Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

Be alert for fresh, reactive slabs of dense, wind-drifted snow at upper elevations. These can be dangerous where they're more than about 8 inches thick or above terrain traps. Minimize your chances of triggering a large avalanche that breaks several feet deep in old weak layers by avoiding likely trigger points, such as steep break-overs or slopes where the snowpack appears thin and rocky. Expect the danger to rise to Moderate at mid-elevations if snowfall exceeds forecasts.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Fresh, reactive slabs of drifted snow pose an isolated hazard at upper elevations. These will be confined to the terrain that collects small amounts of wind blown snow from large areas, such as the tops of couloirs and chutes, or slopes downwind of large, open fetches. These could be more widespread in areas where near-surface crusts are thin and fragile, in areas that have received more snow above these crusts (the Ten lakes and Jewel Basin areas), or if snowfall today exceeds forecasts.

    Be alert for pockets of dense, wind-blown snow. They can pose a danger where they are 8 inches or more thick, or can carry you into or over something that increases the chances of burial or injury. Cracking in these is an unambigious sign that they're sensitive to the weight of you or your snowmachine.

    (12/20/2024)

    The crown of a rider-triggered hard slab avalanche composed of drifted snow is visible near the ridgeline. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    The possibility of triggering avalanches that break on deeply buried weak layers continues to diminish. You can stumble into a trigger point on steep slopes with variable snow depths and convex profiles. These will often be pocked with boulders or rock outcrops, or where the entire slope isn't visible from the top or bottom because of abrupt changes in slope angle. 

    Any slide triggered on these weak layers will be large and dangerous. With that in mind, choose slopes with planar or concave profiles that have runouts free from terrain traps. The more consequential the terrain, the more important your safe travel practices. Ride one at a time, keep your partners in sight, and stop only in spots safe from debris coming down from above. 

     

    (1/6/2025) Photo: Michael Reavis

Avalanche Discussion

The hunt for snow that's not topped with a breakable crust may take you into the terrain where you're most likely to find small but reactive slabs of wind-blown snow. That's because areas where recently-deposited rime or freezing rain crusts are thinnest  - or topped with 4 to 6 inches of recent powder- are also the areas with the most snow available for transport. And winds have finally increased to speeds where they can blow snow around, with upper-elevation stations reporting hourly averages of 15 to 30 mph overnight and gusts of 20 to 60 mph. These winds are blowing loose snow around and creating isolated slabs of drifted snow. Sticking to sheltered, mid-elevation slopes will offer the most pleasant, lowest-risk riding today. 

We continue to see evidence that deeply buried weak layers are strengthening and becoming unreactive to the weight of a rider or machine. No propagation in snowpack tests, few reported signs of instability since New Year's Eve, and no reported triggered avalanches on these layers since Dec. 29th, even with people riding steeper terrain. 

 I'm still leery of some slopes, however. Tuesday, my partner and I looked down the steep, southerly chutes dropping down from the Tunnel Ridge weather station. The chutes were a series of short snowfields above rollovers where the slopes disappeared from view, many with rock outcrops dotting the sides. Windward, sunny terrain with variable snow depths and lots of trigger points. Just the kind of terrain where one is likely to get surprised. No thanks. 

Tuesday's snow surfaces are getting buried today in many locations. Many of these -surface hoar, near-surface facets, facets under thin rain or rime crusts - could form future weak layers, particularly if they are buried initially with thin, soft slabs. Pay attention if you're out the next few days to how these layers are behaving. They won't pose a hazard now unless they're buried by drifted snow. But they may be a future problem where they're breaking or collapsing easily in snowpack tests or on test slopes. If today's storm overproduces, the future could be now; expect a widespread Storm Slab problem to develop at mid-elevations. 

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