Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
Auf der Karte ansehen

Lawinengefahr

Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
Open full map
Veraltet — dieser Lagebericht ist 10 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

MODERATE avalanche danger exists on N aspects steeper than 30 degrees in all terrain above 8000 ft. due to possible persistent slabs. MODERATE avalanche danger also exists on NW-N-NE-E-SE aspects in near and above treeline terrain on slopes steeper than 35 degrees due wind slabs. Large persistent slab avalanches are possible on true N aspects in the northern portion of the forecast area.

Lawinenprobleme

  • Wind Slabs

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

    Wind slabs that formed yesterday may remain sensitive to human triggering today. These stiff hard wind slabs may feel firm on the surface and sound hollow when traveled on. In addition to breaking when weighted at the top, these wind slabs can break after one has committed to the slope or when a person reaches the edge of the slab where a thinner slab exists. Most of these wind slabs should remain relatively small and not extend too far downslope from where they form, but even a small avalanche in these low tide conditions would have serious consequences. Some of these wind slabs could have grown large enough to bury a person in the most heavily wind loaded areas. Despite the fact that yesterday's winds already moved most of the snow available for transport, today's S and SW winds could continue to add to these wind slabs. Wind slabs will most likely exist on the near and above treeline NW-N-NE-E-SE aspects near ridgelines. Some wind slabs may exist in unusual places as the S winds take over today. This avalanche problem could exist on slopes 35 degrees and steeper. As more snow and wind impact the forecast area tonight wind slabs will become larger and more widespread.

    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.

  • Deep Persistent Slabs

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

    While triggering a persistent slab avalanche may have become slightly more difficult for today, a persistent weak layer (PWL) still 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 this PWL has enough snow above it to form a slab, snowpit tests have indicated that if the PWL does break the resulting fracture could propagate along the weak layer and cause a persistent slab avalanche. 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. As more snow loads the snowpack tonight, these persistent slabs may become easier to trigger.

    Unfortunately, the areas most likely to hold this persistent avalanche problem also represent the areas with the most snow coverage. 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. Dig into the snowpack to see if this layer exists 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.

Avalanche Discussion

Yesterday observations on Castle Peak revealed significant wind transport of the recent snow. Several hard wind slabs existed on the leeward aspects near ridgelines, and large cornices had formed above some of the more exposed N-NE aspects. Ski kicks on wind-loaded test slopes caused some shooting cracks and one wind slab failure that measured about 8 to 12 inches at its deepest and extended downslope for about 10 to 15 ft. Possible debris from what may have been a wind slab avalanche also existed in one of the NE facing gullies on the north side of the Peak. Due to conditions, confirming any details on this possible avalanche proved unrealistic so a great deal of uncertainty exists about this.

Snowpit data showed that the 11/16 facet layer still exists on N aspects above 8300 ft. in the Castle Peak area. Tests targeting this layer yesterday indicated that it may have become more difficult to break in some areas, but if it does break the resulting fracture could easily travel along that layer. Snowpits on more E and NE aspects found that this layer of facets was either non-existent or much less well developed. On the NE aspects above 8000 ft. snowpit observations and tests showed that a thin layer of weaker snow may be forming at the base of the recent snow on top of the first ice crust. 

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.

Abgedeckte Regionen