Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Areas of MODERATE avalanche danger exist on all aspects at all elevations on slopes 35 degrees and steeper due to a combination of wind slab, persistent slab, and loose wet avalanche problems. Small to large human triggered avalanches remain possible today. Don't loose sight of how human factors may positively or negatively affect decision making in avalanche terrain.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    With the shift and speed increase of ridgetop winds from the SW to NE, and fresh round of wind slab formation is expected to have occurred last night. With light winds during nearly all of the snowfall event, significant amounts of new snow were available for transport and redistribution in near and above treeline areas. New wind slabs will exist today on SE-S-SW-W-NW aspects near and above treeline. The strongest NE winds are expected to focus along the Sierra Crest. New wind slab development may be minimal along the east side of Lake Tahoe.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    3

    Snowpit data collected yesterday indicated that the slab of storm snow deposited on top of near surface facets lacked sufficient cohesion to show instability issues. As the new snow settles and gains strength, it is possible that the area of greatest relative weakness within the snowpack may shift down to the base of the storm snow. In near treeline and below treeline areas above 8,000' on NW-N-NE aspects where near surface facets have been observed at the old/new snow interface, this may actually lead to a decrease in stability over the coming days as compared to what was observed yesterday. This also means that previous tracks on a slope from earlier in the storm cycle will have no bearing on current slope stability. Handpits will easily detect near surface facets at the base of the storm snow. Snowpit tests will indicate if/when this layer interface is becoming problematic. Glossary of terms

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    1

    As air temperatures approach freezing and the increasingly strong solar input from the early March sun shines through breaks in cloud cover, small areas of loose wet instability are expected to occur. Roller balls and pinwheels are expected as the vast majority of activity as snow falls off of rocks and trees. In an isolated sun exposed area, an actual loose wet avalanche may occur on a slope steeper than 37 degrees.

Avalanche Discussion

Small natural and skier triggered avalanches occurred yesterday near Grouse Rocks (Ward Canyon area) and on the East face of Mt. Judah (Donner Summit area). In all cases, avalanche activity occurred on N-NE-E aspects both above and below treeline on slopes steeper than 35 degrees. The snowpack was noted as failing on a subtle change in density within the storm snow. All crown heights were observed at less than 1 foot.

Snowpit data collected on Elephant's Hump (Carson Pass area), Grouse Rocks (Ward Canyon area), and Andesite Ridge (Donner Summit area) indicated that the overlying slab of storm snow was very soft, had low cohesion, and was therefore difficult to test using compression based techniques. Bonding of new snow to old snow surfaces of crust or near surface facets was moderate in strength. A number of very small loose dry skier triggered avalanches were observed and reported from around the forecast area with snowpack failure occurring within the storm snow. Wind slab formation was minimal yesterday with one 20' long skier triggered shooting crack observed yesterday afternoon near Grouse Rocks in a lee area along the summit ridge. Some concern exists that as the storm snow settles and gains strength, failure on near surface facets at the base of the storm snow will become more of an issue and avalanche size will increase. Glossary of terms

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