Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Thu
Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

Copious amounts of loose surface snow and continued wind loading have created very dangerous avalanche conditions. A buried persistent weak layer has added complexity to a dangerous situation. It will be difficult to maintain adequate safety margins today. Travel in, near, and below avalanche terrain is not recommended.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1.5–3

    While continued snowfall is expected to taper off throughout the day, strong and shifting winds will continue to load leeward terrain features on all aspects and at all elevations. Wind-transported snow can accumulate and overload the snowpack much more quickly than storm snow alone.  While the largest and most concerning deposits exist near and above treeline on northerly and easterly aspects, extreme winds can distribute snow in unexpected ways. Fresh wind deposits will be possible to find at lower elevations and further down slopes than you may typically expect.  It will be important to monitor for continued wind transport and be prepared for deteriorating conditions on solar aspects as winds shift to the north this afternoon. With good coverage at lower elevations, very large avalanches could run down to valley floors and into lower-angle below treeline terrain.  It's important to be wary of overhead hazards and avoid the runouts of large avalanche paths today.

    (2/28/2023)

    Large to very large avalanches on Laurel Mountain

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2.5–3.5

    Over 100" of new snow and over 9” of water equivalent have been recorded in the Mammoth area over the past 10 days. While the sheer weight of our new load may have been enough to overwhelm buried weak layers in some areas, in others, things may be hanging in the balance. The weight of a rider may be all that is needed to trigger a large and destructive avalanche. Remember that persistent slab avalanches often propagate across and beyond terrain features that would otherwise confine surface instabilities such as wind or storm slabs and persistent slab avalanches can be remotely triggered from adjacent slopes. Persistent slab problems require extensive evaluation and deliberate terrain choice. The best management strategy is to put a wide margin between yourself and the terrain where the problem may exist. When in doubt avoid being on or underneath slopes greater than 30° in steepness.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Storm instabilities, both slabs, and sluffs are expected to settle relatively quickly as precip rates decrease today. However, our recent snow accumulations are large and it will still be possible to see surface instabilities in sheltered areas near and below treeline.  Steep and sheltered terrain features over 30° will be the potential problem areas today. Be cognizant of your exposure to terrain features that increase the consequences of avalanches. This includes road cuts, gully features, cliff bands, and mid-slope depressions.

Avalanche Discussion

This week’s storm event continues to be extraordinary. Heavy snowfall and strong winds have overwhelmed emergency services, leading to power outages, travel difficulties, and evacuation orders. Many large and very large avalanches have been recorded during this cycle including several that have impacted highway 395. This loading event has not finished yet, however as the skies begin to clear this afternoon it will be important to take stock. Snow events like this one, especially when paired with an active persistent weak layer, are notorious for producing avalanches that surprise even professionals. Usual "safe routes" should be carefully re-evaluated for exposure to overhead hazards. Better than usual snow coverage at lower elevations, heavy loading rates, and buried persistent weak layers make it possible for avalanches to exceed “typical” max run-outs.  

In addition,  there may be unexpected areas with increased avalanche hazards today. Large drifts on exposed roof lines and large and steep snowbanks in town are areas to keep an eye on. Furthermore, very deep and unconsolidated snow will make for difficult travel conditions and has increased the risk of deep snow immersion. Travel in pairs if you venture outside today. Be mindful of unusual hazards and keep an eye on pets and small children.

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