Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Tue
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Limite del bosco
2 · Moderato
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In sintesi

Above 6,000 feet, you can trigger an avalanche where winds have drifted new snow into fresh slabs. Blowing snow signals active loading, while shooting cracks clearly indicate unstable snow. If you observe either, give a wide margin to nearby steep slopes. 

On slopes above about 6200’, slabs may be sitting on buried weak layers, meaning avalanches could break wider and have more serious consequences.

Problemi valanghivi

  • Wind Slabs

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

    Continued snowfall combined with increasing winds will drift snow into slabs that may be sensitive today, especially above 6000’ in favored, wind-loaded areas that receive the higher end of forecasted snowfall. Slabs deeper than about 8 inches can be large enough to bury or injure a person.

    Expect slabs to form on the downwind sides of ridges and saddles, at the tops of gullies, and below cliff bands. You can look for textured or wind-affected snow and feel for denser snow over softer layers, but I’d recommend keeping it simple today—let blowing snow guide your travel decisions, as it clearly highlights where the problem is developing.

    Adding uncertainty, weak snow surfaces that existed before this storm may now be buried. In places where wind slabs rest on surface hoar or faceted snow, avalanches could be triggered remotely—from the side, from above, or even from low-angle terrain below the slope. We don’t yet have enough information to confidently rule this in or out, so plan for that possibility in your terrain choices today.

    Storm Slabs:

    Incremental snowfall over the past few days may have formed thin slabs in sheltered locations. Keep an eye out for these cohesive features if you plan to travel on steep slopes.

    (4/23/2025)

    Slopes with wind-textured surfaces or below cornices are where you are most likely to trigger sensitive slabs of wind-drifted snow. 

    Photo: Rob Millspaugh

Avalanche Discussion

Today marks the fourth consecutive day of light snow and wind. These incremental loads can act like the frog in a pot of warming water—changes happen slowly enough that it’s easy to miss how much stress is being added. Though the total snowfall over the past few days is modest thus far (around 6–8 inches at 6,500 feet, increasing with elevation), the cumulative weight of this snow has become meaningful, with up to 1.7 inches of SWE added to the snowpack.

That amount of snow alone is enough to support wind slab development, and potentially storm slabs in more sheltered terrain. The unanswered question is what the new snow landed on—and how that should influence our terrain choices.

We know there were widespread weak snow surfaces before this storm cycle. What remains unclear is if, where, and how consistently those surfaces were preserved beneath the recent snow. Field observations remain mixed. Rob and I found dry snow above 6,200 feet in the Swan Range yesterday, but did not encounter buried surface hoar along our route, which made us comfortable skiing steep slopes. Over near WMR, Mike and his partner found knocked-over surface hoar with no signs of instability. At the same time, other parties reported upright surface hoar along with collapsing and cracking.

At this point, we don’t yet have enough information to confidently confirm or rule out a persistent weak layer beneath the new snow. If present, it would most likely be found on east- through north- to west-facing slopes above roughly 6,200 feet.

Given that uncertainty, I’d approach travel today with an extra layer of caution beyond what I typically use for a wind slab problem. If considering slopes where wind slabs overlap with the potential for a persistent weak layer, I’d seek out multiple safe, representative test slopes at similar elevations and aspects, and dig into the snowpack to look for weak snow and test it before committing. If that level of uncertainty feels like more than you want to manage, there are good alternatives: upper-elevation south-facing slopes, or terrain below about 6,200 feet on any aspect, where this problem is unlikely to exist.

Additional storms this week, along with your continued observations, should help clarify if and where this problem exists as the week progresses.

Regioni coperte