Avalanche Advisory: Sunday - Dec 9, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON December 11, 2018 @ 6:16 am
Avalanche Advisory published on December 9, 2018 @ 6:16 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

S to SW winds continued to blow through the night.  Human triggered wind slab avalanches are likely on exposed SE-E-N-NW facing terrain on the leeward side of ridges, cross loaded gullies and around other features that promote drifting at upper elevations. Be on the lookout for freshly loaded pillows and ribbons of snow and treat steeper complex terrain carefully.  Early season obstacles exist!

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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Mid to upper elevation winds increased Friday night and continued through Saturday for the first time since up to 8” of light snow fell earlier this week.  The low density snow combined with wind speeds of 25-30 is optimal for loading leeward slopes.  On Saturday there were several skier triggered avalanches both in the backcountry and inbounds that occurred in constrictive chutes that in some cases ran long distances. (See attached Observations).  Several of the wind slabs found by skiers yesterday were triggered part way down the couloir they were skiing-- not directly at the entrance of the feature they skiied. The old snow surface is a mix of wind board, near surface facets and in some places potentially some surface hoar. Fresh, firm wind packed slabs deposited on these surfaces are often are just waiting for a trigger, and that was evident by yesterday’s events.  Avoid skiing above terrain traps and pick your lines wisely as conditions have changed from a few days ago when conditions were more benign.

Avalanche Problem 2: Cornice
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The recent winds from the S and SW have loaded leeward slopes SE-E-N-NW and have started forming cornices at ridgeline above what was some of the most favorable skiing at this point in regards to overall coverage. Although cornices are neither that extensive, nor sizeable yet, they are likely to be sensitive as they are newly formed. Pay particular attention to these features that form right adjacent to ridgeline and look like a suspended wave.  Make sure you’re always on terra firma when evaluating conditions as cornices are one of the greatest dangers in the mountains.

advisory discussion

With an underlying very stable snowpack, don’t be lulled into complacency with the new surface wind slabs that started developing Friday night and through Saturday. Although the amount of snow available for transport will start drying up, there will still be plenty of opportunity for wind slabs to develop in the steeper leeward alpine terrain as S-SW winds are forecasted to continue to blow. There also may still be a bit of slope warming and potential for loose wet snow to become unstable on sunny aspects, but the increasing clouds should moderate this issue.  The avalanche incidents that occurred Saturday are clear warning that even when skiing in-bounds wearing a beacon is a good idea and paying attention to your surroundings is imperative in regards to people skiing above or below you. It’s also a good reminder that the winter season is young and as most of us are super anxious to start getting some turns, don’t let the skier fever cloud your judgment and wise terrain choice selection. Early season obstacles exist!  Plenty of rocks, logs, and tree stumps are lurking just under the surface, which has led to injury and broken equipment already.  Take your time, be careful, and don’t end your season before it barely begins! 

recent observations
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12/8 -Mammoth Area - Mammoth Mountain Ski Area - skier triggered windslab avalanche

12/8 - Mammoth Area - Mammoth crest - windslabs reactive to skier trigger

12/8 - South Fork Bishop Creek-South Lake TH - conditions, blowing snow in the alpine

Weather and CURRENT CONDITIONS
weather summary

Increasing clouds are on tap for today. Temperatures are expected to remain quite warm at mid elevations in the 36-41F range and a bit cooler in the upper elevations at 31-36F. The biggest driver concerning avalanches and wind slab development will be the continuation of unabated SW Winds that will be blowing at perfect speeds to transport loose snow. 10-15mph with gusts in the 30’s are expected at mid-elevations with 15-25mph winds with gusts of 40 in the upper elevations. SW winds are expected to continue to blow and gust up to even higher speeds (50's) through tonight  into Monday.  Some snow flurries may occur Monday through Monday night as the weak low pressure system passes through the area.  Be aware of potential cold  wind chill values (in the negatives) in the mountains Monday if your out and about. Sunnier skies with continued, but lighter winds are expected on Tuesday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy Mostly cloudy then becoming partly cloudy Partly cloudy. Slight chance of snow in the afternoon
Temperatures: 36 to 41 deg. F. 16 to 22 deg. F. 31 to 37 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy then becoming mostly cloudy Mostly cloudy then becoming partly cloudy Partly cloudy. Slight chance of snow in the afternoon.
Temperatures: 31 to 36 deg. F. 13 to 18 deg. F. 25 to 30 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
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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