Eastern Sierra Avalanche Forecast - 2/14/14

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THIS AVALANCHE ADVISORY EXPIRED ON February 15, 2014 @ 7:10 am
Avalanche Advisory published on February 14, 2014 @ 7:10 am
Issued by Sue Burak - Inyo National Forest
bottom line

The avalanche danger is MODERATE on steep north to east facing slopes in alpine terrain in the Mammoth Basin. Human triggered avalanches are possible on slopes steeper than 35 degrees where weak facets and depth hoar dominate the lower half of the snowpack. 

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

Bottom Line

The avalanche danger is MODERATE on steep north to east facing slopes in alpine terrain in the Mammoth Basin. Human triggered avalanches are possible on slopes steeper than 35 degrees where weak facets and depth hoar dominate the lower half of the snowpack. 

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

Avalanche Problem 1: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Trend ?
    Decreasing Danger

An unusually thick facet and depth hoar layer exists on most north to northeast slopes above tree line in the Mammoth Basin. Observations from the Mammoth Crest area yesterday reported similar snow structure to the slopes above Hammil Lake observed on Tuesday- a slab of storm snow from last weekend resting on up to 2 feet of weak facets and depth hoar. 

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.  

 

Photo credit. Preston Few

Avalanche Problem 2: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Windy conditions in the alpine zones yesterday formed shallow wind drifts. With high west and southwest winds forecasted for the next few days, areas of wind drifted snow and wind slabs will form and may become large enough to create an avalanche problem in isolated areas. These will found at tree line and below rock ribs and ridges. 

Avalanche Problem 3: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Same

Small point releases of wet unconsolidated snow have been observed within layers of wet snow near the surface of the snowpack below about 9,500 ft. on north, east and on the west facing slopes of the Sherwin Ridge.  These surface instabilities will be watched to see if the size increases and poses a danger of stepping down to deeper faceted layers.

advisory discussion

The snowpack in alpine terrain on east and northeast facing slopes continues to warrant respect. It’s still winter in the alpine- north to east facing slopes do not receive much direct sun this time of the winter and the snowpack cools at night under clear night time skies. Daytime highs at the top of Mammoth at 11,000 ft. only reached 35 F yesterday compared to the balmy 52 F at 9,400 ft. at Mammoth Pass.

Mid elevations in the Mammoth Basin range from 9,000 to around 10,000 ft. and receive more direct sun. Sheltered slopes in glades still hold dry snow from the storm, and stability tests show no cracking or propagation. The storm snow is dense because the bonds between snow grains are becoming more numerous. The weak facets below take much longer to become more dense –that is why they are called persistent weak layers. The recent naturally occurring avalanches in the Mt Humphrey's area are in your face reminders that the Sierra snowpack at high alpine elevations is capable of producing avalanches.

recent observations

An observer reported two natural avalanches in the northeast facing bowl under the Checkered Demon near Mt Humphrey’s. One avalanche started at 12,700 ft and ran around 800 vertical feet, pulling out smaller crowns on the steep rocky terrain. A second smaller avalanche released from an ESE slope under rocks and pulled all the snow out to bare ground. Thanks to David Enloe for the report and picture. See his post and pictures in the Observation section.

Natural avalanche releases days after the storm are often seen in the snowpack in Colorado where the snow is notoriously weak. The slides reported yesterday are clear warning that high elevation snow is capable of producing avalanches. 

 

Reports from the Mammoth Crest noted small wet point releases on sun baked slopes. Stability tests had no results and the snow structure is similar to the Hammil Bowl area- 2 feet of facets with 12-15’ of dry slab above the facet layer.

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 24 deg. F.
Max. temperature in the last 24 hours: 35 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 25 mph
Maximum wind gust in the last 24 hours: 62 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 40 inches
weather

Unusually warm days are in store for the eastern Sierra  over the next two days. Sierra ridgetop wind gusts will exceed 100 mph today, Saturday and Saturday night. Snow showers are possible over the crest. A significant storm mid week remains a possibility. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Friday Friday Night Saturday
Weather: clear clear partly cloudy
Temperatures: 47 deg. F. 31 deg. F. 46 deg. F.
Wind direction: SW SW SW
Wind speed: 20-40 20-30 20-30
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Friday Friday Night Saturday
Weather: partly cloudy clear clear
Temperatures: 40-45 deg. F. 30 deg. F. 43 deg. F.
Wind direction: W W W
Wind speed: 40-55 40-50 40-55
Expected snowfall: 0 in. 0 in. 0 in.
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