Bollettino valanghe

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Pericolo valanghe

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

In sintesi

Dangerous avalanche conditions will develop during the day at middle and low-elevation slopes. The threat of wet snow avalanches will increase as the day warms up. These slides are most likely on very steep slopes that directly face the sun but are possible on shaded slopes. Additionally be on the lookout for small wind drifts in very steep, upper-elevation terrain, and cornices on ridgelines. Don't linger below large avalanche paths and avoid areas with wet, soggy snow.

Problemi valanghivi

  • Loose Wet

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

    Last night was the second night of below-freezing (but not for very long) temperatures. Skies remained mostly clear overnight, which aids in a harder refreeze. The snow is melting during the day, but warmer-trending overnight temperatures are preventing the snowpack from refreezing completely at night. Warm temperatures and intense sunshine today will quickly melt the crusts that formed at the snow surface overnight. If you find yourself traveling on steep slopes with wet, slushy snow on top, triggering one of these slides will be likely. These slides will start at a point and fan out as they travel downhill and entrain additional wet snow. They may gouge deeply, creating large avalanches. Monitor the snow surface as you travel—hop off your sled or out of your skis. If you are finding wet snow it's a good idea to avoid steep slopes or go looking for colder snow to play in. Triggering one of these slides is most likely on very steep, rocky slopes. These slides are also possible on shaded slopes at lower elevations.

    Large cornices exist along many ridgelines in this zone. Warm temperatures are stressing these monsters from their perches and eventually they will tumble off ridges. This can be dangerous in its own right, but can also trigger loose snow and slab avalanches on the slopes below.

    (4/9/2023)

    Wet loose avalanches occurring ~2 PM at lower elevations despite rapid melt occurring and bare ground present; wind-loaded slopes will still have enough snow volume to move longer distances. 

    Photo: Antonioli

  • Persistent Slabs

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

    While increasing temperatures can worsen this problem initially and lead to some warming-induced slab avalanches, there are signs that it's beginning to help. With enough time, the warm temperatures help these previously weak layers to bond together and gain strength. This process will be happening most slowly on the highest and shadiest slopes. While triggering a persistent slab avalanche has become unlikely, given the rap sheet of this snowpack, this problem is still on the menu and worth consideration when entering steep terrain. March 20 and March 29 exist 1-3 feet in depth, and remain in the top meter of the snowpack. 

    ADDITIONAL IMPORTANT DISCUSSION:The most prominent weak layers in this zone (primarily the Wyoming Range) are depth hoar near the ground (buried 4-6' deep) and crusts and surface hoar that formed in March (buried 2-3' deep). Buried weak layers, warm temperatures, and weak overnight freezes are the recipe for generating wet slab avalanches. These types of slides are both unpredictable and destructive. Dealing with persistent weak layers is tricky business, particularly when trying to infer what is happening with them through freeze-thaw cycles. Snowpack test results become more difficult to interpret and obvious signs of instability are absent. The first clear sign of an unstable snowpack may occur when you trigger a large avalanche. The best way to avoid the above problems is to avoid avalanche terrain during the heat of the day and steer clear of slopes with wet or moistening snow.  

    (4/6/2023)

    Cornice-triggered avalanche in the Greys Zone on an east aspect ~9,500'.

Avalanche Discussion

The overlap of the seasonal transition and buried weak layers produces plenty of fodder for avalanche forecasters. I'll mention wet slabs here not necessarily because I think they are going to be happening today, but because they are likely to be a problem in at least one of the zones, and it is good for us all to still keep persistent layering in mind. At this seasonal stage, slab avalanches are very interesting. A personal observation I've made over time is that some slab activity at this transition never really appears fully wet or dry. Moist slabs?

On upper and middle-elevation slopes that don't directly face the sun, the snowpack is still cold and dry. At lower elevations and on all slopes that face the sun, the warm temperatures and direct sunshine of the past two weeks have begun to push meltwater down into the snowpack. When the snowpack is freezing solidly at night we like this; it behaves in a more predictable diurnal pattern of melting during the day and then refreezing at night. This pattern usually means that avalanche activity is restricted to surficial wet loose avalanches. However, when ambient air temperatures stay near freezing overnight this re-freeze is much weaker. Without a solid freeze over the span of several days, significant amounts of meltwater can accumulate in the snowpack. If you are out in the mountains today and you are finding a snowpack that is either saturated with water OR recently dry snow surfaces that are now becoming moist on shaded aspects, you will be well served the think about both the possibility of wet avalanches and the possibility of awakening any persistent issues.

As this is my last forecast for the season, I'd like to thank the community surrounding the Bridger-Teton Avalanche Center for their observations, support, and adaptation to some major changes. It has been a pleasure forecasting here and striving to build community and provide a useful product for users. Thanks and as always, keep it tight in the mountains and have a fun spring!

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