Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

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

Dangerous avalanche conditions exist at upper elevations today. Skiers and riders are likely to trigger wind slab avalanches on north and east aspects. Avalanches could also 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 in slope angle. 

Problemi valanghivi

  • Wind Slabs

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

    Very strong southwest winds combined with fresh snowfall drifted significant amounts of snow onto north and east aspects yesterday and last night. Skiers and riders are likely to trigger large wind slabs at upper elevations today. Fresh wind slabs will be smaller at middle elevations. 

    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, although light snowfall today may bury and hide 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, and buried surface hoar. 

    The increased load from recent snowfall and wind loading has stressed the buried weak layers. Last night’s snowfall could have tipped 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

    A propagating snowpit test result on a buried layer of facets below a crust. 

Avalanche Discussion

Happy March! Although it has recently felt a lot like spring down in the valleys, upper elevations are still holding on to winter. 

Yesterday’s cold front favored the Bitterroot for snowfall, but all zones experienced strong southwest winds. The Bitterroot picked up 5 to 10 inches of new snow, while the Rattlesnake and Seeley Lake zones received 3 to 4 inches. Unfortunately, the Point 6 weather station (our best upper elevation wind data for the region) was rimed up for most of the day until the cold front passage in the evening. However, during and after the cold front, it showed strong winds out of the west. Observations yesterday confirmed this and observed significant blowing snow and wind loading at upper elevations in all zones. 

Due to warm temperatures, observers found wet snow and rollerballs up through middle elevations yesterday. This limited the wind loading and snow available for transport at middle elevations. Temperatures dropped during the cold front passage last night, and the snow should be frozen in most areas today. There may be some smaller wind slabs in the fresh snow at middle elevations, but upper elevations are the primary concern. 

SWE totals since Sunday morning (2/25/24) are as follows:

Seeley Lake (North Fork Jocko Snotel): 2.7” SWE

Rattlesnake (Stuart Snotel): 1.7” SWE

Bitterroot (Twin Lakes Snotel): 4.1” SWE

That is a significant load of recent snow, especially in the Bitterroot. We haven’t observed any avalanches on buried weak layers in the last several days, 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, while other observations continue to find propagation in snowpit tests. Either way, it is enough load to cause concern, and last night’s snowfall could have tipped the scales. If you do trigger a persistent slab, it will be large and consequential. You are more likely to trigger a wind slab today but keep in mind that wind slabs could step down to buried weak layers. 

Today’s wind slabs will be largest and most sensitive in areas that received the highest snowfall totals last night, like the central Bitterroot. Remember that each danger rating is a spectrum. Today, upper elevations in all zones are rated Considerable, but we are at the lower end of Considerable in the Rattlesnake and Seeley Lake zones and the upper end of Considerable in the Bitterroot zone. 

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

Ski and ride safe.

Regioni coperte