Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Tue
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2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
2 · Moderato
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In sintesi

Today, it's all about the crusts. You can still trigger avalanches on steep slopes where there's no crust that supports your weight. At upper elevations, the hazard remains wide, thick slabs that break on weak layers buried in late January. At low elevations, it's sluffs of loose, wet snow near the snow surface. Both hazards are isolated at mid-elevations. A storm snow avalanche hazard will develop as new snow accumulates overnight and Tuesday. 

Problemi valanghivi

  • Persistent Slabs

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

    Steep slopes where the snow surface remains dry are the terrain where you're most likely to trigger a large, wide avalanche. These slopes are where weak facets and surface hoar buried in late January survived recent warming intact. They're mostly above about 6000 feet on shaded aspects, and above about 6500 feet on sunny aspects. You can minimize your risk of triggering these slides by sticking to slopes less than about 35 degrees from top to bottom, that aren't pocked with scoured or shallow spots, and aren't above terrain traps. Because falling cornices can act as triggers for these weak layers, don't linger under steep start zones capped with cantilevered cornices. 

    (3/4/2021) A snowpit at 5100' produced propagating results in snowpit tests. This shows that our Persistent Slab structure is still capable of producing slab avalanches.

  • Loose Wet

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

    On steep slopes where you find more than a few inches of moist or wet snow near the surface, you can trigger wet loose avalanches that can drag you into gullies, rocks, and trees, or over cliffs. This hazard is growing more isolated; cooler temperatures and reduced solar radiation are limiting the amount of liquid water in the near-surface layers of the snowpack. You can minimize your exposure to this hazard by sticking to slopes where the snow surface remains frozen, or slopes less than about 40 degrees, where sluffs of wet snow move slower and entrain less debris. Gullies are the terrain that collects enough debris to makes small slides disproportionately dangerous.

    (3/7/2021) Large natural avalanches on north facing terrain in Glacier National Park.

Avalanche Discussion

Last week's sun and warm temperatures had an obvious impact on the near surface layers of the snowpack. The effects on deeper layers in the snowpack are less pronounced and less certain. The processes that heal weak layers that are buried 3 feet or more deep are slow, and they're not uniform. Compared to 10 days ago, a person is far less likely to trigger slabs that break on the surface hoar preserved in late January, or the facets above the mid January crust. Those general trends don't help much on individual slopes, because we can't determine which slopes retain collapsible weak layers, or where on a slope a person can affect them. To reduce our risk of triggering these dangerous avalanches, we're left with identifying and avoiding the terrain most likely to harbor intact weak layers and trigger points. Those would steeper than 35 degrees, convex in profile or shape, and likely to have variable snow depths due to scouring or rocky bed surfaces. 

All our automated weather stations showed below freezing temperatures this morning, for the first time in a week. The snow surface refroze at most elevations, though the crust may not be very stout between 3500 and 4500 feet. The crusts will break down during the day, though broken skies and mild temperatures will limit the amount of meltwater near the surface, and with drainage already established, it won't saturate the near-surface layers on most slopes. 

A plume of moisture will flow from southwest to northeast across the region late today and tonight. Upper elevations will see enough snow that by tomorrow, I expect storm snow hazards to develop in areas favored by the snowfall - mostly the southern half of the forecast region. Winds will be light with moderate gusts, so any wind slabs should be isolated. 

Regioni coperte