Eastside Region

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  2–3.5

  20-30” of new snow has fallen in the last 24hours, with strong to extreme winds last night out of the south to west reaching over 100mph over ridgetops and 60mph at lower elevations. Another 1-2ft of snow is possible today before tapering off in the afternoon.

  Remember winds can deposit snow many times faster than it can fall from the sky. Large dangerous Wind Slabs will continue to grow especially on northerly to easterly facing terrain at all elevations. They will be very sensitive to human triggering and could likely fail on their own. The same avalanche path could fail multiple times throughout this storm.  A weak sugary underlying snowpack could make avalanches even more sensitive, susceptible to remote triggering, and end up being much larger than expected. Avoid being on or under wind loaded slopes steep enough to slide, as today’s avalanches could run a long distance.   

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  2–3

  With the amount of new snow and the weak underlying sugary snowpack, soft Storm Slab avalanches in sheltered terrain could easily be large and deadly today as well.  A denser layer of new snow sandwiched between lighter layers was found yesterday, which will also increase the likelihood that a steep sheltered slope could fail as a large slab rather than sloughing, even in terrain without old snow coverage. Even small slopes that end in a gully could result in a deep burial.  These may also be remotely triggered from a distance today. Again, avoid travelling on or under sheltered slopes steep enough to slide, and be especially aware of steep mid-slope convexities.      

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3.5

  Early season snowfall from October and November sat around for a solid month on shaded E-N-W facing slopes above ~9,200’. Most other slopes where significant wind load didn’t occur became bare ground.  Many areas where this old snow remained developed heavily faceted sugary weak layers, just waiting for a heavy new load of snow to cause avalanches.  Well, that new load is here, and those persistent weak layers have already been failing resulting in large avalanches. Yesterday, Mammoth Mtn reported two sizeable avalanches that failed near the ground in old snow layers, one failing naturally, and the other human triggered (fortunately no one was injured). This problem will overlap and exacerbate the other two problems of Wind Slab and Storm Slab, but will be limited to slopes with prior old snow coverage (mostly E-N-W facing slopes above ~9,200’).  Failures in persistent layers will result in much larger avalanches than expected. A smaller Wind Slab or Storm Slab avalanche could be the force required to trigger one of these deeper layers, or they could be triggered out-right to begin with, either naturally or by a human trigger.  Give suspect slopes a wide berth, as a resulting avalanche will likely be very large.