Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
View on map

Avalanche danger

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Out of date — this forecast expired 5 years ago.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

Highlights

Conditions are complex, despite the yellow icon and moderate danger at all elevations. You can trigger slides that range from small sluffs of wet surface snow to slabs three or more feet thick that break hundreds of feet wide. Keep it simple; stick to terrain where you're not exposed to the overlapping hazard. Your best bets are shaded, mid-elevation slopes less than 30 degrees that aren't below steep start zones or prone to collecting drifted snow. 

Avalanche problems

  • Loose Wet

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

    A wet snow avalanche hazard is developing at low and mid-elevations as air temperatures and solar radiation warm the cold snow at the surface. Today, you can trigger sluffs of wet snow on steep slopes where the snow gets sticky. These can be dangerous above terrain traps. Very steep slopes where trees and rocks shed snow can produce natural slides of wet debris. With plenty of loose snow at the snow surface, triggered and natural slides can entrain surprising amounts of debris once they get moving. This debris can be hard to escape. Large slides are possible on very steep slopes with continuously steep tracks. 

  • Persistent Slabs

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

    Slabs failing on surface hoar and facets buried over a month ago still pose a hazard? Yep, as shown by at least one large, remotely-triggered avalanche this weekend, very large triggered and natural avalanches last week, and large columns regularly propagating in tests at mid-elevations. These layers are typically buried about three feet deep. You're most likely to collapse them and trigger an avalanche on slopes steeper than 35 degrees. If you do trigger a slide, it will likely break the full width of the terrain feature and involve large or unsurvivable amounts of debris. The most reliable way to reduce your exposure to this low probability but high consequences hazard is to avoid steep slopes - those over 30 degrees. Steeper than that, the risk increases substantially. You can pick lines that avoid likely trigger points, like convex rollovers and slopes with patches of shallow snow where you're more likely to affect the weak layers. That's a high-stakes card game, and the house eventually wins those games. 

    (2/27/2021) Skier investigating an unintentionally-trigggered slide in the northern Whitefish Range. Avalanche triggered by another party; the two riders apparently escaped unharmed. The slide broke on weak snow buried in late January.

  • Wind Slabs

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

    At upper elevations, you can trigger slabs of recently-drifted snow on steep slopes. These can be dangerous where the slabs are 8 inches or more thick or above terrain trap. These can step down and release thicker slabs that fail on weak layers near the Jan. 13 crust, creating far more dangerous slides. Cracking in dense pockets of wind-blown snow is a clear sign you've found this hazard. This morning, it's localized to parts of the region that have seen gusty winds the past few days, like Marias Pass, the Lake McDonald valley, and the eastern side of the Whitefish Range. As winds pick up today, expect it to become more widespread.

    (2/28/2021) Crown of an avalanche involving drifted snow on a leeward slope just below a ridgeline. Note the cornice. The slab may have stepped down and released on weak layers buried in mid-January.

Avalanche Discussion

Conditions are complex. A warming trend starting last night, the lurking threat of triggering slabs that break 3 feet thick and the full width of start zones, and localized winds and blowing snow the past few days. The hazards overlap, and the likelihood of each is rising but at different rates. Reduce the complexity and stay out of trouble by sticking to slopes where you minimize your exposure to the suite of hazards. That's shaded, wind-sheltered terrain less than 30 degrees that's not below steeper start zones. On those slopes, the surface snow will be slower to warm, it'll be harder to trigger thick slabs on deeply-buried weak layers, and wind-blown snow will be minimal. If you venture away from these safe harbors, aim to expose yourself to one hazard at a time. 

The warming: There's plenty of loose snow on all aspects this morning; this snow will be prone to sluffing as it wets from warming temperatures and solar radiation. Rollerballs fanning down a slope are a clear sign this process is happening. They're most likely to coalesce into a wet loose avalanche on slopes steeper than about 30 degrees that receive direct sun. The size of these slides should increase through the week if the weather models are correct. The likelihood of wet slab and glide slab avalanches may also rise as meltwater percolates below the surface. 

Persistent slabs: While snowpack tests are producing inconsistent results, there's plenty of evidence this hazard is still lurking on steep slopes, particularly above about 6000 feet where the late January surface hoar is well preserved. Some tests vary with elevation, some with site across the region. In the Whitefish Range, a skier-triggered a large avalanche Saturday while many tests Sunday near Whitefish Mountain Resort weren't producing propagation. People have also reported collapses from near Marias Pass and a remotely-triggered slide above Skyland Sunday. And, of course, there are the very large natural and triggered slides from earlier this week. The likelihood of these slabs running naturally this week is slowly rising; falling cornices could act as triggers, and the warming trend will likely make them more susceptible to a person's weight.

Wind slabs: Some observations reported blowing snow Sunday, while others didn't. It seems more prevalent in the Swan Range and - surprise! - near Marias Pass, where a remotely-triggered avalanche on a leeward slope may have involved only wind-drifted snow, or may have broken on the late January weak layers. As winds pick up today, expect this hazard to become more widespread. Winds will diminish over the next few days, and it will become less of a hazard as the week progresses. 

Regions covered