Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  As the visibility improved yesterday, more reports of wind slab avalanches came in. NE winds continued to blow snow yesterday and last night. Expect to find slabs of wind-blown snow on the leeward sides of ridges and gullies, especially near and above treeline. People could still trigger wind slab avalanches if they travel in wind-loaded areas. Some wind slabs will be firm and dense and may not break until someone gets out onto the slab or hits a trigger point where the slab is easier to break. The most fragile wind slabs will exist on SE-S-SW-W-NW-N aspects, but some of the wind slabs that formed earlier in the storm may still linger on other aspects or isolated areas below treeline.

  Areas with wind slabs will look smooth and filled-in, may feel firm on the surface, and may sound hollow underneath. Look for signs of instability like recent avalanches, cracking, or unstable test results on small test slopes or edges of these slabs where they might be thinner. If signs of instability or uncertainty about a slope exist, move away from that area. Traveling in terrain sheltered from the wind can help bypass potentially unstable wind slabs and locate more consistent snow conditions. 

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–2.5

  The recent snow has pushed the buried weak layers below the surface to the brink of failure. Unlike typical storm slab issues, these layers will take longer to adjust to the load above them. Yesterday, several whumpfs occurred as the buried weak layers collapsed under a person's weight on top of the snow. These weak layers include weak feathery snow (surface hoar) and sugary snow (facets) buried about 18 inches below the surface. A graupel layer also exists near the base of the recent storm snow.  Avalanches large enough to bury or injure people remain possible in places where these weak layers exist. The weakest snow exists on open, wind-protected NW-N-NE aspects near and below treeline, but some weak snow may also exist in isolated areas on other aspects.

  These avalanches often break wider than expected and wrap around terrain features. People can also trigger them from lower-angle terrain below or to the side of a slope.  Look for signs of instability like recent avalanches, shooting cracks, whumpfing, and collapsing. Sometimes digging into the snow can help identify areas where weak layers exist.

  Use low-angle, non-avalanche terrain not connected to steeper areas above or beside it if you see signs of instability. If you have doubts about whether or not the weak layer exists, play it safe and assume the snowpack is unstable. 

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–1.5

  Sun, warmer weather, and less wind today may allow enough wet snow to form on sunny slopes for a wet loose avalanche to occur. Places near sunny rocks and trees will experience the most warming. Warming may slow down this afternoon if enough cloud cover develops to block the sun. 

  Look for signs of unstable wet snow like rollerballs, pinwheels, or sticky wet snow. Once these start to happen, it is time to avoid steep slopes and move into lower-angle terrain. Long steep slopes can produce larger wet loose avalanches since they gather more snow as they move downhill.