Lawinenlagebericht

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Kernpunkte

You're most likely to trigger a large avalanche today on steep, rocky, wind-affected slopes where weak layers are nearest the surface, and where fresh wind slabs sit. Weak snow close to the ground is most easily impacted in areas with a thin snowpack or by large triggers like falling cornices. Beware of cornices and watch for cracking and drifting in wind affected terrain.

Lawinenprobleme

  • Wind Slabs

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

    Wind slabs 1- 2 feet thick exist in exposed, wind-affected terrain particularly above ~9,000'. Up to 10 inches of low density snowfall over the past two days, combined with gusty winds up to 30 mph today could drift more snow into triggerable slabs. 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 wind-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.

    (3/13/2024) A wind slab on the ENE aspect of Powder Peak in the Snake River Range that likely occurred mid-storm March 2nd. There is currently alot of loose snow available to drift into similar wind slabs.

  • Deep Persistent Slabs

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

    In much of the Greys forecast zone at middle and upper elevations, weak snow from December is buried up to 5 feet deep. The layer is very stubborn to human or snowmachine triggers in most locations but it continues to produce avalanches with each round of new snow, wind-loading or cornice fall. These rapid changes in stress to the snowpack appear to be the most likely way to trigger an avalanche but don't overlook the potential to step on or ride over a thin spot which then causes a crack to propagate widely across a slope. Watch for cornices as you travel and evaluate convex slopes and thin, steep rocky, and wind-affected areas carefully. Terrain configuration and wind affect over time really matter. Check out recent observations here and here for other examples of this problem.

    The best way to manage this avalanche problem is to build in a large margin for error in your intended route. Increase your safety margin by 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 show signs of instability like cracking or collapsing. Furthermore, tracks on a slope are not conclusive evidence of stability.

    (3/12/2024)

    Cliff Creek 6700' NW aspect

    Photo: Alex Drinkard

Avalanche Discussion

We changed persistent slab problem to a deep persistent slab problem and added the lower elevations this week due to the average depth of the weak layers, it's stubborness, and high consequences of triggering this problem.  We have not had a human or snowmachine-triggered avalanche on these weak layers reported in over a month. 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.  

The following is a summary of 2 situations where the avalanche hazard is elevated:

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.

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