Bollettino valanghe

Whitefish Range

Flathead Avalanche Center
Vedi sulla mappa

Pericolo valanghe

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

In sintesi

Today you can trigger avalanches that may be dangerous on slopes steeper than about 35 degrees with more than about 8 inches of new and drifted snow. Those conditions won't be widespread unless the storm arrives earlier than forecast. Shooting cracks, fresh natural avalanches, and whumpfing collapses are all clear signs the party is over and it's time to head home. Dangerous avalanche conditions will develop tonight as snowfall and winds intensify. 

Problemi valanghivi

  • Storm Slabs

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

    As snow accumulates today, you're increasingly likely to trigger slides on steep slopes. These can break a foot or more thick in the new snow or at the old snow surface. These slides will involve enough debris to bury or injure a person at mid and upper elevation slopes; slides this size will be isolated at low elevations. Stick to slopes less than 35 degrees if you find more than about 8 inches of new and drifted snow. That's less than boot-top high. Slabs may be more sensistive to a person's weight where they've been stiffened by the wind, or where the storm snow feels upside down - denser at the surface than below. 

  • Persistent Slabs

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

    The snow forecast for the next 36 hours may overload weak snow that's been dormant recently. This weak snow includes facets adjacent to the Dec. 3 crust, depth hoar near the ground on windward slopes with shallow snowpacks, and a thick layer of facets above a mid-December rain crust at low elevations. The likelihood of triggering slabs on these layers will rise as storm snow accumulates. Avoid likely trigger points for these layers, like convex rollovers and windward slopes with variable snow depths.

Avalanche Discussion

If I were to list the ingredients that would create a spike in the avalanche danger, they'd include: sustained snowfall, rising temperatures, and increasing wind speeds. Let those simmer for 18 to 36 hours, and you'd get 1- to 3-foot thick slabs of dense or drifted snow over soft, low-density snow, all sitting on weak or hard old snow surfaces. And indeed, that's what's in store for today and Friday, as mild, wet Pacific air overruns the cold Arctic air smothering the forecast region. One note about the recipe: warm too fast, and you'll get freezing rain. 

As snow accumulates today, the chances of triggering soft slabs a foot or more thick will rise. So will the potential size of triggered slabs. As temperatures climb today, the density of the falling snow will increase, creating dense snow over soft snow on all aspects. Slopes prone to collecting drifted snow might see some slightly thicker and stiffer slabs that will be more sensitive to a person's weight. The storm favors the southern half of the forecast region today; the Whitefish Range may be late to the party.

Snowfall intensifies tonight, and wind speeds increase. I expect dangerous to very dangerous conditions Friday, particularly at upper elevations. If the storm developes as forecast, the region will see 1 to 2 feet of new snow, with 1 to 2 inches of water, before snowfall tapers off late Friday. The likelihood of natural avalanches should peak Friday morning.

Drifting snow should form thick, hard slabs at mid and upper elevations, and with plenty of new snow to entrain, natural and triggered avalanches may run into valley bottoms. The chance of slabs breaking into old, weak snow will rise as new and drifted snow overload old snow layers. These include soft snow below hard layers of drifted snow, faceted snow on windward slopes, and rounding facets near the Dec. 3 Melt-Freeze crust that's buried roughly a meter deep on shaded, mid and upper elevation slopes. On Sunday just north of our forecast area, a rider triggered a slide on this layer his second time down a slope. The snowpack on many low-elevation slopes is shallow and harbors weak facets above a mid-December crust. Slides breaking on this weak snow will be possible if a foot or more of new snow accumulates on these slopes. 

Backcountry travel will become increasingly risky as the storm progresses. Choose low-consequences objectives with little hazard of slides overrunning you from above. Be prepared to call it early if snowfall and winds are higher than forecast. 

Regioni coperte