Eastern Sierra Avalanche Advisory - 12/7/17

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THIS AVALANCHE ADVISORY EXPIRED ON December 8, 2017 @ 6:59 am
Avalanche Advisory published on December 7, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center
bottom line

Isolated areas of stubborn wind slab above tree line remains the primary avalanche concern through Friday.  While unlikely, it may still be possible for a human to trigger one of these wind slabs in steep terrain where these dense hard wind slabs lie on top of weak sugary snow.  These are most likely to be found on non-southerly facing slopes, will take a rider hitting just the right trigger spot, and could result in a release above the rider.  Be wary of areas with smooth, firm, hollow sounding snow.

How to read the advisory

Bottom Line

Isolated areas of stubborn wind slab above tree line remains the primary avalanche concern through Friday.  While unlikely, it may still be possible for a human to trigger one of these wind slabs in steep terrain where these dense hard wind slabs lie on top of weak sugary snow.  These are most likely to be found on non-southerly facing slopes, will take a rider hitting just the right trigger spot, and could result in a release above the rider.  Be wary of areas with smooth, firm, hollow sounding snow.

Avalanche Problem 1: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Although little significant snow movement and deposition has occurred since the weekend, there remains isolated areas above tree-line where stiff wind slabs laying ontop of weaker sugary snow exist where it may still be possible for humans to trigger a small avalanche.  It has been more than a few days since these slabs have formed, which is typically enough time for them to bond and stabilize.  However, in some locations, weak sugary snow which has formed as a result of cold temperatures over the past week lies underneath and has made it hard for these slabs to bond well, as some test results on Wednesday have shown.  While it will remain hard to trigger these isolated areas due to the strength of the overlying slab, triggering a slide is conceivable and could result in taking a rider off their feet for a nasty ride in steep terrain. 

Be wary of smooth hollow sounding snow, do your own stability assessments of these wind slabs, and realize that results can vary tremendously over short distances.

advisory discussion

A relatively high snowline still exists for much of the Sierra, with little snow coverage below 9,000’ for the most part.  Heavy dense warm November storms gave the high country surprisingly good supportable coverage, however early season obstacles abound.  The cold temperatures over the past week have resulted in many areas of weak sugary faceted snow.  Strong winds last week and into last weekend have resulted in a patchwork of wind stripped and wind deposited areas above and at tree line.  The greatest concern for avalanches exists where these patches of dense wind deposits sit on top of weak sugary snow.  Cold clear nights will continue this facet formation and persistence of weak underlying snow.  Besides the avalanche concern, early season obstacles abound, as do firm surfaces which could result in a nasty slide after a fall. 

weather

Sadly a high pressure ridge remains firmly in place over the Sierra, and the greater west-coast, likely at least thru next week, meaning high and dry weather.  Temperatures have begun to rise from the chilly temperatures earlier this week, and will continue to do so into the weekend, with some moderate NE winds over ridgetops, but mostly calm at lower elevations. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 40 to 45 deg. F. 17 to 25 deg. F. 43 to 48 deg. F.
Wind direction: Light Light Light
Wind speed: Light Light Light
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 37 to 43 deg. F. 19 to 24 deg. F. 40 to 45 deg. F.
Wind direction: NE NE Light
Wind speed: 20 to 30 mph with gusts to 40 mph decreasing to 15 to 20 mph with gusts to 30 mph in the afternoon 10 to 15 mph with gusts to 25 mph in the evening becoming light light
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer
This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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