Eastern Sierra Avalanche Advisory - 12/23/17

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THIS AVALANCHE ADVISORY EXPIRED ON December 26, 2017 @ 6:59 am
Avalanche Advisory published on December 23, 2017 @ 6:59 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The primary avalanche concern thru Monday will focus on Wind Slabs near and above treeline on W-S-E aspects that formed as a weak storm moved through the region (12/21) depositing 1 to 4 inches of new snow with moderate to strong Northerly winds. Forecasted moderate Southwesterly winds Sunday (12/24) are expected to form additional sensitive Wind Slabs near and above treeline on NW-N-NE-E-SE aspects.  These should be generally isolated in nature.  

Below ~9,000’, natural and triggered releases are unlikely due to little or no snow (below threshold).​

 

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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Primary concern during the forecast period will focus on recently formed Wind Slabs from a weak storm system that moved through the region Wednesday (12/21) with 1 to 4” of snow reported. The system moved into the region with moderately strong pre-frontal Southwesterly winds, then veering to Northeast as the system continued its eastward progression. The Northeasterly winds formed potentially sensitive Wind Slabs on NW-W-S-SE-E aspects, near and above treeline. Sunday into Monday, the wind is forecasted to increase from the Southwest forming new Wind Slabs on NW-N-NE-E-SE aspects. Normally, Wind Slabs tend to bond to the underlying snow within a few hours to couple of days. However, the recent extended period of cold clear nights and a shallow snowpack has formed weak faceted (sugar) snow in the upper snowpack, especially on northerly facing aspects, which slows the normal bonding and strengthening processes. Though Wind Slabs are expected to be shallow and isolated in nature, a slide is conceivable 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. To date, observations have been limited, so it’s important to do your own stability assessments of wind slabs encountered and realize stability can vary tremendously over short distances. Below ~9,000’, 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, possibly 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 recent cold overnight temperatures have created a moderate to strong temperature gradient in the upper snowpack, producing weak sugary faceted snow in the upper snowpack, especially in sheltered areas. Previous wind events formed pockets of dense wind slabs, near and above treeline that are resting on weak faceted snow. Normally, a few hours to a couple of days is enough time for Wind Slabs to strengthen and stabilize but the faceted upper snowpack is inhibiting bonding and slowing the normal strengthening processes. Additionally, previous Wind Slabs are showing signs of decomposing and loosing strength, increasing the potential for triggered release. The snowpack near and above treeline alternates between soft facts in sheltered areas, wind stripped, wind deposited areas, and melt / freeze patches. Good skiing can be found on sheltered Northerly aspects or on Southerly aspects with enough coverage that are reportedly offering good spring-like skiing in the limited areas. Below treeline, the snowpack continues to be exceptionally thin. As the winter progresses, the facets in the upper snowpack may form a weak layer, once buried, while the melt-freeze crusts may act as a prime bed surface for future avalanches.

 

Weather and CURRENT CONDITIONS
weather summary

Saturday thru Sunday - Low amplitude ridge continues to stream mid and high-level clouds across the region this weekend. Winds will remain breezy across the Sierra ridgetops today with gusts of 50-60 mph possible this afternoon before weakening this evening. Another weak wave will brush northwest Nevada on Sunday will again bring some gusty winds across the Sierra ridgetops but no precipitation.

Mon and beyond – models continue to flatten the persistent "dirty" ridging with a weak systems moving into the Pacific Northwest with little or no precipitation expected. Friday into next weekend, a stronger system may move into Oregon and may have a better chance of showers. 

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: Mostly cloudy Mostly cloudy Mostly cloudy
Temperatures: 41 to 49 deg. F. 24 to 32 deg. F. 42 to 50 deg. F.
Wind Direction: West Southwest Southwest
Wind Speed: 10 to 15 mph around 10 mph 10 to 15 mph
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy Mostly cloudy Mostly cloudy
Temperatures: 35 to 40 deg. F. 27 to 32 deg. F. 37 to 43 deg. F.
Wind Direction: West Southwest Southwest
Wind Speed: 10 to 15 mph 10 to 15 mph with gusts to 35 mph. 20 to 25 mph with gusts to 50 mph.
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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