Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Many avalanches have occurred in the last 48 hours in our forecast region.  The recent storm snow overlies previous weak sugary surface snow that has led to large avalanches occurring.  Strong winds will continue to create slabs of wind blown snow in open areas.  Active loading of new snow and wind will occur today with avalanches remaining likely.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2.5

    Weak near surface facets formed during the multi-week dry period since the Thanksgiving holiday on northerly aspects at all elevations.  The recent Thursday-Saturday storm cycle deposited 2 to 4 feet of dense snow on top of this weak faceted snow surface.  Multiple large avalanches have been reported, many of these avalanches occurred naturally sometime on Saturday at the tail end of the storm cycle.  A snowmobile triggered avalanche occurred  yesterday in the Mt. Rose area that is thought to be a persistent slab avalanche.  Many other reports of collapsing and whumpfing sounds have also been submitted, further pointing to this weak layer remaining reactive.  This persistent slab avalanche problem is expected to continue, with more avalanches occurring.  

    In more isolated areas in the southern and eastern portions of the forecast region, basal facets on the ground could be reactivated by the recent storm snow.  These basal facets exist on NW-N-NE aspects above around 8000-8500', where a shallow and weaker snowpack structure formed.  No recent avalanche activity has been reported on this layer in the backcountry at this point. 

    Recent avalanches, snowpack cracking, collapsing, or whumpfing sounds will indicate that this avalanche problem is reactive in your area.  These avalanches could be large, with wide propagation, and could be remotely triggered from a distance away.

    (12/15/2024)

    80’ wide 18” to 30” crown.  Snowmobile triggered avalanche on Sunday, Dec 15.  Relay Peak, Mt. Rose area.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    South-Southwest winds continued throughout yesterday in the strong range actively transporting snow in near and above treeline areas.  SW winds have increased overnight into today and will remain gale force near 100mph along with some additional new snowfall for wind transport.  Wind slabs will continue to build today and will remain reactive.  

    Look for recent avalanche activity, blowing snow, cornice formation, wind pillow development, and snow surface clues as to where these wind slabs are forming.  Large cornices have formed along many ridges and could remain fragile again today.  Move around corniced areas and steep wind loaded slopes with caution.     

    (12/15/2024)

    An overview showing six different wind slab avalanches along the Castle-Basin Ridge, Donner Summit area.

    Photo: Sierra Avalanche Center

Avalanche Discussion

A persistent slab avalanche cycle started to occur at the end of the Thursday-Saturday storm that dropped 2 to 4 feet of snow.  Several reports have been received of large persistent slab avalanches with wide propagation that occured naturally.  A report of a snowmobile triggered avalanche yesterday in the Mt. Rose area is also suspected to have failed on the near surface facet layer.  Many reports of collapsing and whumpfing sounds also came in, further pointing to this weak layer remaining active. The Dec 12 persistent weak layer is made up of near surface facets, but varies greatly in strength and location throughout the forecast region.  This weak layer will gain strength at different rates from area to area, and will take time to fully adjust to the recent large storm load.  Where this layer is well developed, avalanche activity is expected to continue.

A large wind slab avalanche cycle also occurred at the tail end of the recent storm cycle.  Some of these wind slab avalanches were large and potentially involved the Dec 12 weak layer.

Regioni coperte