Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  The NE winds continue to move snow across the ridgelines forming drifts of snow on leeward aspects (NW-W-SW-S-SE) and scouring the windward aspects (N-NE-E). In many cases, the slabs of wind drifted snow formed by the NE winds remain small and confined to terrain near ridgelines, but some larger ones may exist on the most heavily wind-loaded slopes. While the prolonged period of NE winds has scoured much of the snow away from the N-NE-E aspects, a few stiff wind slabs formed by the earlier SW winds may still linger on some of these slopes. Human-triggered wind slab avalanches may remain possible today, especially on heavily wind-loaded slopes or in complex or extreme terrain. Large cornices also exist above many slopes.

  Blowing snow, cornices above a slope, firm hollow-sounding snow, wind pillows/drifts, ripples in the snow surface, and other wind created textures can all help identify where wind slabs may exist. These wind slabs will be hard and potentially stubborn today. Traveling in more sheltered terrain where soft snow may still exist would provide better recreation conditions and help avoid the wind slabs and scoured slopes, but you will still need to assess the potential for the deep persistent slabs mentioned below. 

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  The last known deep persistent slab avalanche occurred on Jan 31 or Feb 1 (~5 days ago). A few test results still indicate instability on the deep weak layers, but in many areas tests targeting the deep persistent weak layers no longer yield unstable results. Data indicates that triggering a deep persistent slab avalanche is becoming more difficult in most places. On the flip side, the consequences of a deep slab avalanche remain severe. Those consequences combined with lingering uncertainty and variability associated with the deep weak layers mean that this problem still warrants caution. Deep persistent avalanches may still remain possible in some places today, but they are becoming a low-probability-high-consequence problem. Most of the areas of concern for this deep slab avalanche problem exist on the Eastern side of the forecast region on NW-N-NE aspects where a weak and shallow snowpack existed before the recent storms. This includes portions of the Carson Range on the NE, E, and SE sides of Lake Tahoe and locations east of the Sierra Crest in the Luther Pass, Carson Pass, and Ebbetts Pass areas.

  Deep slabs represent an unpredictable and difficult to assess avalanche problem. Sometimes digging deep holes or probing can help determine whether or not the buried weak layer exists, but often triggering a large deep slab avalanche is the first and only sign of instability. Sticking to slopes less steep than 30 degrees that are not connected to steeper slopes represents the best way to avoid this problem. Avoid complex terrain with multiple trigger points and unsupported slopes.