Central Sierra Nevada

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  1–2.5

  Intense snowfall rates and weaknesses in the storm snow, like a graupel layer at its base, will make storm snow avalanches very likely today. Unstable storm slabs could exist on any steep slope including below treeline slopes and other typically sheltered areas. These could involve more than enough snow to bury or injure a person since they could entrain all the snow that has fallen since yesterday. As more snow accumulates today, storm slab avalanches will increase in size and become more widespread.

  Areas that people typically think of as "safe" on a storm day may hold unstable storm slab conditions. Today, avoiding travel on, near, or below any slopes steep enough to avalanche is recommended. 

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  1.5–3

  New slabs of wind-blown snow exist near ridges, open wind-exposed slopes, and gullies. These wind slabs will continue to grow in size and distribution as more snow and wind impact the region today. The largest wind slabs will exist on near and above treeline NW-N-NE-E-SE aspects, but some wind slabs will also form in open areas below treeline due to the strength of the winds. Large to very large wind slab avalanches will be very likely today. Wind slab avalanches could start high on a mountain and run into lower elevation areas. 

  Traveling in, near, or below avalanche terrain is not recommended today. If you do get out, give yourself an even larger buffer to avoid avalanche terrain since blowing snow and poor visibility will make it hard to see what terrain may lurk above or nearby. 

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3.5

  Another 1+ feet of snow has accumulated during this storm, and up to 2 more feet could fall before the storm's end. This additional weight may again overload the facet/crust layer (Feb 21 weak layer) that exists on NW-N-NE aspects. The possibility exists that other large triggers like avalanches or huge cornice failures could break this layer. If this layer fails, the resulting avalanches would be wide, deep, and likely unsurvivable since this layer may be 6 to 8+ ft below the surface. This problem should be less widespread during this storm due to many slopes having already avalanched, less overall accumulation, and stronger snow above the weak layer. Because It will take a large trigger or load to break the deeply buried weak layer, avalanches will become more difficult to trigger once the storm stops loading the snowpack and the storm snow starts gaining strength.

  Travel advice for this problem boils down to avoidance, just like today's other avalanche problems. Today is not a day to push boundaries. Being in, near, or below avalanche terrain is not recommended.