Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

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

Dangerous avalanche conditions exist today due to very strong winds and ongoing snowfall. Human triggered wind slab avalanches are likely at middle and upper elevations. Natural avalanches are possible.

Avalanches could break deeper on persistent weak layers in the middle of the snowpack, resulting in larger, more destructive avalanches.

If you encounter recent avalanches or see shooting cracks around your skis or snowmobile, stick to slopes less than 30 degrees steep.

The avalanche danger could reach HIGH at upper elevations in the Bitterroot this evening due to increased snowfall. 

Problemi valanghivi

  • Wind Slabs

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

    Skiers and riders are likely to trigger wind slabs today. Strong west winds have drifted snow over the last several days, and the winds will get even stronger today, gusting up to 60mph out of the southwest. The wind will continue to drift snow into wind slabs at middle and upper elevations, especially as snowfall rates increase this evening. 

    Wind slabs often form along ridgelines, in gullies, and below convex rollovers. However, strong winds often build wind slabs in unexpected locations like low on slopes and in cross-loaded terrain. Their smooth, rounded appearance can often identify them. If you see recent avalanches, shooting cracks, or collapsing, stick to slope angles under 30 degrees. 

    (2/24/2024)

    Active wind transport of snow is visible in this photo. Snow is blowing off the edge of the cornice and loading the slope below.

    Photo: Payton Schiff

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Large persistent slab avalanches are possible on upper elevation slopes today. There are a variety of persistent weak layers buried in the snowpack, including rounding facets near the ground, small facets above and below a crust in the middle of the snowpack, a thicker layer of buried near-surface facets above a crust, and buried surface hoar. 

    The increased load from recent snowfall has stressed the buried weak layers. Today’s wind loading and snowfall will continue to add stress and could tip the scales. If you do trigger an avalanche on these layers, it is unlikely that you will see obvious signs of instability like whumfing, collapsing, or cracking before the avalanche occurs. 

    Dig in the snow, and if you find weak snow around crusts, or loose sugary snow below a firmer slab, recognize the potential for a larger, more dangerous avalanche in these areas. Smaller avalanches have the potential to step down and trigger a larger avalanche on persistent weak layers. You can use slope angle to avoid the problem by sticking to slopes under 30 degrees.

    Photo: Cam Mackenzie

    Snow pit with multiple buried weak layers in the southern Bitterroot. 

Avalanche Discussion

It has been a windy week and today will be even windier. Strong to extreme southwest winds will gust to 60mph and peak during the cold front passage this evening. The cold front is forecast to deliver 4 to 12 inches of snow to the region, favoring the Bitterroot. Portions of the central and northern Bitterroot could receive even higher totals as snowfall amounts during the day today are uncertain and could overproduce. Thus, the avalanche danger in the Bitterroot zone could reach HIGH this evening. 

Snow showers continued throughout the week after the strong storm on Sunday night. SWE totals since Sunday morning are as follows:

Seeley Lake (North Fork Jocko Snotel): 2.4” SWE

Rattlesnake (Stuart Snotel): 1.4” SWE

Bitterroot (Twin Lakes Snotel): 3.2” SWE

That is a significant load of snow, and today’s snowfall will add to it. We haven’t observed any avalanches on buried weak layers since this load was added, but we have also had limited visibility into upper elevation start zones. There is uncertainty about the sensitivity of these weak layers. Some recent observations have found that the snowpack is adjusting to the load with extended column tests not producing propagation. However, it is more than enough load to cause concern. Will today’s snowfall tip the scales? Or have they already tipped? We don’t know. Either way, the snowfall has provided ample snow for transport by the ongoing strong winds, and large wind slabs remain the primary concern. 

It will be warm today, with temperatures reaching into the 40s in the mountains. Snow levels will rise up to 5500 ft before the cold front brings them back down this evening. With such warm temperatures and the potential for light rain up to middle elevations, keep an eye out for small wet loose avalanches at middle and lower elevations. If you see rollerballs progressing to point releases, consider moving to lower angle slopes. 

Please let us know what you see out there today by submitting an observation. 

Ski and ride safe.

Regioni coperte