Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

MODERATE avalanche danger exists on N aspects in all terrain above 8000 ft. and on NW-N-NE-E-SE aspects in near and above treeline terrain on slopes steeper than 30 degrees due to a combination of wind slabs and persistent slabs. Large persistent slab avalanches are possible on true N aspects in the northern portion of the forecast area.

 

Lawinenprobleme

  • Deep Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2

    A layer of weak sugary snow (facets) exists below the recent snow on N aspects above 8300 ft. on the Sierra Crest and above 9000 ft. in the Mt. Rose area. In places where these facets have enough snow above them to form a slab, snowpit tests have indicated that if the facets break the resulting fracture could propagate along the weak layer and result in a persistent slab avalanche. So far tests have yielded unstable results on N aspects along the Sierra Crest north of Tahoe City especially in the Castle Peak area and in the Mount Rose area. Where this weak layer exists it crosses over sub-ridges, spines, and other features connecting multiple avalanche paths and start zones together. Given the persistent nature and depth of the weak layer, any avalanches that occur could be wide, large, and destructive. This avalanche problem could exist on slopes 30 degrees and steeper.

    Unfortunately, the areas most likely to hold this persistent avalanche problem also represent the areas with the most snow coverage and many informal observations will not prove reliable to determine which slopes are safe to travel on. Often actual avalanches represent the only clue other than snowpit data that persistent slab avalanche problems exist on a slope. These kinds of avalanches can occur after several people have already used a slope so previous tracks do not provide good clues to stability or instability. Use your shovel and perform snowpit work to determine if this avalanche problem/weak layer exists in the particular area of interest or go the easy route and avoid N aspects that meet the location and elevation criteria for the presence of this avalanche problem. Keep in mind that this is the type of avalanche problem that could be triggered from low on the slope or even remotely from low angle connected terrain.

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    As the winds increase today, they could transport additional snow onto leeward slopes and add to the previously existing wind slab or create some new fragile wind slabs on the near and above treeline NW-N-NE-E-SE aspects. Some of these wind slabs could grow large enough that if they do break the resulting avalanche could involve enough snow to bury a person. This avalanche problem could exist on slopes 35 degrees and steeper.

    Avoid areas of recently drifted snow by identifying cornice features and wind pillows. Avoid venturing into steep terrain in areas where human triggered snow surface cracking is observed around the edges of wind slabs.

Avalanche Discussion

While we did not receive any observations from Thanksgiving, a party did report a small avalanche near Grouse Rocks (South of Alpine Meadows) on a NE aspect at ~8000 ft. on Wednesday morning. This avalanche released in a rocky area a short distance away from the party that triggered it. While the avalanche only measured about 30 ft. wide and less than 1 ft. deep, it ran downslope for approximately 300 ft. over some rocks. This party also reported some wind slab formation near the summit of Grouse Rocks on Wednesday morning. Another party on Andesite Ridge (Donner Summit) on Wednesday, found small wind slabs in wind loaded NE-E facing terrain near the summit of Andesite. They reported some cracking of these wind slabs while skinning. Across the Lake on Relay Peak (Mt. Rose backcountry), observations indicated that a layer of weak sugary snow (facets) exists underneath the recent snow. This layer was buried on Nov. 16 and has continued to weaken since then. Snowpit tests on a N aspect at ~ 9600 ft. on Relay Peak showed that if this layer breaks the resulting fracture could travel along this persistent weak layer. Snowpit data and observations in this area looking for the surface hoar layer that existed on the snowpack on Tuesday 11/22 did not find any signs of instability associated with a possible buried surface hoar layer, but they were inconclusive as to whether or not a buried surface hoar layer existed. 

The usable snowpack within the forecast area exists along the northern portion of the Sierra Crest. Areas above 7,000' and north of Tahoe City have sufficient amounts of supportable snow for quality over snow travel (2 to 4 feet). Along the Sierra Crest south of Tahoe City, snowpack depths decrease dramatically, even at the highest elevations. In these areas the snowpack is unusable for skiing/boarding/snowmobiling. The Mount Rose area in the NE portion of the forecast area has decent snowcover (~2 feet) in areas that have held snow/ice since mid Oct. This is generally N aspects above 9,300'. Below 9,300' on N aspects or on all other aspects in the Mount Rose area, over snow travel is possible, but impact with rocks is likely with snow depths around 1 foot.

Due to the wide variances in snowcover between the northern and southern portions of the forecast area, this advisory is heavily biased towards the northern areas where the snowpack is usable for over snow travel.

The avalanche center is still working on a limited number of observations so please submit observations through the "submit observations" buttons found throughout this website.

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