Eastern Sierra Avalanche Advisory - 12/9/17

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THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2017 @ 6:52 am
Avalanche Advisory published on December 9, 2017 @ 6:52 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The primary avalanche concern thru Monday will continue to be stubborn wind slabs near and above treeline on W-N-E aspects. While natural avalanches are unlikely, human triggered avalanches remain possible in steep terrain where wind slabs overlay weak faceted (sugar-like) snow. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. The wind slabs may not be big enough to result in burial, they can entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Below ~9,500’, natural and triggered releases are unlikely due to little or no snow (below threshold).

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Persistent Slab
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Persistent Wind Slab

A weak but windy storm system last weekend formed shallow wind slabs on W-N-E aspects, which was followed by light winds and little snow transport or new deposition through this past week. Typically, this would be enough time for wind slabs to bond and stabilize. However, near and above treeline, there are isolated pockets of wind slab overlaying the faceted (sugar) snow that formed as a result of the recent cold temperatures. The well-developed facet layer has been slow to bond to the overlying wind slab, as some test results have shown. While the wind slabs are stubborn to trigger due to the strength of the overlying slab, a slide is possible and could entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Typically, wind slabs may be encountered: below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. Do your own stability assessments of wind slabs encountered and realize stability can vary tremendously over short distances. Below ~9,500’, natural and triggered releases are unlikely due to little or no snow (below threshold). 

advisory discussion

The snowpack in the Sierras remains thin and mostly confined to elevations above ~9,000 to 9,500’ around Mammoth, and higher elsewhere. Dense early season storms in November deposited enough snow for good coverage, however early season conditions exist with numerous obstacles and many hazards just under the snow surface. The cold temperatures this past week created a strong temperature gradient in the snowpack, producing weak sugary faceted snow throughout the upper snowpack. Moderate to strong winds last week and into last weekend formed pockets of dense wind slabs, near and above treeline, on top of the weak faceted snow at the surface. Cold clear nights continues to drive facet formation in the upper snowpack. The snowpack near and above treeline alternates between wind stripped, wind deposited areas, and firm melt / freeze patches. 

 

Weather and CURRENT CONDITIONS
weather summary

Sat thru Monday - Quiet weather will persist as an amplified ridge of high pressure sits over the Sierra and western NV. Overnight lows will continue to be cold, dipping into the teens and lower 20s, with single digits for colder valleys near the Sierra. Highs mainly in the upper 40s and lower 50s. Relatively warmer temperatures with less diurnal fluctuations can be expected along the mid and upper slopes of the Sierra. Light easterly winds will continue for the next few days, due to the low level pressure gradient extending from the surface high across the northern Great Basin.

Tuesday onward...

Little change is expected in the overall weather pattern through next Thursday as high pressure remains in control of the region. Inversions will keep afternoon temperatures near average for the lower valleys, with the Sierra around 5-8 degrees above average. Nights will remain cold (a bit below average) with the dry air mass, light winds and little to no cloud cover. Generally light winds through the middle of the week. A weak upper disturbance is forecasted to drop south across the Rockies nudging the ridge axis back toward southern Oregon and the northern CA coast with possible moderate east ridge winds.  

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny
Temperatures: 42 to 47 deg. F. 22 to 32 deg. F. 44 to 49 deg. F.
Wind Direction:
Wind Speed: Light winds Light winds Light winds
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Clear Sunny
Temperatures: 38 to 43 deg. F. 25 to 31 deg. F. 42 to 48 deg. F.
Wind Direction:
Wind Speed: Light winds Light winds Light winds
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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