Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Sun
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Out of date — this forecast expired 5 years ago.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

Two developing avalanche hazards can be trouble today. At upper elevations, freshly formed slabs of drifted snow can be dangerous above rock bands, gullies, or trees. The likelihood of triggered and natural wet snow avalanches is rising with warmer temperatures. Direct sun can rapidly increase the danger posed by these slides. Have options so you can avoid these hazards - low angled slopes sheltered from the wind and direct sun. The risk of triggering dangerous hard slab avalanches lingers on steep, upper elevation slopes with variable snow depths and convex shapes.

Avalanche problems

  • Wind Slabs

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

    Small, thin wind slabs developed Friday at upper elevations, and with increasing winds today, these will grow wider and thicker. Look for dense pockets of drifted snow on the lee sides of ridges, downwind of saddles, and cross-loaded into chutes. Test slopes (steep but short and with minimal consequences) can give you a sense of their size and sensitivity. Don't underestimate small slabs; debris and people rocket down steep slopes surprisingly fast. Avoid being under steep start zones where you see active loading. 

    (3/26/2021) Gusty winds drifted the recent snow.

  • Loose Wet

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

    You can trigger sluffs of heavy, wet debris on steep slopes below about 6000 feet today. The likelihood and size of these slides will increase on slopes that receive more than a few minutes of direct sun. They will be most dangerous where the snow is getting wet for the first time and on slopes with long, steep pitches where sluffs can entrain more debris or gouge below the surface. Rollerballs that fan down a slope are a sign that conditions are ripe for these slides. Stick to slopes less than about 38 degrees or where the snow is dry.  Avoid steep slopes above terrain traps or that leave you few escape options. 

    (3/26/2021) Wet rollerballs initiated with ski turns at low-elevations. These did not coalesce into wet snow avalanches.

  • Persistent Slabs

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

    Tune your spidey senses to the factors that increase the likelihood of triggering a dangerous hard slab that breaks on old snow buried in January. These factors: steeper than 35 degrees and convex or unsupported in shape, with variable snow depths. Slopes like these tend to be pocked with trigger points where the weak layers are closer to the surface, and the slabs are continuous enough to propagate a fracture. The more of these factors that exist on a slope, the riskier it is to ride. 

    (3/26/2021) The 1/13 crust found 28 inches below the surface on a SE aspect at 6300 feet. Two thin layers of facets sandwich this crust.

Avalanche Discussion

While riders have been enjoying winter-like conditions, two weather factors are driving the development of new avalanche hazards. These are increasing winds and increasing temperatures. Wind Slab and Wet Loose avalanche problems are now the primary avalanche concerns, with the Persistent Slab avalanche problem lurking on isolated slopes. The Wind Slab and Wet Loose hazards are mostly small and easy to recognize, with straightforward mitigation options. There's far more uncertainty around the lurking Persistent Slab hazard, so it's best mitigated with a conservative, status-quo mindset.

Friday in the Middle Fork, several parties reported gusty winds and light to moderate blowing snow. I saw a few small drifts around 6500 feet; a party in John F. Stevens Canyon reported small, thin slabs that were sensitive to a person's weight. Windward slopes have plenty of loose snow available for transport, and forecast wind speeds today are enough to collect snow from fetches into the tops of chutes and the sides of gullies and couloirs. The winds may harden the snow surface on windward slopes, so freshly-developed slabs may be concentrated in the terrain with the best riding conditions - shady, leeward slopes above about 6500 feet. The strong winds forecast for Sunday will exacerbate this problem.

I've seen the line between wet and dry snow at about 5200 feet the past two days. It will likely rise higher with warmer temperatures overnight and today. Sustained sun on a slope will push it even higher, so be prepared with an exit strategy that doesn't expose you to the runouts of steep, mid and upper elevations start zones that face east through south to west. Below about 4000 feet, the snow is isothermal and mostly unsupportable, triggered sluffs in this terrain can gouge deep and entrain surprising amounts of debris.

Settlement has reduced the likelihood and potential size of Loose Dry snow avalanches. If you're considering a  40 degree or steeper slope at upper elevations, check that you can escape fast-moving sluffs. Find another option if that looks difficult. 

Shady, upper elevation terrain still harbors weak layers buried in late January. They appear to be sintering into the snow above and below them.  Where these are still intact, they are typically buried deeply enough that a person is unlikely to collapse them and start a slide that propagates widely. The exceptions are shallow spots, so avoid terrain pock-marked with thin snowcover. The drier areas of the forecast region are where trigger points are most widespread. 

For a quick, visual summary of the week's conditions, check out Clancy's video summary here or in the gallery below.

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