Mammoth Basin Snowpack Summary - 2015-12-08 09:54

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THIS ADVISORY EXPIRED ON December 10, 2015 @ 9:54 pm
Avalanche Advisory published on December 8, 2015 @ 9:54 pm
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Most of the Eastern Sierra areas are below threshold (too little snow to cover the ground anchors) with the exception of the: Mammoth Lakes Basin, near San Joaquin Ridge, Virginia Lakes, and some of the higher peaks in the range. Snow conditions are variable on most aspects with the most consistent soft snow on northerly protected aspects. Elsewhere, conditions are a mix of soft snow in shaded areas mixed with melt freeze crusts with varying strength and supportability. The most supportive crusts are on Southeast to Soutwest aspects. On northerly and leeward slopes of 35 degrees and steeper, there's the potential for isolated unstable pockets and wind slabs from Sunday's wind event. Given the avalanche fatality in November, areas with significant snow depth are highly suspect and should be treated with caution. The Summary will be updated Thursday AM.

Snowpack Discussion

The Eastern Sierra Avalanche Center is officially operational for the 2015 - 2016 season with the forecasting team in place. The Center's board has hired Josh Feinberg and myself (Doug Lewis) to staff the Center and provide the backcountry community with: snowpack summaries, avalanche observations, manage the center's daily operations, community outreach and avalanche safety programs. We will be posting Snowpack Summaries three times a week (Tues, Thurs, and Sat) for the month of December and adding a fourth Snowpack Summary in January as the season ramps up further. We encourage the backcountry community to help us provide the most accurate and complete regional snow assessment by posting, whenever possible: on the community forum, submitting snow and avalanche observations, or incident accounts. All submissions are appreciated and encouraged.

A special shout-out: I’d like to  thank everyone who attended the Season Kickoff event last weekend. The Avalanche Rescue and Terrain Assessment clinics were very successful and well attended. Also a special Thank You to Todd Offenbacher, who was most highly entertaining. Best of all we raised record amounts for the future of the new ESAC.

Snowpack and Weather History: Limited snow and thus snow data available currently but based on the Mammoth Pass Snowtel site the first significant snowfall occurred on November 3rd, depositing 16". This storm came in cold with temps as low as 9 degrees but as the system cleared temps quickly rebounded with highs reaching the 50's. This system helped break down the high pressure ridge dominating the region and opened the door for more weather systems to move in.  The next storm to visit the area was on November 10th depositing 17"; over two days with temps dipping down to as low as 11 degrees but rising rapidly into the low 50's as high pressure settled in. High pressure dominated the weather cycle until mid-month when the next system came through on November 16th depositing 6" of snow with lows in the teens and highs into the 50's. From Nov. 25th thru the 28th, an additional 12" was deposited with more winter like temps, lows were in the single digits and highs only in the teens. Temps have slowly climbed since then but remain closer to seasonable. The most recent system moved in on the December 2nd depositing only 2" of snow, followed by strong westerly winds. To date the region is 70 percent of normal to date.

recent observations

The warm post-storm temperatures have created extensive supportable sun crusts on southerly aspects at mid to lower elevations. Strong Westerly winds Sunday scoured many westerly aspects, while forming isolated drifted pockets and wind slabs on leeward slopes in the upper elevations on top of decomposing grains mixed with graupel.

A series of stability tests above Mammoth Pass show moderate to hard failures in the upper snowpack on a 20 and 27 degree slopes approximately 20 cm deep from surface and are consistent with sudden collapse failure. Additionally, we encountered 2mm facets on a melt freeze crust at the ground interface which could act as a bed surface for slides initiating in the upper snowpack and possibly stepping down to this layer.

Overall the snow pack is shallow and variable, primarily consisting of facets at or near the surface that may not support the anticipated snowfall from the approaching storm system, which may produce as much as 2 feet of new snow with high winds.

weather

Wednesday - winds will begin to increase, especially in higher elevations, to 55 to 70 mph while temperatures remain well above normal. A strong storm system is on track for Wednesday night and Thursday with very windy conditions, colder temperatures, and significant snow forecast for the Sierras. Anticipating high wind potential with gusts near 150 mph possible for the highest peaks over 13,000 feet. Many lower elevations will receive a period of moderate to heavy rainfall, changing to snow by late Thursday afternoon or evening.

Precipitation - snow levels to remain rather high through Wed night (8000-9000 feet). Thursday morning precipitation is forecasted to increase while snow levels will continue to fall through the day.  The National Weather Service has issued a Winter Storm Warning for Mono County with 1-2 new feet of new snow forecasted for the Crest and surrounding mountains. Thursday night - snow levels will continue to drop to most valley floors with additional moderate accumulations likely in the Sierra as a secondary shortwave moves into the re

Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.

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