Avalanche forecast

Tetons

Bridger-Teton Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

The combination of recent snow, wind, and buried weak layers make human triggered avalanches possible today. Cracking within wind drifted snow and audible collapsing of the snowpack are signs of instability. Carefully assess the snowpack before entering steep, rocky terrain, especially below large cliff bands. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Persistent slabs that are 2-4 feet thick exist at the upper and middle elevations. These slabs sit on top of weak snow that was buried dating back to the end of November. No avalanches on the November 27th layer have been observed with the most recent loading event, but layers of less developed facets exist closer to the surface of the snow.  The November 27 layer is now buried over a meter deep at the upper elevations and is certainly stubborn if not unreactive. Southerly and Easterly aspects at the upper elevations seem to have the best slab/weak layer combinations for avalanches to occur. Areas where wind slabs sit on top of persistent weak layers are of primary concern, especially in and around large cliff bands. 

    Persistent slab avalanches at the lower elevations are unlikely at this point and limited to isolated areas. Steep north facing terrain with lots of recent wind loading are an example of these isolated areas.  

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Wind slabs that are 1-2 feet thick exist at the upper elevations. With low temperatures dipping down towards -20 degrees Fahrenheit over the last few nights, these slabs should be on the stubborn to unreactive end of the spectrum. Extreme winds associated with the most recent storm created unusual distribution patterns, with lots of snow ending up well below traditional starting zones. 

Avalanche Discussion

Yesterday was mostly sunny with frigid temperatures in the Tetons. Winds were relatively modest and westerly around 10,000 ft, but definitely had more bite and a northerly component at elevations above 11,000 feet. Around an inch of settlement occured in the Tetons yesterday, and clouds were on the increase throughout the night as pacific moisture approached the forecast area. 

Reactivity of recently formed wind slabs is expected to be minimal due to the influence of the Arctic air mass. The November 27th layer appears to be largely dormant at this time with no avalanches observed on this interface from the most recent loading event. Other persistent weak layers exist in the snowpack above the Nov. 27 layer, but they are less well developed and their distribution patterns remain uncertain. The extreme winds from December 20/21 caused abnormal loading patterns, with much of the snow ending up downslope from traditional start zones. Aprons below complexes of large cliff bands are of primary concern, as are mid slope areas on south and southeast aspects that may have been cross loaded.   

The "Bomb Cyclone" from December 20/21 dropped around 12-15 inches of snow and a little over 1 inch of SWE at the upper elevations in the Tetons. Fierce winds associated with frontal passage on the morning of the 21st caused a natural avalanche cycle. Natural avalanches were observed in the side country surrounding both Jackson Hole Mountain Resort and Grand Targhee Resort. Naturals were also reported in Grand Teton National Park and the Teton Pass area. Many of these slides seemed to have decent propagation but likely failed on interfaces above the November 27th layer. Most of these avalanches appeared to be size 2 avalanches (~2 ft deep) on south and east aspects. 

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