Bulletin d’avalanche

Bitterroot

West Central Montana Avalanche Center
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Danger d’avalanche

Wed
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Human triggered and natural wet snow avalanches are possible at mid and low elevations in areas where rain has fallen. Triggering a wind slab avalanche in new snow at upper elevation is possible today. 

Uncertainty exists on the elevation of the rain/snow line. If you encounter wet unsupportable snow at low elevations or observe fresh loading and cracking at upper elevations, step back to slopes under 35 degrees.

Problèmes avalancheux

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Warm overnight temperatures and a rain on snow event have made wet loose avalanches possible at mid and lower elevations. Temperatures dipped below freezing this morning for the first time in 36 hours at 6000’. Some low elevation spots have not seen a freeze in 48 hours. Cooling temperatures this morning will help alleviate this problem, but hazard may rise again this afternoon as temperatures rise. 

    Look for wet or unsupportable snow. Slopes with a supportable crust at the surface may hold wet snow underneath. If you experience any collapsing or find yourself trenching through wet snow, step back to terrain under 35 degrees.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–1.5

    2-4 inches of new snow fell at upper elevations overnight. Shifting overnight winds created shallow wind slabs along ridgelines. While these avalanches will be small, expect them to run easily on the firm bed surface of old sun crust. Even a small avalanche can have high consequences if you are dragged through rocks or trees, and you will struggle to self arrest on the firm bed surface. 

    Watch for cracking and collapsing along ridgelines. Wind slab can be identified by its smooth rounded appearance. Step back to terrain under 35 degrees if you find signs of recent wind loading. 

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1–2

    In the southern Bitterroot, a buried crust and facet combination is buried by a slab 1-2 feet deep. Snowpit tests have shown mixed results, with only some tests showing propagation. How far north this layer occurs is still uncertain, but a recent slab avalanche on south facing terrain in the Lost Horse drainage possibly failed on this problem layer. 

    You are most likely to encounter this layer at mid and upper elevation on south aspects in areas with previously buried sun crusts. If you plan to ski or ride in steep south facing terrain, dig down, and if you encounter buried crusts, be suspicious of weak snow around the crust. 

Avalanche Discussion

Spring storms bring an inherent amount of uncertainty as only a few degrees of temperature change can mean a shift from snow to rain. Precipitation types can change across a mountain range, even within the same elevation band. As a result, avalanche hazards can vary significantly across the same zone.

Temperatures have been warm the past few days. All zones saw highs in the mid 40s yesterday at mid elevation. This morning is the first below freezing temperatures many areas have seen in two days. Yesterday, observers noted wet snow at mid elevation with the distribution of wet snow dependent on how much the wind was able to help cool the snow surface. Overnight re-freezes were superficial at best. 

Last night, the entire zone had a precipitation event. A few inches of new snow fell at upper elevations, and rain fell down low. What happened in the middle? Well… temperatures were right on the edge of rain versus snow, and without solid weather station data, we are not sure. With such high uncertainty, step back to the basics. If the snow is wet and unsupportable, stay out of avalanche terrain. 

We have a new persistent weak layer in the southern Bitterroot and potentially further north. Small facets have developed just above a buried melt freeze crust. Snowpit tests have been varied with no propagation in extended column tests but propagation to the end in propagation saw tests. Dig down at upper elevations on previously sunny slopes and look for small angular crystals just above the crust. It is a poor snowpack structure and should be approached with suspicion. 

Please take the time to submit an observation of what you see in the mountains today. We are especially interested in where you think the rain line was overnight and how much snow fell where you are traveling. 

If you have extra time this morning, check out the Mountain Rain or Snow project hosted by the Desert Research Institute. This group is working to improve rain versus snow forecasting and relies heavily on public observations during conditions like this. 

Ski and ride safe. 

Régions couvertes