Eastern Sierra Avalanche Forecast - 2/19/14

The backcountry is more accessible than it has been in years. Please help!  ... Click Here to Find Out How

THIS AVALANCHE ADVISORY EXPIRED ON February 20, 2014 @ 7:05 am
Avalanche Advisory published on February 19, 2014 @ 7:05 am
Issued by Sue Burak - Inyo National Forest
bottom line

A MODERATE danger exists on north facing slopes steeper than 35 degrees in exposed alpine terrain. Strong winds today will form fresh deposits of wind drifted snow on top of areas of unstable  snow from Sunday's snowfall.These avalanches may be confined to the upper 8 inches of the snow pack or may step down to deeper layers.

The other avalanche concern is the persistent two foot layer of facets and depth hoar at the bottom of the snow pack. On some steep slopes, it remains is possible to trigger an avalanche on the buried weak layer.  of snow or recent storm snow, while other slopes might need a big trigger like a cornice drop to trigger a slide. Windy conditons will create areas of wind drifted snow on northeast and easterly aspects above tree line. The avalanche danger rating for mid elevations in the Mammoth Basin is generally LOW. 

How to read the advisory


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

Bottom Line

A MODERATE danger exists on north facing slopes steeper than 35 degrees in exposed alpine terrain. Strong winds today will form fresh deposits of wind drifted snow on top of areas of unstable  snow from Sunday's snowfall.These avalanches may be confined to the upper 8 inches of the snow pack or may step down to deeper layers.

The other avalanche concern is the persistent two foot layer of facets and depth hoar at the bottom of the snow pack. On some steep slopes, it remains is possible to trigger an avalanche on the buried weak layer.  of snow or recent storm snow, while other slopes might need a big trigger like a cornice drop to trigger a slide. Windy conditons will create areas of wind drifted snow on northeast and easterly aspects above tree line. The avalanche danger rating for mid elevations in the Mammoth Basin is generally LOW. 

  • 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 ?
    Same

Two feet of facets and depth hoar are now buried under one to two feet of recent storm snow. It’s well past the time when obvious signs of deep instability such as cracking and whumpfing give unmistakable warnings. The absence of whumpfing or avalanche should never be interpreted as evidence the layer is not active. Observations showing limited to the recent new snow and crust combination do not address deep instability. Dig down to the old layer and look for increasing hardness- the layer is still fist to 4 finger in steep N and NE facing slopes above 10,000 to 10,500 ft.  Slabs that involve deeper persistent weak layers  could be large and wide, similar to the Hammil Lake avalanche http://esavalanche.org/content/hammil-bowl

 and the large natural reported in the Checkered Demon area, http://esavalanche.org/content/avalanches-near-checkered-demon

 One observer dug a pit that was 5 feet deep in a wind deposition area near Red Cone yesterday and found 2 feet of buried facets at the bottom. He found instability 6-8 inches down on the old snow crust layer. The snow was much deeper than his saw and ECT tests were not done.

These layers can go dormant and reactivate days or weeks later.  Surface condition is not indicative of persistent weak layers. This is what makes the current conditions so tricky above treeline.

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

Recent storm snow instability was reported by many people over the last two days. Sunday morning's snow fell on crusts of varying thickness in the Red Cone area. One compression test failed on isolation of the column on facets under the most recent suncrust. Others reported easy to moderate compression test results on the same layer. One observer reported snow depth of 5 feet in a wind deposition area near Red Cone. Compression tests in this location were similar- moderate test results on the old snow/new snow boundary about 8" down from the snow surface. Wind loading today will form new wind drifts and wind slabs. It will be possible to trigger an avalanche on steep north and east facing slopes.

advisory discussion

If you've spent time in the backcountry this winter, you noticed how soft and unsupportable the snowpack was until recently. Depth hoar exists at the base of the snowpack on slopes where early season snow stuck around. Above the depth hoar, there are well-developed facets that repeatedly formed during extended clear sky conditions. In addition, numerous crusts exist in the lower snowpack as a result of solar warming. Each crust is embedded between facets that developed either during crust formation or after burial.

All these weak layers are now buried under late January, Feb 7-9 and last Sunday's snowfall. Expect near surface faceting and even surface hoar formation in spots. If the weather pattern turns wet next week, future loads will create an interesting situation where near-surface instabilities could step down to the deeper persistent weak layer.

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: 46 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 50 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 44 inches
weather

Daytime temperatures cool down 10 degrees today after yesterday’s mild temperatures in the upper 40’s.  Night cool off also, with night time lows around 20 degrees. Strong southwest and west winds gusting to 75 mph are expected today at the 8,000 to 10,000 ft elevations and gusts to 90 mph are expected over the ridgetops.

Weather forecasters are watching a storm develop in the Pacific that could produce snow for 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: windy windy
Temperatures: 35 deg. F. 23 deg. F. 38 deg. F.
Wind direction: NW N W
Wind speed: 25-35 15-20 10-15
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: very windy windy breezy
Temperatures: 323 deg. F. 20 deg. F. 36 deg. F.
Wind direction: NW N W
Wind speed: 30-40 20-25 15-20
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

ESAC receives support from ...