Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 6 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Use caution where drifting snow deposits fresh wind slabs below ridgelines. On upper elevation slopes without a stiff near-surface crust, you may be able to trigger a large persistent slab avalanche. Sheltered, planar slopes less than about 35 degrees are a safer terrain choice. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    Ridgetop winds will increase to snow-blowing speeds today. They will have several inches of recent snow to drift onto leeward aspects. Watch for areas of blowing snow near ridgelines and below saddles. Look for fresh, round pillows below cornices and in the tops of chutes. Soft wind slabs may bond poorly on top low density or faceted snow, crusts, and a new crop of surface hoar. Small avalanches can be dangerous on slopes where terrain traps below magnify the consequences of getting caught. Sheltered terrain will have softer snow, higher quality riding, and safe conditions.

    (1/18/2021) These cracked just below ridgelines above 6,000'

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2

    We are primarily concerned about avalanches failing on layers of surface hoar. One is buried about a foot down, and another 2 to 3 feet below the surface. We have had no reports of avalanches failing on persistent weak layers since the 13th. However, stability tests across the region continue to show that they are capable of propagating a fracture across the slope. In areas where you find a stout crust near the surface, you are unlikely to impact the weak layers deeper in the snowpack. Treat upper elevation slopes where the crust is thin, or missing entirely, with suspicion. You can use terrain to limit the chances of triggering one of these notorious weak layers. Stick to planar slopes less than about 35 degrees without terrain traps below if you want to avoid the problem.

    (1/18/2021) Surface hoar buried ~1 foot

Avalanche Discussion

A round of small storm slab avalanches ran in the Swan Range Sunday as a storm quickly dropped 1.2 inches of new water and 8 inches of snow in a 7 hour period. Storm totals were modest elsewhere - around 3 inches. The last large, and persistent slab, avalanches occurred during the atmospheric river event on the 12th and 13th.

Monday, observers experienced small collapses and cracking in wind drifted snow near ridgelines in the Swan Range and the northern Whitefish Range. They also found 3 to 6 inches of soft snow available for wind transport. Increasing winds will build small new drifts in leeward terrain today. These new pillows will develop on top of older, stiffer drifts, a hard crust, and possibly a patch of surface hoar or near-surface facets. Use caution on steep alpine slopes if you see blowing snow or cracking in the snow around you. 

Deeper in the snowpack, layers of buried surface hoar and faceted crusts continue to make their presence known. They usually appear as a grey stripe in snow pit walls. There's one about a foot down and another 2 to 3 feet below the surface. Crusts and facets from December are down 4 feet deep. Stability tests show that these layers can still carry a fracture across the slope. The good news is that the crust that formed on the 13th is generally thick and hard. It covers over these buried weak layers in many locations. The exceptions are where the crust is thinner - on some northerly slopes - and where it peters out at upper elevations. If you're unsure about the snowpack beneath you, or on the slope where you'd like to play, give yourself a margin for error. Slopes with lower angles, uniform snow cover, and a concave or planar shape are safer than big, complex terrain. 

You'll notice the forecast looks different today. Check out our new Blog post to find out why.

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