Bollettino valanghe

Bollettino valanghe

Payette Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering a persistent slab avalanche on older, buried weak layers remains possible today. You are most likely to encounter this problem on NW, N, NE, and E-facing slopes between 7000-7500 feet. Yesterday, S/SW winds were drifting soft snow into slabs on N/NE-facing aspects above 7000 feet. Clear skies and strong solar radiation will increase the likelihood to wet loose avalanches on south-facing slopes, especially in the afternoon.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2

    Yesterday, the PAC investigated a snowmobile-triggered avalanche NW of Box Lake that occured Thursday (02/09). This avalanche failed on a weak layer of buried surface hoar from January. It was 1-2' deep, on an east aspect, at about 7500'.

    Monday, we observed several snowmobile-triggered avalanches near Council Mountain (read observation here). These avalanches ranged in size, but most were large enough to bury, injure, or kill a person. Common characteristics that we found were:

    • Avalanches failed on a layer of buried surface hoar from January that ranged in depth from 40 cm (16") to 110 cm (3.5 ft.)
    • These avalanches were on NE-facing slopes between 7200-7400 feet
    • These avalanches were in pockets of wind-sheltered terrain(pictured at right) that had been loaded by Sunday's storm 

    Buried weak layers, particularly surface hoar, are spotty and can be difficult to find. When traveling in the mountains this weekend, you really need to think like a weak layer. When looking at particular slopes, look for openings or clearings where weak layers (surface hoar or facets) would form. Now, think about pockets that are protected just enough from wind and sun that these weak layers would stay preserved. It's a lot to pay attention to but if you want to avoid a persistent slab avalanche, you need to be thinking about these terrain features. The weight of a snowmachine is much more likely to impact these buried weak layers than the weight of a person.

    (2/6/2023)

    Avalanches such as these are possible in pockets that were protected just enough from the wind to preserve older weak layers 

    Photo: Studley

  • Wind Slabs

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

    Yesterday (Friday), winds out of the S/SW were drifting and blowing snow, especially near ridgetops (photo to the right). Sensitive wind slabs may have formed on N and NE aspects. With the recent snow this past week, there has been plenty of snow available for transport.  Last night, winds shifted out of the NE. You're most likely to encounter a wind slab on slopes above 7000 feet, particularly on north and east-facing aspects. Local terrain features can influence upper level winds and wind direction can vary by zone. This leads to a level of uncertainty with where exactly wind slabs could be lurking. All wind-exposed slopes should be carefully evaluated for the presence of wind slabs.

    Pay attention to clues like shooting cracks from your skis or machine. If you feel a dense or hollow feeling in wind-exposed terrain, you've found evidence of a wind slab. Find aspects that are not presenting these clues to avoid wind slab avalanches.

    (2/10/2023)

    Moderate SW winds were transporting snow on ridgetops Friday (02/10)

    Photo: Studley

Avalanche Discussion

Today's avalanche forecast is sponsored by Mile High Powersports of McCall. Thanks for supporting the Payette Avalanche Center.

Bluebird skies, warm, sunny weather, and soft snow will draw lots of people into the mountains this weekend. The weather might make us feel confident, but the snowpack does not always share the same level of enthusiasm. If you've been reading the forecast this season (yes, I'm talking to you!), then you should be familiar with two avalanche problems we've been discussing: wind slabs and persistent slabs.

  1. Wind slabs, although manageable, can catch people by surpise. Wind slabs require paying close attention to recent weather (wind speed and direction) and how the snow feels. Look for places the wind can blow and deposit snow (beneath ridgelines and cornices, across gullies and depressions, over cliffs and rocks). Wind slabs may also be disguising problem #2. A discussion with your travel partners should include how to manage wind slabs.
  2. Persistent slabs, these sneaky weak layers have caught experienced snowmobilers by surprise this week. Persistent weak layers are very spotty and are difficult to find. Currently, persistent slab avalanches are large enough to bury, injure, or kill a person. I highly recommend a solid travel plan of how to identify, manage, and/or avoid terrain where persistent slabs are possible.

With another day of clear skies, warmer temperatures, and solar radiation, small, loose snow avalanches may increase. Dry loose snow (sluff) may be possible of steep, shaded, north aspects. The likelihood of wet loose avalanches will increase this afternoon on south-facing slopes, especially near rocks, cliffs, and trees in steep terrain.