Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Wed
Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

Heavy loading by new snow and wind will create very dangerous avalanche conditions throughout the day. Expect the size and distribution of storm slabs and wind deposits to grow as the storm ramps up. In isolated areas, sliding snow could step down to deeper weak layers resulting in very large avalanches. It will be difficult to maintain adequate safety margins today. Traveling in and below avalanche terrain is not recommended.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2–3

    Wind-drifted snow accumulates more rapidly than direct snowfall alone and will add to cohesive slabs that already exist over weak underlying snow. New slabs will form and the problem will become more widespread throughout the day. They will be most common near and above treeline, below ridges, and on the sidewalls of gullies. Blowing snow and cornice development will point to the most dangerous slopes. Shooting cracks in dense snow are red flags. Traveling in steep leeward terrain and the runouts of avalanche paths with wind-loaded start zones is not recommended.

    (3/20/2023)

    Large wind slab avalancehs that caught riders by surprise in the Sherwins

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1–2

    Snowfall should become heavy by mid-morning and storm instabilities will become more sensitive throughout the day. Soft, cohesive slabs of new snow and sluffs fanning out from a point will be likely in steep sheltered terrain. Convexities and confined terrain features like chutes are most concerning. Terrain traps like cliffs, creek beds, or deep tree wells increase the consequences of getting caught in an avalanche. Avoid travel on or below slopes greater than about 30 degrees.

    (3/14/2023)

    Natural storm slab on a snowbank in town

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2.5–4

    Deep Persistent slab avalanches failing within older layers of snow buried in February have become a low likelihood/high consequence scenario. Recent snowpack collapses south of Bishop and in a shallow-snowpack area near Mammoth are keeping the problem in the back of our minds. Heavy loading will again test those remaining areas where the problem exists. Human-triggered deep slabs are unlikely, however, there is the potential for large wind slab avalanches to step down. If triggered, expect deep slabs to propagate widely, connect multiple start zones and terrain features, and run long distances to valley floors. Features of concern include shallow trigger points, steep complex terrain, convexities, and slopes with overhead hazards like cornices or big start zones. Uncertainty is high and avalanches could be unsurvivable. Avoidance is the best strategy, especially during periods of heavy loading.

Avalanche Discussion

The mountains were talking around Mammoth and near the Sierra Crest yesterday. We have reports of 4 large rider-triggered avalanches on north and east aspects near and below treeline in the Sherwins. In a few cases, people were caught and carried. Fortunately, there were no reported injuries. Natural avalanches also ran below ridgelines. Observers in the Knolls reported deep whumpfs from the snowpack. The activity was not limited to the Mammoth area. A forecaster in Bishop Creek remotely triggered a small wind slab as he approached a test slope. 

Another strong storm moves into the region this morning. Moderate to heavy snowfall and increasing winds will add to the size and distribution of already sensitive slabs. New slabs will form atop the most recent storm snow, near-surface facets, and melt-freeze crusts. Expect stability to deteriorate throughout the day. 

There is a high level of uncertainty over facets buried in mid-February. While snowpack tests in the Mammoth area and north since the 14th have indicated that weak snow buried in mid-February is becoming more unreactive, deep collapses where the snowpack is thinner are a warning sign that the problem may not be totally played out. On slopes far from the Sierra Crest, and in the southern half of the forecast area, avalanches stepping down to deep layers are more likely. 

This short-duration, high-intensity storm is expected to impact communities, infrastructure, and road crews. Considerate travel choices and strategic parking will be appreciated if you decide to venture out today. With good coverage at lower elevations this winter, recent avalanches have been running further, impacting roadways and buildings in the communities of Virginia lakes, Mono City, June Lake, and Aspendell. It is important to practice good run-out awareness. Triggering an avalanche above infrastructure could have devastating consequences.  Remember that it often takes multiple passes to clear roadways and trailhead parking. Road crews are working overtime to allow access to communities that have been stranded by heavy snow, avalanche activity, and flood damage. Blocking snow drifts, travel routes, and snow piles makes it impossible for road crews to continue their work.

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