Avalanche Advisory: Saturday - Dec 1, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 2, 2018 @ 6:39 am
Avalanche Advisory published on December 1, 2018 @ 6:39 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

As 4-10” of new snow falls throughout the morning and into the afternoon accompanied by strong SW winds, human triggered wind slab avalanches will become increasingly likely, and natural avalanches possible on the leeward side of ridges, cross loaded gullies and around other features that promote drifting at mid to upper elevations.  Evaluate snow and terrain carefully.  Early season obstacles exist!

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Avalanche Problem 1: Wind Slab
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Wind slabs sensitive to human triggering will continue forming today on mid to upper elevation exposed leeward slopes (SE-E-N-NW facing) as 4-8” of new snow is expected accompanied by moderate to strong SW winds.   Wind slabs began forming in the same types of areas yesterday as the result of moderate SW winds transporting some of the 1-3’ of light new snow that fell Wednesday thru Thursday.  Be on the lookout for denser fresh wind deposits, and use clues such as blowing snow and cornices to determine where these may be.  Do your own localized assessments. 

Avalanche Problem 2: Storm Slab
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In sheltered areas that receive greater amounts of new snow isolated low density storm slabs may become possible for humans to trigger on steep slopes greater than about 35 degrees.  This will be for all aspects and elevations, and will become a concern only for areas that meet or exceed the upper end of the forecasted new snow amounts.   

advisory discussion

Winter has begun! Before Thanksgiving the mountains from Virginia Lakes to Bishop were bare of snow.  Now there is solid white down below 7000’, with 1-3’+ of coverage above 8000’.  Not counting a mid-October storm which brought a couple inches of snow to high elevations which then completely melted away, Thanksgiving was the first real storm of the season.  Dropping a foot+ of snow above 9000’, it paved the way for this week’s Wednesday-Thursday storm which dropped another 2-3’ of snow, with highest amounts recorded around Tioga Pass.  Snow levels for this storm reached down near 6000’.  While the coverage is still thin in the back-country, people have been getting out with their skins and finding some blissful turns in between the occasional rock scrape and bash.  

Now for talk on the stability of this new snowpack:  Local entities around the Mammoth area reported sensitive storm slab conditions on Thursday morning in steeper mid to upper elevation terrain.  Since then Friday brought moderate SW winds to the Mammoth area south toward Bishop for the mid to upper elevations.  Significant snow banners could be seen blowing off the higher peaks along HWY 395 until late afternoon on Friday  (click here for picture), after which the winds died down considerably.  Very fresh evidence of small cornice collapses resulting in small wind slab failures were found just below the ridgetop of the Sherwins on Northerly facing slopes yesterday (Friday) evening (click here for observation).  

Much in contrast to last season, the underlying snowpack to the ground this season at this time does not have suspect persistent weak layers lurking, since all of the snow on the ground has fallen so recently.  Our biggest avalanche concern for today will be surface instabilities in the form of wind slabs at mid to upper elevations on the leeward sides of ridges and cross loaded steep exposed slopes.  These slabs will become more widespread and concerning throughout the day as SW winds pick up and 4-10” of additional snow falls throughout the day.  

And again, the coverage is thin!  Rocks and tree stumps and logs are lurking just under the surface.  Don’t end your season with an injury before it really even begins!

recent observations
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-11/30 - Sherwins - Recent snow and natural small cornice collapse triggering small wind slab.

-11/30 - Afternoon banners and wind transporting new snow visible from 395 between Mammoth and Bishop:

-11/30 - June area - Some very small sensitive wind slabs just below upper elevation ridges.  Cold and light winds.

-11/29 - Mammoth area - New snow and sensitive morning storm slabs

Weather and CURRENT CONDITIONS
weather summary

A Winter Weather Advisory exists from now until 3pm from Rock Creek and to the north.  A cold fast moving storm will bring fresh snow today (4-10” to the mountains, much less south towards Bishop). Heaviest snow is expected in the morning beginning after 7am, and then light snow showers in the afternoon. This will be accompanied by strong SW winds with gusts into the 70s at upper elevations.  Temperatures are expected to reach the mid 20s below 10,000’, and the mid teens above 10,000’, and then drop into the single digits tonight.  These cold temperatures will keep snow levels down to 5000'.  

For Sunday expect sunny skies, continued cold temperatures, and lighter winds.  A chance for more snow returns on Wednesday.

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. Snow developing in the morning. Partly cloudy. Scattered evening snow showers, then isolated snow showers after midnight. Sunny.
Temperatures: 21 to 27 deg. F. 6 to 12 deg. F. 18 to 24 deg. F.
Mid Slope Winds: Southwest West North
Expected snowfall: 80% probability of 4 to 8 inches. 20% probability of 8 to 12 inches. in. 70% probability up to 1 inch. 30% probability of 1 to 3 inches. in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy. Snow developing in the morning. Partly cloudy. Scattered evening snow showers, then isolated snow showers after midnight. Sunny.
Temperatures: 14 to 24 deg. F. 3 to 8 deg. F. 11 to 16 deg. F.
Ridge Top Winds: Southwest West Northwest
Expected snowfall: 80% probability of 5 to 10 inches. 20% probability of 10 to 14 inches. in. 70% probability up to 1 inch. 30% probability of 1 to 3 inches. 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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