Eastern Sierra Avalanche Forecast - 2/16/14

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THIS AVALANCHE ADVISORY EXPIRED ON February 17, 2014 @ 7:15 am
Avalanche Advisory published on February 16, 2014 @ 7:15 am
Issued by Sue Burak - Inyo National Forest
bottom line

The avalanche danger is MODERATE on steep north facing slopes in alpine terrain-wind drifts and wind slabs have been forming and building since yesterday.  Human triggered avalanches are possible on wind loaded slopes above treeline. Wind slabs and new storm slabs are loading the buried weak facets and depth hoar that dominate the lower half of the snowpack above treeline.

At mid-elevations in the trees, the avalanche danger is LOW; although unlikely, it remains possible to trigger avalanches on very steep slopes. 

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 is MODERATE on steep north facing slopes in alpine terrain-wind drifts and wind slabs have been forming and building since yesterday.  Human triggered avalanches are possible on wind loaded slopes above treeline. Wind slabs and new storm slabs are loading the buried weak facets and depth hoar that dominate the lower half of the snowpack above treeline.

At mid-elevations in the trees, the avalanche danger is LOW; although unlikely, it remains possible to trigger avalanches on very steep slopes. 

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

Sustained winds of 50-60 mph gusting to over 100 mph continue to form wind slabs and drifts along the Mammoth Crest. Yesterday’s winds blew snow high above ridgelines and along rock ribs in swirling columns. Wind drifts formed everywhere except right under the ridgetops where snow usually creates rounded pillows. Wind deposited snow in the middle of slopes, down the slope near the bottom, and across slopes yesterday. Wind loading continued through the night and will probably continue through most of today.

 These drifts will be most common on north through east facing slopes and will be sensitive to any additional load.  Over 36 hours of sustained violent storm and hurricane force winds have loaded steep slopes that have up to 2 ft of buried depth hoar and facets at the bottom.   

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
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  • Trend ?
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An unusually thick facet and depth hoar layer exists on most north to northeast slopes above tree line in the Mammoth Basin. Recent sun and periods of high thin clouds added a lot of energy to the upper half of the snowpack above and below treeline,  and now the old storm snow is a cohesive slab- it might be a hard slab, or sastrugi after this wind event, but the important thing to know is that this layer is being loaded again.  The return of below freezing temperatures to alpine regions will activate the facet machine, possibly destabilizing these buried weak layers in rocky, high elevation, shaded north facing terrain. 

advisory discussion

Snow conditions in the Mammoth Basin were um, kinda ugly yesterday. Strong downslope winds were gusting to 50 mph at 9,000 ft. and wind drifts formed on any open area exposed to the wind. Pine needle drifts often stopped skiing progress.  Frozen icy crusts were supportable in most places, and it was difficult to predict where skis would break through. I saw an unusual snow grain yesterday- rounded and clusters of rounds below the crust. The last week of ridiculously high temperatures, thin high clouds and mid-February solar radiation assisted in creating more bonds and a stronger snowpack below treeline. The avalanche danger is Low at middle elevations but isolated areas of instability may exist in steep terrain.  

At treeline, the snow surface was becoming sastrugi, wave like ridges of snow, 2 ft deep. Two stability tests produced no results on the buried weak layers. The avalanche danger remains MODERATE above treeline because of  continuous wind loading. New snow measured on study plots this morning will be hard to find at exposed elevations.  

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 22 deg. F.
Max. temperature in the last 24 hours: 30 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 70-80 mph
Maximum wind gust in the last 24 hours: 93 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: inches
weather

Strong west and southwest winds are blowing this morning. A few inches of snow accumulated last night on study plots located in the trees. Another inch or two of new snow could fall today but by the afternoon, the storm will move east and skies may clear in the late afternoon. Last night, temperatures were below freezing at 9,000 ft.-the first time in days. Howling winds will die down today from gusts of 80-90 mph yesterday and last night, decreasing to 45-50 later today. Seasonal temperatures return this week with nights cooling off to below freezing.  

There is a chance for a colder storm middle of this week. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Sunday Sunday Night Monday
Weather: clouds and snow showers showers sun
Temperatures: 38 deg. F. 26 deg. F. 40 deg. F.
Wind direction: W SW SW
Wind speed: 30-45 15-20 15-20
Expected snowfall: 3 in. 0 in. 0 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: clouds, wind and showers
Temperatures: 39 deg. F. 22 deg. F. 39 deg. F.
Wind direction: W W SW
Wind speed: 45-55 15-20 20-25
Expected snowfall: 3 in. 0 in. 0 in.
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