Bollettino valanghe

Tetons

Bridger-Teton Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

The combination of recent snow, wind, and buried weak layers make human triggered avalanches possible today. Heightened avalanche conditions exist where wind affected snow lies on top of slick crusts in steep terrain, especially on the southern half of the compass. Pay attention to signs of instability such as cracking within wind drifted snow and moist snow rolling down steep slopes. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    The snow that fell between the 26th and 28th of January has settled into a 2-3 foot soft slab. This snow lies on top of weak snow and slick crusts that formed prior to the major storm event. Multiple human-triggered avalanches, some of them triggered from a distance, have been reported on this weak layer. This January 26 layer is especially pronounced on west to south to southeast facing slopes, but could be present elsewhere. The likelihood of triggering this layer has decreased over the last few days but still remains possible. 

    Your best line of defense against a persistent slab avalanche within the top 2-3 feet of the snow surface is probably to dig a pit and perform an extended column test. Monitor the effects of the sun today on the snow surface. 

    Deep Persistent Slab: A weak basal structure highlighted by the November 27 crust/facet layer is present on previously wind scoured aspects (W-S-SE). This layer is 20 to 60 cm above the ground and 70 to 130 cm down from the surface in areas that correspond to the highest likelihood of human triggering but appears to be dormant at this time. While finding the trigger point is unlikely, the resulting avalanche will be very large.

  • Wind Slabs

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

    Moderate to strong westerly and south-westerly winds have moved low density snow onto lee slopes since the storm cleared on Monday. Cold weather immediately after the storm helped to preserve these slabs which may remain sensitive to human triggering today. Slabs that are 1-2 feet thick may be hard and stubborn in some locations and softer and more sensitive in more sheltered areas. Look for cracking or hollow sounding hard slabs indicating soft and weak snow beneath.  Very steep, unsupported slopes or convex terrain features are especially suspect.

    Monitor the wind direction and look for signs of instability such as cracking within wind drifted snow. Moniter the effects of the sun today on the snow surface. 

     

Avalanche Discussion

A moisture starved shortwave passes through north-western Wyoming today.  A bigger trough shows up on the 5th (Sunday). Current avalanche hazards are primarily due to wind slabs in the alpine and persistent slab avalanches at the middle and upper elevations. Uncertainty is most prevalent in the spatial distribution of wind slabs above tree line and the reactivity of both the Jan 26 and Nov 27 persistent weak layers. 

As the days without new snow go by, the avalanche problems continue to change in character. Wind slab distribution has increased throughout the alpine but overall sensitivity has decreased. In other words, things are getting firm on many (if not most) aspects above 10000 feet. The Jan 26 PWL is changing as well. On the southern half of the compass the top of the crust is bonding well to the new snow, while the bottom of the crust has continued to facet. So it seems that the sensitivity has decreased as well while the distribution has remained the same. However, propagation potential could remain the same or increase if the failure plane moves to the facets below the crust. This has not been observed in recent avalanche activity. No recent avalanche activity has been observed on the thin layer of small near surface facets on the northern half of the compass. 

Recent avalanches have involved persistent weak layers at the upper elevations (Jan 26 layer) and wind slabs in the alpine. The November 27 layer appears to be dormant at this time. 

Current snow surfaces are composed of wind affected snow at the upper elevations, sun crusts on southerly aspects, and surface hoar and near surface facets on protected northerly aspects. Current snow depths are around 90 inches (228 cm) at upper elevation sites in the Tetons, with low elevation sites closing in on 3 feet. Snotel sites range from 90-108% median snow water equivalent in the Tetons. 

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