Bollettino valanghe

Whitefish Range

Flathead Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

You can stumble into trouble today. You can easily trigger large, dangerous avalanches that break 1 to 2 deep on preserved weak layes near the mid-January crust. This hazard is most widespread on colder slopes (northwest through northeast to southeast). Triggered and natural avalanches involving slabs of drifted snow are likely near ridgelines and summits. Enough loose, dry snow has accumulated on steep, sheltered slopes that sluffs can be large enough to be dangerous. Find your fun on slopes that are less than about 30 degrees. 

Problemi valanghivi

  • Wind Slabs

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

    Sustained winds haven't accompanied this week's snowfall. But they've been gusty enough to drift snow near ridges and into the tops of chutes and on the lee sides of gullies and create some dense slabs. Expect to find these slabs in atypical locations, thanks to wind switching from the prevailing southwest to northerly. This hazard will grow more widespread as winds increse this afternoon. Debris from triggered wind slabs and falling cornices can trigger much wider and deeper avalanches as it runs downslope. Avoid being on or under steep start zones where you see blowing snow and active loading. 

  • Loose Dry

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

    Enough low density snow has accumulated that sluffs can be dangerous on very steep slopes that are sheltered from the wind and don't harbor buried surface hoar. This hazard poses the greatest danger at low and mid elevations, on slopes where recent snow has not consolidated into a slab above the mid-January crust. Don't let your sluff push you into a tree well or other terrain trap. 

  • Persistent Slabs

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

    As slabs consolidate above the mid-January crust, the likelihood and potential size of triggered and natural avalanches increases. Steep northwest through northeast to southeast slopes harbor a buried surface hoar layer that has been reactive near ridges at mid and upper elevations. On other slopes, facets around the mid-January crust have not been as reactive, though tests indicate the potential for triggering slides persists. The most reliable way to mitigate this hazard and its variable distribution is to avoid slopes steeper than about 35 degrees, and stick to slopes that are less than that angle for their entire length. Mid-slope convexities are likely trigger points for these slides.

    (2/2/2021) Failed on buried surface hoar.

Avalanche Discussion

The conditions I've seen in the past two days of fieldwork are remarkably unforgiving. I expect them to get worse, with more snow forecast for today and winds spiking this afternoon. Unless you are very skilled at avoiding avalanche start zones and runouts, we recommend staying out of the backcountry.

You might think I'm overstating things if you're traveling in the northern half of our forecast region. That half of the region is just late to the party; the same structure (soft slabs over buried surface hoar and the 1/13 crust) exists on shaded slopes. However recent snow does not seem to have consolidated into stiff slabs yet, so they're less reactive. For now. As more new and drifted snow accumulate in that part of the forecast region, expect the slabs to become more reactive and conditions to grow more dangerous. 

Southerly slopes have not produced the same kind of avalanche activity in the past week. You can find weak facets around the 1/13 crust, and tests are still producing propagation. That's enough info to keep me off of, and out from under, slopes steeper than about 35 degrees on these aspects. They will grow increasingly dangerous when winds turn to the north and start top-loading and cross-loading these usually windward slopes. 

Conditions like these are unusual in northwest Montana. Rarely do slopes remain reactive to the weight of a person or snowmachine for so long. The weather of the past week has incrementally loaded the weak snow around the 1/13 crust, steadily increasing the danger. Until the storms end, the danger will not taper off. 

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