Bulletin d’avalanche

Tetons

Bridger-Teton Avalanche Center
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Danger d’avalanche

Wed
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Fresh snowfall and moderate winds are forming fresh wind drifts a foot or more thick at upper elevations. These will become larger and more dangerous as the day progresses. There's a good chance that your weight could trigger one of these slabs—avoid slopes where you find drifted or pillowed snow. Underneath these drifts, weaker snow buried deeper in the snowpack continues to pose a threat. To reduce your risk, steer clear of slopes where the wind has stiffened the snow surface.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Over the past week, the wind has continued to work our snowpack. With clear skies and warmer temperatures, the amount of snow available for transport has depleted. Throughout the day today, reinforcements will arrive in the form of new snow; due to the old, slick surfaces, slabs of this new snow could be touchy. With additional thanks to the shifting wind directions, the slabs that will form may be distributed in an irregular pattern in upper-elevation terrain. 

    Up to 10 inches of new snow may accumulate by this afternoon. This snowfall, combined with moderate northerly winds, will form fresh, soft drifts a foot or more thick in upper elevation, exposed terrain. These will be forming on a variety of slippery surfaces including icy crusts and facets, so I would expect them to be easily triggered on steep slopes. You'll want to use extra caution on—or avoid—steep slopes where you find fresh drifts.

    ADDITIONAL DISCUSSION

    Today's avalanche conditions are directly tied to the weather forecast—which has some uncertainty. If you're finding more than 8 inches of new snow has accumulated, or the winds are blowing harder than forecast, expect the wind slab problem to be more dangerous. Choose terrain wisely as any new snowfall will likely move easily on the variety of slick, old snow surfaces--even in wind-sheltered areas. 

    Small wind slab avalanche on No Name Canyon where a skier was caught and carried requiring rescue from TCSAR.

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    1.5–2.5

    Triggering a 2-4 foot thick persistent slab avalanche on weak snow deeper in the snowpack is unlikely, but not impossible. You'll do well to keep them in mind and continue to factor them into your decision-making. It would take a dose of very bad luck, but you could trigger a large avalanche in very steep (over 35 degrees), alpine terrain that is more likely to have thin spots where your weight may reach the weak layers.  The weak layers we've struggled with most of the winter have not gone away, but they've become less reactive to the weight of a skier or rider over time. Due to their depth in certain areas, they've also become very difficult to impact with your weight. 

    Triggering a persistent slab avalanche remains possible in terrain with the following characteristics:

    • Steep and Southerly: slopes in prime-time avalanche terrain around 35-45 degrees ranging E-S-W on the compass.
    • Wind-affected: heavily wind-loaded by past or recent wind events.
    • Alpine flavor: complex, windswept terrain with rock ribs, cliff bands, or convexities.

    Deep Persistent Slab: In addition to the persistent slab problem type, weak snow near the ground persists, primarily on the southern half of the compass in the middle and upper elevations. This layer is considered dormant at this time with the last activity observed on January 16th. 

    (2/11/2023)

    Teewinot 10,300' SE aspect. Snowpit where we experienced large collapse. No cracking. Great example of why to avoid thin, shallow areas. 

    Photo: Drinkard

Avalanche Discussion

In the Tetons over the past several weeks, our attention is slowly shifting from buried persistent weak layers to storm-related surface instabilities. Today's storm is looking more promising than it was a few days ago, with up to 10 inches of new snow could accumulate in these mountains by the end of the day. With the low-confidence weather forecast, this storm could produce even more (or of course, less). Moderate westerly winds shifting northerly today will easily transport this new snow into fresh, soft wind drifts on upper-elevation ridgelines and other prominent terrain features. These wind slabs will form on old snow surfaces that include anything and everything—wind-hardened surfaces, stiff drifts, sun crusts, and near-surface facets. All of these tend to be slippery, and today's fresh wind slabs will likely be touchy on steep slopes. If the storm produces on the higher end of what is forecasted—say, greater than about 8 inches—we may begin to see a storm slab problem on very slopes that are sheltered from the wind.

It's easy to get cavalier about these surface instabilities after dealing with persistent slab problems all winter. When faced with these "manageable" problems, you may find yourself in consequential terrain where even a small slide can be every bit as dangerous as a persistent slab. Remember that the avalanche danger is only one part of the equation, the other components are your level of exposure to the avalanche hazard and the consequences of triggering a slide in your terrain of choice. Often our overconfidence in managing surface instabilities leads us into the terrain where the exposure and consequences can be dangerously high.

Régions couvertes