Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Mon
Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

Extreme winds and 20 inches of new snow have built strong slabs over weak early-season snow. Large to very large natural and human-triggered avalanches are very likely today. Travel in avalanche terrain is not recommended.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    1–3

    Ridgetop winds were out of the SSW overnight with gusts as high as 130 mph. Wind slabs will be present at all elevations and all aspects with significant snow. Today, in addition to areas near ridgelines, wind slabs can be found lower on mountainsides and in terrain features such as dense forests and low-elevation clearings. Valley winds sometimes flow perpendicular or opposite to the ridgetop wind direction causing cross-loading of terrain features such as gullies and ridges. Be aware of overhead hazards from avalanche paths in the terrain above you.

  • Persistent Slabs

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

    A persistent weak layer of faceted snow exists 2-4 feet below the snow surface. Wind slab and storm slab avalanches may step down into these older layers today. If triggered, they can propagate around terrain features that would otherwise confine new snow instabilities like storm slabs, or wind slabs creating larger-than-expected avalanches.  Build wide safety margins into your tour plan and use conservative terrain choices and careful route finding to limit your exposure.

  • Storm Slabs

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

    Heavy snowfall will continue to build storm slabs throughout lower elevations today. Watch for an "upside down" profile in the new snow, with denser more cohesive snow over lighter weaker snow. Storm slabs will be most dangerous in steep terrain where avalanches run into terrain traps such as benches, gullies, and creek beds where a deeper burial is possible. These sorts of features exist even in relatively simple terrain below treeline.

Avalanche Discussion

Uncertainty levels are high surrounding the reactivity of the persistent slab problem. During the last round of loading on December 4th wind slab avalanches in the Mammoth Lakes area stepped down into the December 1st persistent weak layer. Whumpfs reported by ski touring parties over the days following the storm indicated that human triggering was possible as well. For areas outside Mammoth Lakes where the snowpack is shallower the persistent weak layer will likely react to this new snowstorm in a similar way. Hopefully, we will have a clearer picture of our Persistent Slab problem on Monday and we can look at patterns of natural avalanche activity. Until then, it is best to give avalanche terrain a wide berth.

Regions covered