Avalanche forecast

Swan Range

Flathead Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 7 months ago. You’re viewing past conditions, not the current bulletin.

Highlights

Human-triggered avalanches are likely today where the recent snow rests precariously on a very weak layer of surface hoar. While wind-loaded slopes near and below the ridgeline are the most likely location for a larger avalanche, anywhere with more than 6" of snow on the surface is supect. 

Avalanche problems

  • Storm Slabs

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

    Up to a foot of recent storm snow is capping a very reactive layer of buried surface hoar. This layer is widespread and can be found directly below ridgelines in normally wind-hammered locations, such as the slopes above Picnic Lake off Aeneas Ridge. There, we remotely triggered an avalanche on this layer yesterday from the flat ridge, despite the very soft slab and limited wind loading. In areas without buried surface hoar, weak faceted snow exists below the recent snow, sometimes in close proximity to a crust. Add it all up, and it’s a bad combination. 

    What this layer has been waiting for is a cohesive slab to form on top of it. Increasing wind speeds yesterday afternoon, combined with recent and ongoing warming, are more than capable of accomplishing this task. Avalanches could break up to two feet deep in wind-loaded terrain and be large enough to bury or injure a rider, even on wind-sheltered slopes. If you are traveling where there is more than 6” of recent snow, consider the consequences of even a small slide.

    Remote triggering is a significant possibility today, and I wouldn't be shocked to hear about a natural avalanche, especially if greenhousing comes into play. Pay close attention to what is above you as you travel, as avalanches could release higher up on the slope and descend onto lower-angle terrain.

    (4/7/2024)

    This avalanche is from a previous season, but it is the type of slide that could fail naturally or be triggered by a rider today. This problem will be exacerbated if the recent snow rests on a weak layer of facets or surface hoar.

    Photo: Jeff Metsky

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–1.5

    Light rain, warm temperatures, and elevated humidity moistened the surface snow at low and middle elevations on Friday. Today’s warmer conditions should affect all elevations, increasing the risk of natural and human-triggered wet avalanches on steep slopes.

    Any prolonged periods of sustained sunshine will increase this hazard on slopes exposed to direct sunlight. With partly cloudy conditions expected, the potential for greenhousing could further moisten the surface on all aspects.

    Tree bombs and water dripping from trees are signs of warming. Roller balls and mushy-surface snow are both indicators that the surface is capable of producing a wet avalanche. Choosing a slope not affected by the sun may help mitigate this problem. Otherwise, default to lower-angle terrain and limit time spent above gullies where wet debris can pile up deep.

    (4/24/2022)

    Natural and human-triggered wet loose slides like this one are possible today.

    Photo: Mark Dundas

Avalanche Discussion

This is a heads-up day for backcountry travel. In the Swan Range, especially—and anywhere you find similar snowpack structure—a conservative travel plan is warranted.

Here’s what we know:

  • Widespread weak surfaces existed prior to the recent snowfall, and in many locations,  some variety of a crust was also in the mix. 
  • In the Swan, large buried surface hoar has been found in multiple locations, including in normally wind-exposed terrain. It was already very reactive yesterday, and that was before the slab stiffened.
  • We haven’t yet found a widespread layer of buried surface hoar elsewhere, but observations are limited. Regardless, near-surface facets are plenty capable of causing trouble on their own.
  • Recent snow totals vary widely across the region. The Central Whitefish Range near Stryker Ridge appears to have received snow totals similar to the Swan (roughly 10–12”), while the Flathead Range likely saw significantly less. Small storm slabs were already becoming reactive near WMR yesterday afternoon, despite less recent snow. Expect more dangerous conditions where snow totals are higher, and especially as you gain elevation into wind-loaded terrain.

Fortunately, this problem should provide obvious feedback as you travel. Shooting cracks extending more than a foot from your feet or machine, or audible collapses, are clear signs of instability. If you experience either, it would be reasonable to expect a human-triggered avalanche on a nearby steep slope and to dial back your terrain choices. In areas with less new snow and an absence of red flags—recent avalanches, cracking, or collapsing—the avalanche danger is likely lower. This may be the case in parts of the Flathead Range today.

In areas with buried surface hoar, which you can likely identify in a quick hand pit as a grey stripe 6–12” below the surface, do not rely on the slab above needing to feel stiff or well developed. This layer was already producing avalanches and propagating results in column tests yesterday, when the overlying snow was barely cohesive.

Finally, warming will be driven largely by solar input. If the sun breaks through and greenhousing occurs, wet loose avalanches will be possible on all aspects, and slab avalanches may become even more reactive. A moistening snow surface is your cue to dial things back another notch.

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