Eastern Sierra Avalanche Advisory - 12/14/17

THIS AVALANCHE ADVISORY EXPIRED ON December 15, 2017 @ 6:59 am
Avalanche Advisory published on December 14, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Stubborn old hard wind slabs sitting on top of weaker sugary faceted snow is still the main avalanche concern thru Friday.  Be wary of this combination on non-southerly facing near-tree line and above slopes.  While unlikely, it remains possible that a rider could find an isolated spot where the slab is thin and this combination could be reactive and a slide could be triggered.      

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

It has been over a week and half since any real snow deposition has occurred either as a result of snowfall or wind.  Despite this, some limited areas likely exist where the wind slabs left by the late November and early December storm/wind events are sitting on weaker sugary faceted snow, where a human could conceivably trigger a slide.  Normally it doesn’t take too long for wind slabs in the Sierra to bond with the underlying snow.  But cold temperatures, clear nights, shallow snowpack, and some buried melt-freeze crusts have led to the formation and persistence of weak faceted sugary snow on non-southerly facing slopes.  Observations have been very limited, so it is important to do your own localized tests of hollow sounding firm snow before committing to steep terrain.  Keep in mind that the most likely spot for this kind of avalanche to be triggered is where the overlying slab is thinner, and if triggered it most likely would release above the rider.

advisory discussion

Higher elevation NE winds picked up to moderate levels over-night (Wednesday), but are expected to decrease to light levels again today.  This would be concerning if more loose snow existed for transport at mid to upper elevation.  But given our current high and dry situation, the high winds associated with and following the significant late November storms, and the smaller early December storm, has left northerly facing mid to upper elevation slopes either wind-scoured or with firm wind slabs with very little loose snow available for transport.  Cold clear nights have been continuing the faceting (weakening) process of the upper snowpack, despite warm daytime temperatures reaching 50 degrees even above 10,000’.  This has resulted in increasingly good near-tree and below skiing on sheltered non-southerly facing slopes.  On southerly facing slopes the daily melt-freeze cycle has been strong, with reports of good spring-like skiing in the limited areas that have enough coverage (time of day dependent).  Some day when we get another snowstorm, all this faceted snow that has been transforming will present itself as a worrisome weak layer, and these melt-freeze crusts will act as a slick sliding surface.  For now, the main thing to watch out for as described above remains isolated areas where old stubborn wind slab sits ontop of weaker sugary faceted snow that may be reacitve to a human trigger.   

weather summary

High pressure ridge.   No moisture.  Sunny and clear. 

Warm daytime temperatures reaching over 50 degrees above 10,000’ will continue, with temperature inversions resulting in quite cold lower-elevation night time temps in the 20s.  Winds increased overnight out of the NE, but will calm throughout the day as they shift back out of the west. 

End in sight??  Weeeellll, a quick fast cold front for the weekend (very little moisture, but maybe a flurry?), and a bigger longer colder front mid-next week (again very little moisture, but maybe two flurries?) doesn’t quite count. 

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: 48 to 53 deg. F. 26 to 36 deg. F. 49 to 55 deg. F.
Mid Slope Winds: - - -
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Sunny Clear Sunny
Temperatures: 46 to 51 deg. F. 28 to 38 deg. F. 44 to 49 deg. F.
Ridge Top Winds: NE Light W
Expected snowfall: 0 in. 0 in. 0 in.
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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