Eastern Sierra Avalanche Forecast - 2/12/14

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THIS AVALANCHE ADVISORY EXPIRED ON February 13, 2014 @ 7:05 am
Avalanche Advisory published on February 12, 2014 @ 7:05 am
Issued by Sue Burak - Inyo National Forest
bottom line

The avalanche danger rating is rated MODERATE at mid elevations and high elevations in the Mammoth Basin. Natural avalanches and widepsread signs of instability such as cracking and collapsing have diminished but the trickiness of determing the stability of exposed alpine slopes has gone up. Right now more slopes are stable than unstable, but human triggered avalanches remain possible. As avalanches become increasingly difficult to trigger, manage the terrain and avoid steep, shaded rocky slopes above treeline, only expose one person at a time to avalanche terrain.  

 

 

How to read the advisory


  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

Bottom Line

The avalanche danger rating is rated MODERATE at mid elevations and high elevations in the Mammoth Basin. Natural avalanches and widepsread signs of instability such as cracking and collapsing have diminished but the trickiness of determing the stability of exposed alpine slopes has gone up. Right now more slopes are stable than unstable, but human triggered avalanches remain possible. As avalanches become increasingly difficult to trigger, manage the terrain and avoid steep, shaded rocky slopes above treeline, only expose one person at a time to avalanche terrain.  

 

 

  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Trend ?
    Decreasing Danger

Recent new snow and prolonged winds triggered large avalanches in the Hammil Bowl area.  An unusually thick facet and depth hoar layer in the alpine zone exists on some slopes in the Mammoth Basin. Results from multiple ECT's showed propagation on the ground and at the old snow/storm snow interface. Other tests did not propagate. Probing, snowpits and stability tests are needed to take the guesswork out of decision-making.  Unstable slabs still exist on steep, cold shaded slopes in the alpine terrain under the Mammoth Crest.  

Today, if you are traveling in alpine terrain, assume every slope has weak facets buried under the new snow unless proven otherwise.  This is not an impossible task but will require careful snowpack evaluation. 

 

Avalanche Problem 2: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Trend ?
    Decreasing Danger

The secondary avalanche problem today are lingering wind slabs that may be sensitive to human triggering. These will found at treeline and below rock ribs and ridges. They may be found resting on top of faceted snow. 

Watch for obvious signs of loading such as cornices, rounded drifts and rippled texture of the snow surface.  Avoiding slopes where these signs are present will be a simple way to stay out of trouble. 

advisory discussion

The recent storm added enough new snow to open up a few more areas in the backcountry for skiers and riders. It is a good news/bad news situation. The snowpack in alpine terrain on east and northeast facing slopes is spooky- poles can disappear during probing, the weak sugary facet alyer is thicker than the recent storm snow and a recent large avalanche is direct evidence that the snow structure and steep terrain is still capable of producing a human triggered avalanche. 

Prior to this most recent storm, the facet layer in the Red Cone area was not as thick or widespread. The situation in the steep rocky east and northest facing terrain iin Hammil Bowl is different. One explanation is that the shaded high elevation slopes in Hammil Bowl had October snow that turned to facets and persisted for the long dry months of November, December and January. The little storms that fell in November and December added to the October facet layer until snow came at the end of January and last week.  We have neve seen a facet layer this thick in this part of the Sierra. 

Warming temperatures, longer days and increasing solar radiation as spring approaches will contribute to stabilizing the snowpack. For now, give steep shaded slopes some more time to stabilize. 

recent observations

Two large avalanches released during the storm in the Hammil Bowl area. One slide reached Hammil Lake. Multiple crowns ranging in elevation from 11,000 to 10,200 ft were strewn across the 1,000 ft wide avalanche. Snowpack structure on slopes adjacent to the avalanche was weak with 20 inches of depth hoar and faceted snow on the ground. There was 15 inches of new snow resting on 20 inches of low density facets and depth hoar. The slide ran on a very thin crust that marked the boundary between the thick faceted layer and more recent snow. More photos and a snowpit profile will be posted in the obervation section later today. 

 

 

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 20 deg. F.
Max. temperature in the last 24 hours: 38 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 20 mph
Maximum wind gust in the last 24 hours: 50 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 39 inches
weather

A strong Paciifc jet stream approaching from the west coast will bring gusty winds to the area today and continue through the end of the week. 

Warm and mild temperatures will also continue through the week. A significant winter storm is forecasted to impact our area next week. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: clear, windy clear, winds clear, winds
Temperatures: 41 deg. F. 30 deg. F. 46 deg. F.
Wind direction: W W SW
Wind speed: 25 20 20
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: windy windy windy
Temperatures: 41 deg. F. 45 deg. F. 48 deg. F.
Wind direction: W W W
Wind speed: 30 30 30
Expected snowfall: 0 in. 0 in. 0 in.
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