Bollettino valanghe

Rattlesnake

West Central Montana Avalanche Center
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Pericolo valanghe

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

Wet avalanches are likely today as the sun comes out and temperatures climb. Natural and human-triggered avalanches could be very large and destructive. Stay off of steep slopes and away from avalanche run-outs.

The timing of this avalanche cycle is uncertain. If the forecast cloud cover does not appear, danger will jump to high for a period of time this afternoon.

Problemi valanghivi

  • Wet Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3.5

    Unseasonably warm temperatures are melting the surface snow and introducing liquid water to the snowpack. Yesterday we found water pooling on buried weak layers, and it is only a matter of time before this leads to instability. Unfortunately, it is very difficult to predict the exact timing of wet slab avalanches. With continued warm temperatures and sun, we could start seeing large natural or human-triggered wet avalanches today.

    Wet loose avalanches are also likely on steep slopes at all aspects and elevations. Size will depend on the depth of wet snow, bed surface, and slope angle, with the largest loose avalanches on steep southerly slopes where wet snow sits on a buried crust. Loose wet avalanches, as well as cornice fall, will be effective triggers for wet slab avalanches.

    The best way to avoid wet avalanches is to avoid wet snow. If you’re sinking into wet snow past your ankles, stay of off slopes over 30 degrees. Remember that large wet slab avalanches can run far - right down to low elevations. Pay attention to what is above you, give steep slopes a wide berth, and cross avalanches run-outs one at a time.

Avalanche Discussion

Hopefully you had a chance to enjoy the powder last weekend, because we’re opening this forecast period with a very different story. Yesterday we found wet, even slushy, snow at all elevations and aspects where we travelled, and we suspect that the surface snow is wet/moist to ridgetop. With widespread wet snow, you should be thinking about all the different avalanche problems typically associated with warming. These include wet slab, wet loose, cornice, and glide.

The largest and most destructive avalanches will be wet slab avalanches, which is why wet slab is listed as our primary avalanche problem. Remember that persistent slab problem we’ve talked about all season? Those buried weak layers are now becoming wet. As water percolates through the snowpack, it pools on crusts and facet layers, plus changes the physical properties of the overlying slab. As this happens, both human-triggered and natural avalanches can quickly jump from unlikely to likely to almost certain. The timing of this jump in likelihood is where uncertainty lies, and is what makes forecasting wet slab avalanches so difficult. We expect a natural avalanche cycle today or tomorrow, but can’t predict exactly when it will occur.

In the Bitterroot compared to other ranges, we’ve got more new snow, more recent rain, higher temperatures, and we will see more sun today. Decreasing cloud cover will be particularly notable in the Southern Bitterroot. This is why the Bitterroot is at High Danger, while the Rattlesnake and Seeley Lake are at Considerable. But, don’t discount considerable danger. Any avalanches you trigger in any region will be large and destructive. Each danger rating level is a range, and we’re at the higher end of considerable. As warm temperatures continue in the coming days, expect these regions to bump up to high as well.

With last week's snowfall and wind event, followed by this week’s warming, cornices are large and particularly sensitive. Today is not the day to travel near or under cornices, as cornice-fall can occur without warning. We’ve also seen evidence of recent glide avalanches on slabby rocks in the Bitterroot. Like cornices, if you see a glide crack, don’t travel underneath it, especially on days like today when liquid water is actively moving through the snowpack.

Often you can avoid wet snow by travelling early in the morning, or moving to northerly slopes as the sun comes out. However, with a limited overnight refreeze and high temperatures across the board, these strategies won’t be effective today. Any overnight surface refreeze will quickly break down this morning, and all aspects are wet due to high temperatures.

If you do make it through the wet slushy snow and somehow find dry snow up high, remember that we just had a big natural avalanche cycle. Triggering a large persistent slab avalanche is still possible.  

If you see or trigger any avalanches, please let us know, and send us a picture if you can. This will help us figure out the timing of the wet slab cycle. Click here to submit an observation. You can fill out as much or as little of the form as you like.

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