Eastern Sierra Avalanche Forecast - 2/5/14

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THIS AVALANCHE ADVISORY EXPIRED ON February 6, 2014 @ 7:05 am
Avalanche Advisory published on February 5, 2014 @ 7:05 am
Issued by Sue Burak - Inyo National Forest
bottom line

The avalanche danger is MODERATE on slopes steeper than 35 degrees and LOW on less steep slopes at middle and upper elevations in the Mammoth Lakes Basin. North facing slopes near tree line (subalpine terrain) are most suspect due to more widespread weak snow at the base of the snowpack.

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 slopes steeper than 35 degrees and LOW on less steep slopes at middle and upper elevations in the Mammoth Lakes Basin. North facing slopes near tree line (subalpine terrain) are most suspect due to more widespread weak snow at the base of the snowpack.

  • 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

Snowpack structure is similar at mid and higher elevations- 12-15 inches of storm snow has densified into a slab and sits on top of 2 to 6 inches of low density facets and depth hoar. The layer continues to produce unstable results in extended column tests on low to moderate angle slopes. The basal facets and depth hoar have gained some strength on some slopes but remain weak and reactive on many others.  The odds of triggering an avalanche vary from slope to slope and within the area of an individual slope. These spots can be near the bottom of a slope, on the edges or even in the middle.

advisory discussion

While there was not enough of a new snow load to cause a natural avalanche cycle last week, extended column tests continue to show that buried depth hoar could still produce an avalanche. The layer continues to produce unstable results in extended column tests on low to moderate angle slopes. The basal facets and depth hoar have gained some strength on some slopes but remain weak and reactive on many others. These stability tests indicate the old weak snow layer continues to be weak and reactive, 5 days after the storm. We don’t base decisions entirely on stability tests and it’s prudent tin these current conditions to consider the snowpack structure. 

We've had and still have - abnormal conditions. But the usual principles still apply-avoid steep slopes where the snowpack is more shallow, making it easier for the force of a skier or rider to reach buried weak snow. Ski one at a time, exposing only one person at a time; don't put yourself above or in terrain traps like cliffs and gullies.

And while it may look the same today as it did yesterday, the snowpack is slowly subtly changing. The cold night time temperatures are faceting near-surface layers where the snowpack is deeper, and faceting the entire snowpack where it's shallower.

recent observations

ECT's done at 8800 and 9600 ft in the Sherwins -collapse on 4 inches of depth hoar at the ground. Total snow depth 30 inches.

ECT's at 9400, north aspect in Mammoth Knolls area, collapse on 2-4 inches of depth hoar on the ground. Total snow depth North facing slope collapsed in open glade in the trees.

ECT tests showed propagation and collapse on the depth hoar layer in two pits in the Hemlock area.

These stability tests indicate the old weak snow layer continues to be weak and reactive, 5 days after the storm.Thanks to Ned Bair for this image of depth hoar taken yesterday, Tuesday Feb 4.

 

 

 

weather

Below normal temperatures continue through the week with a few inches of snow expected Thursday. A more significant storm could impact our area on Sunday. Daytime highs will be in the 20’s  with lows in the teens and single digits. Today will be mostly sunny with light west winds.  

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: mostly sunny clouds snow
Temperatures: 26 deg. F. 17 deg. F. 25 deg. F.
Wind direction: W W SW
Wind speed: 10 10-15 15-20
Expected snowfall: 0 in. 0 in. 3 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: mostly sunny clouds snow
Temperatures: 29 deg. F. 14 deg. F. 27 deg. F.
Wind direction: NW W W
Wind speed: 10-15 20-40 30
Expected snowfall: 0 in. 0 in. 4 in.
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