Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering an avalanche at mid and upper elevations is possible today on both wind slab and persistent slabs. Pockets of considerable danger exist in the southern Bitterroot, where new snow is sitting on wet snow, creating a poor interface. 

Wet loose avalanches are possible on sunny aspects this afternoon.

Collapsing or whumfing are signs to stick to terrain under 33 degrees. If you see rollerballs or point releases, wet snow avalanches are likely to follow. The best skiing and riding will be found on northern aspects in areas less affected by the sun.

Problemi valanghivi

  • Loose Wet

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

    It is mid March, and it doesn’t take much direct sun to create wet snow avalanches on sunny slopes with fresh snow.  Today, we are expecting sun and minimal wind, with wet loose avalanches possible by the afternoon. These avalanches will run easily in areas where a crust is buried at the new snow interface.

    Look for rollerballs and point releases off of dark objects like trees or rocks. These are early indicators that the snow is becoming wet, and that wet snow avalanches are possible in the near future. Be cautious traveling under avalanche terrain during a wet snow avalanche cycle. Avoidance is the best management strategy for wet snow. 

  • Persistent Slabs

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

    A persistent weak layer of small facets around buried crusts is sitting below a slab 8-24 inches deep. Anywhere new snow fell on top of wet snow are areas of concern. Yesterday, widespread whumfing and collapsing in the southern Bitterroot kept forecasters well out of avalanche terrain. Pockets of considerable avalanche danger likely still exist in the far southern Bitterroot where more snow fell Wednesday morning. 

    At mid elevations, there is still wet snow buried within the snowpack. Any slab avalanche failing below this layer will be dense and unmanageable if you are caught in it. 

    You are most likely to encounter these buried persistent weak layers at mid elevation, especially on south aspects where the new snow fell onto warm and wet snow. If you experience collapsing, dig down, and if you find a crust at the interface or buried wet snow, be suspicious of weak snow and increased avalanche potential. 

    A common scenario for developing a faceted crust: warm, wet surface snow is buried by cold snow and facets because of the strong temperature gradient. This process is called melt-layer recrystallization. Credit: Bruce Tremper

    Photo: National Avalanche Center/ Bruce Tremper

  • Wind Slabs

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

    New snow and wind created wind slabs on east aspects at upper elevations. Wind loading is also possible on all aspects where terrain has funneled winds, resulting in shifting wind directions or cross loading. 

    Wind slab was touchy for skiers and riders yesterday, with skiers able to remotely trigger avalanches. Sensitivity will decrease with the warming temperatures today. 

    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. 

    (3/15/2023)

    A small intentionally triggered wind slab on an east aspect at around 7000 ft in the Rattlesnake. 

    Photo: Arden Feldman

Avalanche Discussion

Today's avalanche danger is largely dependent on what surface the new snow fell onto on Wednesday morning. At upper elevations, it fell onto dry snow and was blown around by the wind creating wind slab. This is a fairly straightforward avalanche problem to describe, identify, and avoid. 

Things become more complicated when dry snow falls onto warm and wet snow. You might think that the new snow will just stick to the wet snow, and initially, it does. Then an interesting process occurs. A crust forms on top of the buried wet snow, right at the interface, and faceting begins to occur above the crust in a process called melt-layer recrystalization. Yesterday, this process was just starting to occur, and we were seeing failure and propagation on the layer. How far the faceting has progressed by today is unknown. Use extra caution while traveling around at mid elevation today. This layer is guilty until proven otherwise. 

We are also seeing a crust and facet combination buried deeper on south aspects. Facets around crusts are a troublesome persistent weak layer because they may not be immediately apparent when looking at a snowpit wall. When the slab fails above the crust, it has a firm and smooth bed surface to run on, creating far and fast running avalanches. 

Digging a snowpit is the best way to identify these persistent weak layers. If you find them in your pit, there is a good chance that they exist in similar terrain. Remember, just because you don’t find weak snow or propagation in a snowpit does not mean that it doesn’t exist just a few yards away. Snowpits are an essential tool when looking for weak layers, but can give a false sense of security if done incorrectly. 

Fresh snow, sun, and light winds should make excellent skiing and riding conditions today. Let us know what you are seeing out there by submitting an observation.

Ski and ride safe. 

Regioni coperte