Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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2 · Moderato
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1 · Debole
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In sintesi

The best riding with the lowest risk of triggering a slab avalanche exists on shaded, mid-elevation slopes. In this terrain, stick to slopes less than 35 degrees that have runouts free of terrain traps. Elsewhere, a sun crust may make for less quality riding. And on upper-elevation, shaded slopes, you can trigger slabs that break one to two feet deep with little prior signs of instability.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2.5

    Triggering a slab that breaks one to two feet deep remains a lingering and serious hazard on steep, upper-elevation, shaded slopes. It's an isolated hazard elsewhere. The slabs sit on a layer of weak, faceted snow and sometimes surface hoar a few inches above a very hard crust or ice layer. The weak layer is not that distinct on some slopes, though the crust and ice are obvious.

    The most reliable way of reducing your exposure to this hazard is to be very picky about the terrain you climb, traverse, or descend. Evaluate slope angles, slope shapes, and runouts. Your risk is lowest on slopes that are less than 35 degrees that are concave (bowl-shaped) or planar, and aren't above trees, rocks, benches, or gullies. 

    (2/19/2024)

    The crown of a natural avalanche on an east to southeast facing slope in the Flathead Range. It likely broke on weak facets above the 2/3 crust. It's a good example of the kind of terrain to avoid to reduce your risk. 

  • Wind Slabs

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

    In upper-elevation terrain that collects wind-blown snow, you can trigger small slabs that can be dangerous if they carry you into a gully, trees, or rocks, or over a cliff. That includes the lee sides of ridges and chutes, slopes below cornices, and slopes downwind of saddles. Most of these slabs have formed over the past two days; the slabs could continue developing in areas that receive more than a few inches of snow today.

    Before climbing or descending this terrain, look for signs of wind-affected snow at the surface - smooth, pillowy drifts or dunes? Check the runout of the slope - free of things that can magnify the consequences of getting caught and carried down?  If the answer to either question is yes, step back and consider alternatives on wind-sheltered, low-angle slopes.

    (12/17/2023)

    The kind of slopes where you can trigger a slab of drifted snow are obvious in this image - almost everywhere with snow. Keep this image in mind as you travel today. Even if slopes aren't bare, it can help you picture what terrain has been stripped and what terrain might harbor small slabs.

    Photo: Mark Dundas

Avalanche Discussion

An innocuous-looking layer of facets and surface hoar continues to pose a danger across the region. It's been the weak layer in at least two accidents - one mine - and two near-misses in the past week. 

While this structure exists across the region, we can narrow down the terrain where you're most likely to get into trouble, based on accident sites and recent observations. The most likely suspects:

  • Face north to northeast at upper elevations
  • Are in the Flathead Range
  • Are more than 35 degrees in steepness and aren't adjacent to steeper slopes
  • Are convex in profile

Again, that's the most likely terrain. One unintentionally-triggered slide occurred on a north-northwest facing slope, and another was triggered from a spot that was 33 or 34 degrees. People have reported the structure on mid-elevation slopes in all ranges, where it's propagated in snowpack tests. During the storms that loaded the weak layer, remotely-triggered slides broke on this layer in the Swan Range. A few naturals have also released on southeast-facing slopes. Several of the unintentionally-triggered slides also appear to have released where the slab was thinner, possibly because winds scoured away some of the slab.

I'm focusing on these characteristics because terrain-based decisions are the most reliable way of avoiding surprises and trouble. The slab above the facets and surface hoar is relatively soft and the weak layer is spatially variable. In those conditions, snowpack tests tend to be less accurate predictors of instability. And none of the parties involved in the past week's incidents reported collapsing or cracking prior to the slides. 

A secondary, short-lived hazard today is triggering small wind slabs. These aren't widespread, and most are shallow and small. And they're definitely more predictable. Nonetheless, they can be dangerous in steep, consequential terrain, like couloirs and chutes and steep faces. Check for this hazard with thorough assessments before committing to steep, consequential terrain. 

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