Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  Strong SW ridgetop winds have been drifting snow and creating wind slabs for the past 30+ hours. Areas of unstable wind slabs formed in lee areas where drifting snow has deposited are possible above treeline and near treeline on NW-N-NE-E-SE aspects. Avalanche size is expected at D1 to D2.

  Continued blowing and drifting snow, cornice formations, and new wind pillows are all clues that can be used to identify slopes where unstable wind slabs may be encountered. Identify the areas of concern and move around them with caution.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2

  Ugh. For the past two days, snowpit data collected either adjacent to or in the crown of persistent slab avalanches from March 25 and 27 have shown significant strength gains on the buried surface hoar persistent weak layer. Just when we thought we were past this avalanche problem, another persistent slab avalanche occurred yesterday afternoon. Spatial variability is at play here with weak layer stabilization occurring at different rates in different locations. This creates increased uncertainty. Isolated persistent slab avalanches failing 1 to 3 feet deep in the snowpack may occur in areas without signs of current instability and with numerous previous tracks. Ugh. NW-N-NE aspects at all elevations remain suspect. Avalanche size is expected at D2.

  Avoiding NW-N-NE aspects or using only slopes of less than 30 degrees in slope angle, not connected to steeper slopes above or to the side are the tools for avoiding this avalanche problem. The places where good powder lingers are also where is avalanche problem is more likely to be found.

    Photo: Yesterday's persistent slab avalanche.

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1

  It's almost April. The sun angle is high enough to affect N aspects and incoming solar radiation is strong. This time of year it does not take much to create snow surface warming and loose wet instability. Warm air temperatures and sunshine through the clouds today will create snow surface warming. Rollerballs and loose wet avalanches are expected today in wind protected areas on all aspects, especially around exposed rocks. Avalanche size is expected at D1 with an isolated D2 not impossible.

  Overnight air temperatures were warm and this avalanche problem is expected to form quickly today. With size D1 avalanches, consequences will come from being pushed into or over downslope terrain features such as rocks, trees, cliffs, and terrain traps. These terrain features could make high consequences from an otherwise small avalanche. Rollerballs and pinwheels are indications that this avalanche problem is present.