Eastern Sierra Avalanche Advisory - 12/30/17

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THIS AVALANCHE ADVISORY EXPIRED ON January 2, 2018 @ 6:50 am
Avalanche Advisory published on December 30, 2017 @ 6:50 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

The primary avalanche concern thru Monday will focus on stubborn isolated persistent Winds Slabs near and above treeline on W-N-E aspects where localized failures may be possible in steeper terrain, especially where wind slabs overlay facets and are not well supported. Though unlikely, a slide could conceivably entrain a rider and carry them over or 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.

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

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Near Treeline

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Below Treeline
Avalanche Problem 1: Persistent Slab
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Primary concern during the forecast period will focus on stubborn isolated persistent Wind Slabs that formed when a weak system moved through the region Wednesday (12/21) with 1 to 4” of snow reported. The system was accompanied by moderate southwesterly winds that formed Wind Slabs on W-N-E aspects, near and above treeline. 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 helped form widespread weak faceted (sugar) snow in the upper snowpack, especially northerly facing aspects, which slows the normal bonding and strengthening processes. Stubborn isolated Wind Slabs may be encountered on steeper windloaded terrain, or where Wind Slabs are laid over facets and not well supported. Typically, wind slabs form below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around firm, hollow sounding slabs on steeper terrain. Though a slide is unlikely, a small release could conceivably entrain a rider and carry them over and into hazardous terrain with potentially high consequences. Below ~9,000’- below threshold, natural and triggered releases are unlikely due to little or no snow. 

advisory discussion

Dry and dusty conditions have mostly prevailed this month due to a high pressure ridge that has remained parked off the California coast and deflecting storms systems well to the north of the forecast region. Finally, the pattern appears to be changing with the ridge beginning to flatten and signs of eastward progression. The combination of an extended dry spell, shallow snowpack, and cool nightly temperatures has produced strong faceting of the upper snowpack, especially northerly aspects, producing sugary weak snow throughout the upper snowpack.  

The last storm to move through the region was a weak system on 12/21. The storm deposited ~ 1” to 4” of snow, primarily from Mammoth to Virginia Lakes and was accompanied by moderate to strong winds, depositing dense Wind Slabs on top of weak faceted snow. Normally, Wind Slabs tend to bond to the underlying snow within a few hours to ~couple of days. However, the widespread weak faceted (sugar) snow in the upper snowpack, especially on northerly facing aspects, has slowed the normal bonding and strengthening processes and maintains the potential for small isolated Wind Slabs to persist. 

The snowpack in the Sierras remains thin and mostly confined to elevations above ~9,000 around Mammoth, possibly higher elsewhere. The snowpack near and above treeline alternates between soft facts in sheltered areas, wind stripped, wind deposited areas, and melt / freeze patches. The recent mid-day warming has produced a Melt/Freeze crust of varying supportiveness on solar aspects. 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. Early season conditions exist with numerous obstacles and many hazards just under the snow surface. As the winter progresses, the facets in the upper snowpack may form a weak layer, once buried.

 

Weather and CURRENT CONDITIONS
weather summary

Sat thru Monday - a "dirty" ridge (mainly high clouds) of high pressure remains in control of northeast California with tempersatures reaching the 40s to low 50s with valley inversions in the 20s to mid 30s. The shallow inversions are currently expected to hold as winds aloft fall off this afternoon as the upper ridge strengthens. Lighter winds aloft (less mixing) Sunday and Monday, as well as increased mid and/or high cloud cover will bring a bit cooler temperatures for many areas.

Tuesday through Friday - models are coming into better agreement as the GFS slows to more closely resemble the ECMWF solution. Both of these models start to bring the first in a series of waves into the region on Wednesday with light precipitation developing early Wednesday then becoming heavier and more widespread Wednesday night into Thursday with fairly high snow levels (7000-8000 feet’). Total liquid equivalent precipitation in the Sierra could top 1 inch between Wednesday afternoon and early Thursday evening before a brief break develops. A secondary wave brings a return to precipitation Friday into early Saturday but the timing and precipitation amounts are in question this far out in time.

 

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: Partly cloudy. Partly cloudy. Partly cloudy then becoming mostly cloudy.
Temperatures: 49 to 57 deg. F. 27 to 33 deg. F. 44 to 52 deg. F.
Wind Direction: Southwest West
Wind Speed: 10 to 15 mph with gusts to 40 mph. 10 to 15 mph with gusts to 30 mph in the evening becoming light. Light winds.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy. Partly cloudy. Partly cloudy then becoming mostly cloudy.
Temperatures: 42 to 48 deg. F. 23 to 29 deg. F. 37 to 43 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 20 to 35 mph with gusts to 50 mph. 15 to 25 mph. Gusts up to 35 mph decreasing to 25 mph after midnight. 10 to 15 mph. Gusts up to 25 mph in the morning.
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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