Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Strong winds, new snow, and rapid warming have combined to create considerable avalanche hazards.

Widespread wind slabs have formed along ridgelines.  Triggering a persistent slab avalanche remains likely on north aspects. Avalanches may be remotely triggered from below, above, or to the side. Wet snow avalanches are likely at low elevations as temperatures rise.

Avoid slopes over 30 degrees today.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Winds for the past 48 hours have been strong from the west. These winds, combined with plenty of new snow available for transport, have created reactive wind slabs on east aspects. Cross loading was also observed on north and south facing gullies and terrain features. In areas with previous east winds, wind slab exists on the west aspect, although these slabs are likely stubborn and less widespread. 

    Be aware of pillows, visibly loaded features, and cornices. Give any cornice growth plenty of space, as they may be touchy today. Strong winds may create wind slabs lower on the slope than expected. Thoroughly assess the situation before dropping in. 

    Avoid slopes over 30 degrees along ridgelines and in cross loaded gullies. 

  • Persistent Slabs

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

    Triggering an avalanche on persistent weak layers is likely on all aspects at mid and upper elevation, especially on north facing terrain. The layers of most concern are depth hoar and a layer of surface hoar buried 3 feet down. These are not the only persistent weak layers, however, and attempting to outsmart the persistent slab issue today is not wise. 

    You are most likely to trigger an avalanche around shallow areas of the slab, convexities, or buried rocks. These avalanches may be up to 5 feet deep and run full track. 

    Stick to slopes under 30 degrees, and be aware of what is above you at all times. 

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Temperatures this weekend could rise to 40 degrees at 8000’. A warm air mass is entering the region from the south beginning today. There is high uncertainty about how far north this warming will occur, but all models show a large mass of warm air entering the region. The southern Bitterroot will see the most warming, but similar 40+ ridge top temperatures are possible through Seeley Lake. Combined with the new snow from this week, any rapid warming will create avalanches.

    At lower elevations, wet slabs are possible as persistent weak layers lose strength. 

    Rollerballs are a sign that the snowpack is beginning to lose strength. When the rollerballs progress to point releases, it is a sign that wet loose avalanches are likely, and it is time to exit avalanche terrain. Any slopes with wet snow on top of dry snow are suspect for slab potential. 

    Be aware of what is above you, especially when traveling under gullies or terrain features that may channel flowing snow. 

Avalanche Discussion

It has been a wild week for weather, and the end is not yet in sight. Early this week, all zones received 1-2 feet of new snow along strong winds. This was followed by record cold temperatures. Friday, between .5 and 1 inch of SWE (snow water equivalent) fell between Lost Trail Pass and Seeley Lake, with the central Bitterroot getting the most snow. Ridgetop winds were strong from the west for the past 24 hours. 

This weather has placed a massive load onto a weak snowpack. Visibility has been poor, so we are not able to rule out a large natural avalanche cycle. One full depth natural avalanche was seen in the Rattlesnake zone yesterday. Observers have seen sensitive wind slabs and storm slabs failing at the new snow/old snow interface. Reports of widespread collapses and whumpfing continue to come in from all zones. 

A list of buried persistent weak layers remains constant across all forecast zones. Most aspects and elevations harbor multiple facet layers simultaneously. Buried near surface facets below the new snow have created a poor interface. Multiple layers of buried surface hoar exist in all zones but especially in the Rattlesnake, Lolo Pass, and southern Bitterroot. With the exception of the deepest snowpacks of the central Bitterroot and Southern Missions, depth hoar has the potential to create full depth avalanches 5 feet deep. In the mid elevation and southern aspects of the Bitterroot, a reactive facet/crust combination adds one more layer of complexity.  With this many problems at once, slope angle is the only way to manage hazards. Do not attempt to outsmart this persistent slab issue, as there are simply too many weak layers to manage. 

Snow does not like rapid change. This week, we have rapidly increased the load on existing weak layers. Record cold temperatures further stressed the snowpack with more change. Today, some areas may see a temperature increase of over 40 degrees in 24 hours. This may be the rapid change that tips the balance, releasing a full depth avalanche cycle. The warmer temperatures get today, the higher the likelihood becomes of a significant avalanche cycle. Be aware of what is above you at all times, and stay out of the runout of avalanche paths. 

Please let us know what you are seeing by submitting an observation! 

Ski and ride safe. 

Regioni coperte