Lawinenlagebericht

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Lawinengefahr

Sat
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Look for and steer clear of drifted, wind affected snow, primarily above 9,000', especially as temperatures rise in the sun today. Slabs may be up to two feet thick in isolated locations; large enough to carry or bury a person in the wrong terrain. A lingering persistent slab problem also exists which in rare circumstances could lead to larger human-triggered avalanches and thicker slabs. Evaluate snow and terrain for signs of wind loading and avoid sunny slopes this afternoon. A lingering persistent slab problem also exists and could lead to larger human-triggered avalanches and thicker slabs. Evaluate snow and terrain for both of these avalanche problem types if venturing into steep terrain today. 

A skier triggered avalanche occurred yesterday on buried surface hoar in a sheltered ENE facing location in the Upper Hobacks with a larger natural avalanche observed in a nearby location. This persistent layer is very sensitive but hard to find in the terrain. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    You may find drifted snow ranging from soft and reactive to firm and stubborn in wind exposed areas at upper elevations that formed during Wednesday's snowfall. These wind slabs have had 36 hours to heal and are not very widespread through the terrain. Today as temperatures rise on sunny slopes wind and storm snow 6-12" deep will become more sensitive to triggering. 

    Look for signs of sun and wind affect on the surface. Shooting cracks or drum-like sounds are an indication that you've found a wind slab. Roller balls, wet loose sluffs from cliff faces and moistening snow surfaces are clues that surface snow may be getting less stable.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    What's the problem? The snowpack contains several layers of crusts with nearby weak faceted snow and buried near surface facets  and in widely scattered, sheltered locations, buried surface hoar 40-60 cm down from the surface. The  weak layer that seems most likely to be triggered, is a thick layer of near surface facets from a dry spell prior to the Jan 26-28 storm. The layer shows a tendency to propagate a crack and while there haven't been reported avalanches on this layer for over a week, we aren't out of the woods yet. Small amounts of new snow, such as we received yesterday, added more strain to the layer.  The buried surface hoar was responsible for one skier triggered avalanche  and one observed D2 natural avalanche in the Upper Hoback range in the Greys River forecast zone both of which occurred 2/8-9. 

    Where: Shallower areas in the snowpack with a slab over weak snow are the main concern.  Steep, rocky slopes peppered with rocks and signs of wind effect are locations to watch as well as solar aspects with thicker crusts which have more, and weaker facets. Alpine terrain exposed to wind scouring and drifting creates a thinner snowpack ripe for the development of facets and should definitely be assessed carefully. You may find buried surface hoar in very well sheltered shady aspects, typically in drainages.

    What to do about it? When in doubt probe and dig or simply find someplace else to ride. Some of the weak snow grains are tricky to identify visually in pits but small, loose and angular grains that aren't well bonded to each other or the adjacent crust are what you are looking for. Triggering a persistent slab avalanche is unlikely but not impossible so choose terrain carefully.

    Deep Persistent Slab: In addition to persistent slab problem type, a  deeper weak layer persists in middle and upper elevations on southeast through west aspects. These layers appear to be dormant but could awaken with a big storm or large load, such as another avalanche. The resulting avalanche could be up to five feet thick and carry severe consequences. 

    (2/9/2023)

    This avalanche was triggered by the fifth skier on the 35 degree, ENE facing slope on a layer of buried surface 40-60 cm down from the surface. The skier was not caught.

Avalanche Discussion

The buried surface hoar in the Upper Hobacks adds some uncertainty when thinking about current conditons and terrain selection. I expanded the aspects for the persistent slab avalanche problem to all aspects though the two recently reported avalanches occurred on ENE aspects. Many other areas in the Greys River zone, particualry windier locations or places with a thicker snowpack may not have this concern but it is well worth it to consider the consequences of triggering this layer. The fifth skier on the 35 degree slope triggered the 200' wide avalanche on buried, 6mm surface hoar which was buried 40-60 cm deep.  

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