Avalanche forecast

Bitterroot

West Central Montana Avalanche Center
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Avalanche danger

Tue
Alpine
4 · High
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Dangerous avalanche conditions exist at middle and upper elevations. A strong winter storm is hitting the area with 2 feet of new snow and strong to extreme winds.

Expect large natural and human triggered avalanches. Travel in avalanche terrain is not recommended. Avalanches may run long distances into mature forests, valley floors, or flat terrain.

Unusual weather can create unusual avalanches, and today's extreme wind speeds are certainly not usual. Be on the lookout for loading or red flags in areas where you rarely encounter avalanches.

 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1.5–2.5

    Expect sensitive wind slabs 1-3 feet in depth today as wind speeds continue to increase. For the past 36 hours, winds have been building in intensity from the west/southwest ahead of last night's storm. Last night's new snow and strong winds increased the size of the previous wind slab even more. 

    One longtime Bitterroot local described yesterday's conditions as "about as windy as I have experienced in the Bitterroot, with strong sustained SW winds and powerful gusts all day transporting lots of snow". With it, he could remotely trigger wind slabs 2' deep and 100' wide.

    Strong winds often build wind slabs in unexpected locations like low on slopes and in cross-loaded gullies. Wind slabs can appear as smooth or rounded pillows, but this may not be obvious today as new snow will quickly bury and hide wind-loaded areas.

    If you see recent avalanches, shooting cracks, or collapsing, step back to slope angles under 30 degrees.

    (2/24/2024)

    Active wind transport of snow is visible in this photo. Snow is blowing off the edge of the cornice and loading the slope below.

    Photo: Payton Schiff

  • Persistent Slabs

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

    There are a variety of persistent weak layers in the snowpack, including rounding facets near the ground, small facets above a crust in the middle of the snowpack, a thicker layer of buried near-surface facets above a crust, and buried surface hoar.  In the case of the facet/crust combination, it has continued to become weaker.

    The increased load from the snow last night has tipped the scale and re-awakened the persistent slab problem. Large avalanches are likely today, breaking down onto weak layers deeper in the snowpack.

     If you do trigger an avalanche on these layers, it is unlikely that you will see obvious signs of instability like whumfing, collapsing, or cracking before the avalanche occurs.

    Dig in the snow, and if you find weak snow around crusts, or loose sugary snow below a firmer slab, recognize the potential for a larger, more dangerous avalanche in these areas. Smaller avalanches have the potential to step down and trigger a larger avalanche on persistent weak layers.

Avalanche Discussion

What an active 24 hours for weather. With that, yesterday's message continues. New snow + strong winds = avalanches.

Yesterday, new snow totals did not reach forecasted levels as temperatures were warmer than expected. With these warm temperatures, we saw widespread low-elevation rain ahead. Upper elevations picked up between 4-8 inches of new snow yesterday, with another foot plus last night.  The sustained high wind speeds have made this not just another winter storm.  Today, we are expecting those high winds to continue through the day.

What has me more concerned than the fresh snow and wind is the persistent slab problem. Persistent weak layers can lay dormant, unreactive, and not produce avalanches until a large amount of snow (heavy weight) falls and stresses them. Then once the scales tip, we see a large and destructive avalanche cycle. Yesterday, observations began to trickle in that this load was tipping the proverbial scales and indicating today would be an active day for large avalanches.

Be patient with this storm.  Elbert Despain was the mailman in the famed Little Cottonwood Canyon, infamous for avalanches well before the time of avalanche control. How did he manage to stay alive so long and not get caught in an avalanche? He would wait two days after a large winter storm before traveling through the canyon. Take his advice to heart and make conservative terrain decisions today.

We are entering an active weather pattern with increasing confidence in another storm Thursday into Friday.

Let us know what you see in the mountains today by submitting an observation.

Ski and ride safe.

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