Avalanche forecast

Eastside Region

Eastern Sierra Avalanche Center
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Avalanche danger

Fri
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

You're most likely to trigger an avalanche where recently deposited slabs sit on steep wind-drifted terrain. To avoid loose wet sluffs, move away from sunny slopes as the surface thaws. Shady, sheltered terrain without wind-loaded slopes above will have softer snow and be a safer alternative.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2.5

    Intense wind loading built soft and sensitive slabs in open terrain at all elevations through yesterday. You may find rippled, dimpled drifts can still crack around you on steep leeward slopes. These will be bigger, and more consequential, the higher you climb. Hollow-sounding drifts, shooting cracks through stiff or textured surface snow, and the snowpack collapsing under your weight are warnings that you have found unstable wind slabs. Assess recently drifted snow carefully before committing to steep terrain. Steep start zones and the sidewalls of couloirs and gullies are the most suspect areas where triggering a dangerous wind slab avalanche is still possible. Shady, sheltered terrain will likely hold softer snow and a safer alternative. 

    Intentionally triggered D1 wind slab avalanche. East aspect, near treeline terrain 8775 ft, Mammoth area. 18 inches thick soft slab broke 50 ft. wide and ran 40 ft. The slab broke on Fist-hard precipitation particles.

    Photo: Clancy Nelson

  • Loose Wet

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

    Light winds will do little to cool soft snow on slopes receiving intense sunshine today. Temperatures may rise above freezing to about 9500 feet. Slopes with shallow snow cover and areas with rocky outcrops or exposed vegetation will warm first and the most. Watch for rollerballs or small sluffs under cliffs and trees as signs that bigger avalanches may be coming. You can limit your exposure to yucky loose wet avalanches by moving away from - and out from under - steep solar aspects as the surface snow gets wet.

    Dense snow baking in the sun below treeline sent short cracks from my skis and triggered a few small rollerballs.

    Photo: Clancy Nelson

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1.5–3

    In areas that have held snow since December, persistent weak layers still exist at the base of the snowpack. There's also a crust/facet combo buried 1 to 2 feet below the surface that has recently shown signs of instability on isolated slopes. Over the past month, the snowpack has gained strength, but in areas with a shallower snowpack, the weight of recently drifted snow or a wind slab avalanche may be enough to overload these weak layers. Persistent slab avalanches can propagate beyond terrain features and connect multiple start zones, making for larger and more destructive avalanches that are difficult to avoid. Areas with deeper snow cover and the variability across terrain with shallow snow cover have made the problem more isolated, stubborn to trigger, and have also increased uncertainty. The safest way to limit your exposure is to stick to terrain with low slope angles and few overhead hazards. 

    (2/20/2024)

    Shooting cracks that broke through a soft slab down to facets above a thin wind crust.

    Photo: Clancy Nelson

Avalanche Discussion

Many natural and human–triggered wind slab and loose dry avalanches were reported yesterday between McGee Creek and June Lake. A good portion of these released overnight or early morning 2/20-2/21. Forecasters and ski patrollers found fresh wind slabs reactive throughout the day as southwest winds drifted low-density snow onto leeward terrain. Very cold temperatures overnight may have slowed the bonding between recently formed slabs and the underlying snow.

Up to 10 inches of soft snow fell overnight on 2/20, and even after lots of wind transport yesterday morning, observers found moderate to large amounts of fluff still sitting on sunny slopes. Solar aspects below treeline became dense and heavy, and then crusty yesterday in the intense sunshine, though no wet loose avalanches were reported. Near and above treeline, where not wind-scoured, solar aspects may hold soft snow sitting atop wind board and melt-freeze crusts.

Under the recently drifted storm snow, the snowpack varies across the region. In the central parts of the forecast area, between Mammoth and June, the mid-snowpack is generally deep and strong. A layer of facets and depth hoar near the ground has been showing improvements, especially in the areas with deep snow cover. The northern and southern reaches of the forecast area have shallower, more diverse snowpacks. These layers are still closer to the surface from Convict Creek to Bishop where snowpack tests have demonstrated that they can still propagate a failure on isolated slopes.

Another notable snowpack layer exists mid-pack across the region. Light snowfall on Valentine’s Day buried several crusts. That snow has since faceted and become weak. Forecasters have begun tracking this interface because localized collapses and snowpack tests are hinting at its potential for instability. It has been most notable in Bishop Creek, but it exists in other drainages as well, though with few signs of obvious feedback. You can identify it on sunny slopes as a melt-freeze crust, and below the northerly sides of ridgelines as a wind crust. Look for weak sugary snow above and below it and factor the small possibility of triggering an avalanche on that layer into your terrain choices. 

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