Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Mon
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 6 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Human triggered avalanches in the new snow will be possible by this afternoon. Triggering a larger persistent slab avalanche is possible all day. Small avalanches have potential to step down and trigger a larger avalanche as well. 

Shooting cracks, collapsing, or fresh avalanches are all indicators of instability but you may not see any red flags or warning signs before triggering a larger persistent slab avalanche. 

If you want to avoid avalanches, stick to slopes less than 30 degrees in slope angle. 

Problemi valanghivi

  • Persistent Slabs

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

    A 1-2 foot slab of snow is sitting on top of persistent weak layers of surface hoar and near surface facets that formed during January.  The strain on this weak layer will be increased today as more snow falls on it but new snow totals in the Bitterroot are less than areas further north and this load will be less. While there has been a decrease in activity on this layer the past few days, this could change and the problem could reactivate with the new load. Upper elevation north facing terrain is most suspect, especially in areas with a stiffer slab from wind loading.  

    Identify buried surface hoar by digging in the snowpack and looking for a thin grey line 1-3 feet below the snow surface. If you find this, be especially cautious as surface hoar avalanches can fail on low slope angles down to 30 degrees. A common trap with surface hoar is to dig high in start zones and not find it, then descend partway down the slope and trigger an avalanche on buried surface hoar. It can get destroyed high on slopes but preserved lower on the slope, creating tricky conditions for slope level snowpack assessment. If you dig a pit high on the slope and do not find buried surface hoar, do not trust that there is no buried surface hoar lower on the slope. 

    To identify buried near surface facets, look for a layer of weak sugary crystals below the most recent snow. If you find this hardness step off, avalanches are possible and terrain management with slope angle is the way to avoid avalanches. 

    Most of the surface hoar and near surface facets on south facing slopes were destroyed during periods of strong sun. Some south facing slopes still have buried crust/facet layers that are less reactive, but still pose a risk of avalanches. 

    With all persistent slab avalanches, it is possible to trigger them remotely from above, below, or to the side. You may not see any red flags or warning signs before triggering an avalanche on these layers.

  • Wind Slabs

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

    Exect fresh wind slab formation in todays snow, primarily on east aspects but cross loading and swirling winds could load all aspects. 

    Expect fresh wind slab formation at upper elevation and along ridgelines. Any wind slab triggered has  potential to step down and trigger a larger persistent slab avalanche. 

    New snow totals increase as you move north and areas up high around Lolo Peak could see new snow totals more inline with the Rattlesnake. Elsewhere in the Bitterroot, forecasted new snow totals are significantly less with 1-4 inches likely. Last night a band of precipitation set up over the central Bitterroot and Twin Lakes (Lost Horse Drainage) recieved 5 inches of snow. This is the outlier for the Bitterroot, and most other areas of the zone recieved little to no snow last night.  With this variation in snow totals will come a variety of wind slab sizes. Be suspect of anywhere you find 6 inches or more of windblown snow. 

    Wind slabs can be identified by their smooth or pillow like appearance, hollow feel, or by red flags like shooting cracks and fresh avalanches. If you find any of these and want to avoid wind slabs, use slope angle and stick to slopes less than 30 degrees in slope angle. 

Avalanche Discussion

A welcome increase in forecasted snow totals for today will bring a refresh to skiing and riding conditions. It will also increase strain on buried weak layers and potentially re awaken an avalanche cycle. 

Current weak layers are showing a high degree of spatial variability. What does that mean? It means that even on the same slope, things like the type of weak layer or slab thickness change. As we move from slope to slope, these things change even more. In conditions like this, careful route finding and slope assessment is critical. Use the forecast as a starting point but make an assessment of the slope you plan to ski or ride. 

Things like tracks on a slope are not indicators of stability. It will be possible for the 2nd or 5th person on a slope to trigger an avalanche. 

Glide cracks are opening up throughout all zones. Glide cracks are very unpredictable and can fail minutes after the crack opens or months after. Avoid spending time under open glide cracks. Cornices have the potential to break off naturally during periods of warming. Minimize the time you spend underneath overhung cornice on warm days. 

Avalanche education is a lifelong journey built on developing sound judgment, gaining experience, and applying repeatable decision-making systems in the backcountry. Our American Avalanche Association (A3) recognized Level 1, Rescue, and Level 2 courses are offered throughout the winter and help you travel smarter in avalanche terrain. Explore current courses and registration on the Education page. 

Please continue to submit observations, and ski and ride safe!

Regioni coperte