Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 5 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Avalanche danger will start at Low, and become Moderate as temperatures warm, the sun comes out, and crusts break down. If you're sinking into wet, mushy snow, it's time to move to shadier aspects where a surface crust remains, or get out of the mountains altogether. While unlikely, you could still trigger a larger avalanche at the highest elevations where dry snow remains.

Problemi valanghivi

  • Loose Wet

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

    Multiple forms of wet avalanche are possible, with wet loose being the most likely. Surface crusts that formed overnight will quickly break down this morning. This will happen first on steep solar aspects, but expect crusts to break down everywhere eventually. Even with cloud cover, today's warming temperatures and the occasional glimpse of sun will be enough to warm the snow.

    Wet Loose: These avalanches start from a single point, such as your skis, snowboard, or snowmobile, and fan out downhill, gaining mass as they travel. They can also start naturally on steep slopes, so pay attention to what’s above you during the heat of the day. Loose avalanches are most likely on solar slopes, but possible on all aspects.

    Wet Slab: While unlikely, wet slabs are notoriously difficult to predict, and we do have the snowpack structure for them. We had a wet slab avalanche cycle a week ago, and we can’t write off these large, destructive avalanches while water is still moving through the snowpack. If you dig down and find wet layers surrounded by dry snow, you’ve found the recipe for a wet slab avalanche.

    Cornice: Snowfall and wind this past month built large cornices. Cornices weaken and become more likely to fail as temperatures warm.

    Glide: Glide cracks have opened up in the Bitterroot, and we’ve seen some glide avalanches. Glide avalanches occur on steep rock slabs and take the entire depth of the snowpack with them. They are most likely to happen while water is actively flowing through the snowpack, which is the case at the moment.

    The best way to avoid wet avalanches is to get out of the mountains, or move to shady slopes, as crusts break down and you start sinking into mushy snow or seeing roller balls. Remember, this can happen quickly as the sun hits steep slopes, so make sure you have an exit strategy in case you need it. Don’t travel below glide cracks or cornices, and give cornices a wide berth when travelling at ridgetop.

    Natural wet loose avalanches in the Rattlesnake. We can expect to see similar debris piles in the Bitterroot region today.

  • Wind Slabs

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

    High elevations in the Bitterroot received 4 to 8 inches of snow three nights ago. While most snow surfaces got wet yesterday, the uppermost northerly aspects likely escaped heating and have lingering dry snow.

    If you get up high today, pay attention to the elevation at which you start finding dry surface snow (and please let us know by submitting an observation here). Once this happens, you’ll find wind slabs in all the likely areas. Think leeward sides of ridges and rollovers, downwind of cliffs and large rocks, immediately under cornices, and in cross-loaded gulleys.

    Wind slab in the Bitterroot two days ago. Some of these wind slabs will remain dry on high elevation northerlies.

Avalanche Discussion

With temperatures only just below freezing last night, expect crusts to break down quickly this morning. In the Rattlesnake, ridgetop temperatures didn’t even get below freezing.

Yesterday I was in the Rattlesnake, and southerly aspects stayed hard until noon (see observation here). I suspect northerly aspects at high elevation didn’t soften at all. With higher temperatures today, the snow is likely to soften earlier. While this moves up our window for good skiing, it also moves up our window for avalanche hazard. South aspects will warm first and most, but high temperatures mean wet avalanches will become possible on all aspects at low and middle elevations.

Forecast cloud cover is the main source of uncertainty in today’s weather forecast, and therefore in today’s avalanche forecast. If we have less cloud cover than expected, the avalanche hazard will rise from low to moderate more quickly, and may reach the higher end of moderate hazard. Cloudy skies in the Rattlesnake, Northern Bitterroot, and Seeley region will temper, but not prevent, afternoon snow surface warming. As you move south in the Bitterroot, expect less cloud cover and more warming.

While wet avalanches are the main hazard today, we can’t fully write off dry avalanches, particularly in the highest mountains of the Bitterroot, Swan, and Mission ranges. The snowpack has not fully completed its spring transition, and buried weak layers remain. These are most prominent on northerly aspects at high elevation. If you dig down and find an obvious weak layer (either wet or dry), consider the consequences of a large avalanche before committing to a slope. In the Bitterroot, the highest elevations will still hold dry snow from Thursday's storm. If you find dry surface snow, watch for lingering wind slabs.

While avalanche hazard often becomes easier to manage in the spring, don’t let yourself get lulled into a sense of complacency. Pay attention to how the snowpack changes today – both spatially and temporally. Look for signs of instability such as recent avalanches, cracking, and rapid warming. These signals are as relevant in the spring as they have been all winter.

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