thinner. Keep those caveats in mind if you climb up that high. And help us get a good start to the season by telling us what you see on any forays into the backcountry, even if you don't travel very high. 

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