Bollettino valanghe

Salt River and Wyoming Ranges

Bridger-Teton Avalanche Center
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Pericolo valanghe

Sat
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

Today it is possible to trigger a large avalanche on steep, rocky, wind-affected slopes in upper elevations. Watch for signs of instability on wind-drifted slopes from strong E winds that transported recent snow. Intense sunshine today will heat snow surfaces increasing avalanche sensitivity and making wet loose avalanches likely at middle and lower elevations. Weak snow near the ground can be impacted in areas with a thin snowpack or by large triggers such as falling cornices.

Problemi valanghivi

  • Wind Slabs

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

    Recent snowfall and moderate to strong E wind formed slabs 1-3 feet thick in exposed terrain at upper elevations. Watch for signs of wind-transported snow such as pillows, textured surfaces, stiffened surfaces, and shooting cracks. Eastern winds are atypical for this area and create tricky avalanche conditions. Keep an eye out for wind-loaded slopes in unusual areas. Ridgelines, steep exposed slopes, and cross-loaded features are the most probable terrain to find wind-loading. In the neighboring Tetons forecast zone, a skier was caught and carried by a 3-foot wind slab. If you find signs of wind-loading, move to more wind-sheltered terrain 

    Cornices are large. As temperatures increase throughout the day they will become easier to trigger. Cornices can break unexpectedly and often further back than expected, even onto flat ridgelines. Cornice falls act as very large triggers, and are the most likely way to trigger a deep persistent slab avalanche. Limit your exposure to ridgelines and slopes below cornices. 

    (3/14/2024)

    Cornice fall was the likely trigger for this wind slab avalanche on an east aspect at 9800' in Hunter Creek. 

    Photo: Cameron Galt

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–1.5

    Mostly sunny skies and warm temperatures will heat recent snowfall resulting in wet loose avalanches. Triggering one of these slides is most likely on steep terrain facing the sun. Monitor the snow surface as you travel. Look for roller balls, pinwheels, and wet snow (easy to make a snowball). While these avalanches will be small, they can easily push you off your feet making them dangerous in consequential terrain. The onset of wet avalanche activity can be sudden. Pay attention to signs of warming and minimize time spent lingering on or under steep, sunny slopes. 

    (3/10/2024)

    Natural wet snow activity up Wolf Creek in the Snake River Canyon at low to middle elevations. 

    Photo: Eric Taft

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    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 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 the 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 stubbornness, and the 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 a Large (Mountain Range) Scale and a 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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