Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

MODERATE avalanche danger continues at all elevations on NW-N-NE-E-SE aspects on slopes 32 degrees and steeper due to a combination of persistent slabs and storm slabs. Human triggered avalanches remain a near daily occurrence within the forecast area. Identify the necessary differences between managing wind slab and persistent slab avalanche problems and adjust terrain selection and travel practices accordingly. Increasing avalanche danger is expected tonight into tomorrow morning.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    Unintentional human triggered avalanche activity continues to be a near daily occurrence in areas where persistent slabs exist. Yesterday's reported avalanche activity occurred on Andesite Ridge (Donner Summit area)(more info below in recent observations). Buried surface hoar is a persistent weak layer that remains unstable for prolonged periods of time. Open areas near treeline and below treeline on N-NE-E aspects hold the greatest concentration of buried surface hoar with 1 to 3 layers buried 2 to 3 feet deep. NW and SE aspects near treeline and below treeline hold isolated areas of buried surface hoar.

    There are 8 different types of avalanche problems. Wind slabs are the common avalanche problem in the Sierra. Instability usually last for hours to days and wind slab location is very predictable. Persistent slabs focused on buried surface hoar layers are an entirely different problem with very different characteristics and predictability in terms of location and length of unstable time period. Travel in the backcountry requires a different approach when managing persistent slabs as opposed to managing wind slabs. With persistent slabs, anticipate the potential to trigger avalanches low on slopes, after slopes have been traveled by others, and in lower angle terrain in the 30 to 35 degree slope angle range. Communicate with your partners and adjust terrain selection and travel practices in accordance with the avalanche problem, not just based on the overall danger rating.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2

    A new round of wind slab formation will begin to occur late today following the onset of snowfall. Above treeline areas will be the first affected with wind slabs forming on top loaded N-NE-E aspects and cross loaded NW and SE aspects. These wind slabs will increase in size tonight and tomorrow morning, becoming large enough to bury or injure a person.

Avalanche Discussion

A report was received of an avalanche incident yesterday on Andesite Ridge (Donner Summit area) where two individuals were involved in an avalanche on a NE aspect slope near treeline around 7,600'. One individual escaped off the side of the slab while the other was able to grab a tree. Multiple layers of buried surface hoar are known to exist in this area.

Observations made and received yesterday from Donner Peak (Donner Pass area), Becker Ridge (Echo Summit area),Waterhouse Peak (Luther Pass area), and from the Bear Valley area all indicated that rain had occurred as high as 8,000' to 8,500'. In some areas rain changed to either mixed precipitation or graupel with up to an inch of accumulation.

Skier triggered collapses were observed yesterday in the Donner Peak area. In some cases the areas where collapse was observed had already be traveled by 7 to 10 people. Continued unstable snowpit test results were noted in the Donner Peak and Waterhouse Peak areas. Obvious instability remains in the Andesite Ridge area, with some parties reporting mix results from snowpit data in this area - A direct indication that where snowpits are placed matters greatly.

Second and third hand accounts of avalanche accidents not reported to the avalanche center continue to reach the ears of the forecasters. Please submit your observations so that facts and rumors remain separate. Without the documented facts, we must treat these reports as rumors. What we must classify as rumors we do not post in an effort to avoid spreading misinformation.

Regioni coperte