Bulletin d’avalanche

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Danger d’avalanche

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

Points saillants

It remains possible to trigger a large avalanche on steep, exposed wind-affected slopes. Weak snow near the base of the snowpack is more easily impacted in areas with a thin snowpack or by large triggers like falling cornices. Identify cornices looming above steep slopes as you travel and watch for cracking and drifting in upper-elevation terrain.

Problèmes avalancheux

  • Wind Slabs

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

    Wind slabs up to 2 feet thick exist at upper elevations in exposed, wind-affected terrain.  With up to 5 inches of snow yesterday and a couple more inches of snow today, combined with gusty winds up to 30 mph, wind slabs may continue to grow. Watch for wind transport, drifting, and stiff wind-affected areas around exposed terrain. Watch for shooting cracks coming from your skis or sled. If you find any of these red flags, move to more sheltered terrain.

    Cornices have grown large. Minimize time spent on slopes with cornices looming overhead. Cornices can break unexpectedly and tend to crack further back along the ridge than you might expect. Warming temperatures increase the likelihood of cornice failures. Cornice falls act as very large triggers and are the most likely way to trigger a deep persistent slab avalanche.

    (3/8/2024)

    Evidence of cornice fall triggering an avalanche near Sheep Pass.

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    The deep persistent slab problem exists on all aspects at upper elevations and west through north through east aspects at middle elevations. In much of the Greys forecast zone at middle and upper elevations, weak snow from December is buried up to 5+ feet. This weak snow is sitting under a stiff slab and is stubborn to human and snowmachine triggers in most locations (see below). Cornices are the most likely way to trigger an avalanche. This is a low-likelihood and high-consequence problem, meaning the chances of triggering an avalanche on this weak layer are low but if triggered, it will be a large and destructive avalanche. Watch for cornices as you travel and evaluate thin, steep rocky, and wind-affected areas carefully.

    (3/7/2024)

    Large D2.5 persistent slab avalanche at 9500' NE aspect in the Snake River Range that likely failed 3/2 during rapid loading. 

    This is the specifically the type of terrain where the hazard is highest: Steep, rocky and wind-affected slopes with looming cornices (large triggers). 

    Photo: Alex Drinkard

Avalanche Discussion

We have transitioned the persistent slab problem to a deep persistent slab problem this week due to the average depth of the weak layers and the stubborn nature of this problem. We have not had a human or snowmachine-triggered avalanche fail on these weak layers in quite some time. However, we have had very large avalanches resulting from cornice falls or possibly heavy wind loading (large trigger) in the last week and a half. 

There are 2 situations where the avalanche hazard is elevated that should be on your radar as you travel:

1. Thin Areas in the Snowpack.  Weak layers closer to the surface are more easily impacted in thin areas. This consideration can be further split into Large (Mountain Range) Scale and Small (Slope) Scale. 

  • Large (Mountain Range) Scale. The Greys forecast zone includes several mountain ranges and the snowpack varies throughout. In general, the Salt River and Snake River Ranges have a deeper snowpack than the Wyoming range. This is a huge generalization and the only way to verify the depth is to dig or probe. As a rule, if you probe less than 6 ft to the ground, the snowpack should be considered thin.
  • Small (Slope) Scale. Steep, rocky, and wind-affected slopes should be evaluated carefully due to a higher likelihood of impacting weak layers closer to the surface. Unsupported slopes above cliffs should be considered suspect. If identifiable, slab margins (edges of start zones for example) are also generally thinner.

2. Large Triggers

  • Cornices. We have evidence that this layer is still reactive to very large triggers as recently as March 3rd, or sooner. In terrain where cornices loom over steep, rocky, slopes on the north half of the compass, the danger (likelihood) is elevated. Cornices can break unexpectedly.
  • Rapid loading. While there are no weather events on the horizon that constitute a large loading event, keeping track of incoming storms will be prudent as long as there is a deep slab problem.
  • Smaller Avalanches (wind Slabs) could have the potential to create enough impact to collapse deeper weak layers.
  • Warming. Warming temperatures increase the sensitivity of slabs and also the likelihood of cornice fractures. With enough warming, water begins to flow in the snowpack creating further instability. As we get further into spring, warming will be important to consider.

 

The best way to manage this problem is to build in a large margin for error in your decision-making around avalanche terrain. This includes traveling where the snowpack is deepest, identifying the presence/depth of the weak layer (probing or digging), and avoiding thin, steep, rocky and wind-affected areas. This problem can be hard to identify and will not have signs of instability. Tracks on a slope may indicate increased stability but they are not conclusive evidence. When in doubt, or if uncertainty is high, choose more conservative terrain.

Régions couvertes