Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Fri
Alpine
2 · Moderate
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 5 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Know what time it is and look up. Are refrozen snow surfaces softening? Are sunny start zones or cornices looming overhead? Loose Wet snow avalanches and Cornice falls are today's primary avalanche hazards. The first will likely develop quickly, and the second is unpredictable. In the highest, coldest terrain, a lingering concern is a hard slab breaking on deeply-buried weak layers. In this terrain, avoid steep, unsupported slopes with variable snow depths. 

Avalanche problems

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Natural and triggered sluffs of wet snow can be dangerous today on steep slopes above terrain traps. This hazard may develop quickly as sun and warm temperatures soften crusts that developed overnight. Rollerballs are a sign that snow is getting wet for the first time, and when this hazard is most acute. Have an exit plan - a way to get to shaded, lower-angled slopes that aren't under steep, sunny start zones. 

    (3/31/2021) D1.5 size debris at the base of a southeast gulley. Sun and warming initiated this slide.

  • Cornices

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Strong winds have built dense cornices which hang over the slopes below. These monsters can suddenly collapse during periods of warming or strong sun. Look above you as you travel, and be aware of what could come down from above. When walking along ridge crests, know what you are standing on. Take a big step back from the roots of cornices, which is where they can fail. Likewise, cornice fall can trigger deeply buried weak layers.

    (3/17/2021) Cornices are sagging in the warm weather, straining their roots at ridgelines. If they break, the falling debris can be a hazard in itself. It can also be a trigger that releases large avalanches that break on deeply buried weak layers.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    On steep, upper elevation terrain, the primary avalanche concern remains the possibility of triggering hard slabs that break on persistent weak layers buried in January. That's also the terrain where you're likely to find cold dry snow and good riding. Pick lines that avoid likely trigger points - spots where the slab is thin and the weak layer is likely to be intact. That's slopes steeper than about 35 degrees with convex profiles and variable snow depths. These conditions are more widespread in the northern Whitefish Range, where slabs are thinner due to less recent snow.

    (3/16/2021) A hard slab of snow sitting over buried persistent weak layers.

Avalanche Discussion

A ridge of high pressure building over the region brought mostly clear skies and above freezing temperatures to about 7000 feet on Wednesday. Weather stations above 5800 feet recorded 6 to 12 hours of freezing temperatures overnight, and with mostly clear skies during the period, I expect snow surfaces that got wet have refrozen into a supportable crust this morning. However, those surface should soften quickly as temperatures rebound to at least 40 degrees near 6000 feet. Above that elevation, moderate southwesterly winds should keep snow surfaces cooler. Well-shaded slopes - mid and upper elevations and northwest through north to northeast - should stay dry today unless a plug of clouds arrives this afternoon. That would enable a greenhouse effect that could wet snow surfaces on northerly slopes.

The upshot is to expect wet snow avalanche hazards to develop quickly in sunny, mid-elevation terrain. Natural and triggered sluffs of wet snow are likely on slopes steeper than about 40 degrees. They'll be largest, and run furthest, on slopes that are consistently steep for long distances. They may be more likely on slopes that face more east and west, where recent snow may be getting wet for the first time. Rollerballs fanning down a slope are the classic sign these conditions are developing. Know what time it is - that is, know where you are in the freeze-melt cycle. Firm and supportable crust? Snowmaching or boards sinking a few inches? Fighting mush?

The Wind Slab avalanche problem of the past two days has diminished. Existing slabs have likely stabilized, and there's little snow available for transport on windward slopes. Cornice falls do pose a hazard, however. The grouper, tuna, and whale-sized blobs at ridgelines are sagging under the sun and warm temperatures. Falling chunks of rock-hard debris can be a threat in themeselves, and they can serve as triggers for small to large slabs of wind drifted snow or hard slab on the slopes below them. Another key practice today would be to look up. If you see large cornices overhead or steep start zones recieving direct sun, move quickly to terrain without these overhead hazards.

And then there's the lingering hazard of large hard slabs failing on mid-winter persistent weak layers. The current warming is a test of whether they remain unstable. Given that the test is ongoing, I'd avoid upper elevation shady terrain that's convex in shape, looks to have variable snow depths, and is steeper than about 35 degrees. 

Pro tip: Carry skin wax, ski crampons, and bear spray today. Ideally, you use only the first two. 

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