Central Sierra Nevada

Lawinenprobleme

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1.5–3

  Wind slabs still exist on slopes near and below ridgelines where blowing snow has accumulated. Data and observations indicate that these wind slabs have started to bond to the snow below them and that triggering a wind slab avalanche has become more difficult. That being said, it may still be possible for a person recreating on a steep wind-loaded slope to cause an avalanche. The most wind-blown snow exists on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects, so that's where wind slabs will most likely linger.  The brief period of NE winds last night may have blown some snow back over the ridgelines and scoured some of these areas or it may have simply made the existing wind slabs stiffer. 

  Signs that a wind slab has formed on a slope include cornices above the slope, wind drifted snow, firm hollow-sounding snow, and other wind-created surface textures. Even though they may be harder to trigger today identifying where the wind slabs may exist and moving around those areas can help find more enjoyable snow and avoid triggering a possible wind slab avalanche.  In some areas buried persistent weak layers may still exist below the wind slabs making them behave more like the persistent slabs mentioned below. 

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  2–3

  It has been two days since the last persistent slab avalanche was reported in the backcountry.  No reports of obvious signs of instability like deep shooting cracks, snowpack collapse, or whumpfing came in yesterday either. Snowpit data yesterday from the West Shore and Carson Pass confirms that the old layers of weak sugary snow still exist. In most places, they are now buried 4 to 5 feet below the surface with strong snow on top of them. Data also seems to indicate that these layers are gaining strength and that triggering a persistent slab avalanche is becoming more difficult. Still, it may be possible for a person recreating on a slope where the persistent weak layers exist to trigger one of these persistent slabs, especially on a higher elevation NW-N-NE aspect. A person finding a trigger point on the slope where less snow exists above the weak layers or a larger trigger like a cornice failure, another avalanche, or multiple people on a slope would have a better chance of causing a persistent slab avalanche.

  Significant uncertainty remains about how quickly these old weak layers will adjust to the new load on top of them and about what it would take to trigger a persistent slab avalanche today. Some things that are more certain include the fact that a persistent slab avalanche would have severe consequences. These kinds of avalanches often break wider and travel farther than expected. People can trigger them from lower angle terrain, from the bottom of a slope, or from adjacent terrain. Due to this uncertainty and the potential consequences of getting caught in an avalanche, it is best to play it safe today. Carefully evaluate any slope before you commit to it especially in areas where old buried weak layers may exist. Focusing your travel plan on slopes less steep than 30° that are not connected to steeper slopes can help avoid this problem.