Eastern Sierra Avalanche Advisory - 1/4/18

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THIS AVALANCHE ADVISORY EXPIRED ON January 5, 2018 @ 7:00 am
Avalanche Advisory published on January 4, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Natural and human triggered avalanches remain unlikely, but not impossible for Thursday.  A patch-work of old hard stubborn shallow wind slabs can be found resting on top of weaker faceted snow at and above tree line on non-southerly facing slopes.  In isolated areas in steep terrain, it is conceivable that a human could trigger one of these slabs.  The light snow forecast for today combined with moderate winds from the south could create small isolated sensitive wind slabs on the leeward side of upper elevation ridges and gullies, especially in areas where the 1” forecast is exceeded.     

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Near Treeline

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Below Treeline
Avalanche Problem 1: Persistent Slab
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A patch-work of old hard stubborn shallow wind slabs can be found resting on top of weaker faceted snow at and above tree line on non-southerly facing slopes.  Although it remains very difficult for a human to trigger, recent snow pack column tests have shown that this combination is reactive in many areas.  The hardness and strength of these overlying slabs have been effectively bridging themselves for the most part, but it is conceivable that a rider could find a weak spot where the slab is thinner and it is unsupported down slope, where an avalanche could be triggered.  These sensitive spots are very isolated and most likely to be found in steep terrain.  Use caution, do your own slope assessments, and realize that variability across slopes can be very great. 

Avalanche Problem 2: Wind Slab
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With an inch of new snow forecasted for today with moderate winds from the south, it is possible that small isolated new wind slabs could form on the leeward side of upper elevation ridges, sidewalls of gullies, and around other features that promote drifting, especially in areas where the snowfall forecast is exceeded.  Pay attention for fresh smooth dense snow.   Even a small avalanche in steep terrain could result in injury by knocking a rider off balance and sending them into a nasty fall, especially given the widespread firm conditions that exist above tree line. 

advisory discussion

The Sierra snowpack remains thin a variable.  Our last meager snowfall occurred 2 weeks ago.  Dry clear conditions have been promoting facet formation in the upper part of the snowpack.  In sheltered north facing areas this has meant some patches of nice soft snow, and in wind affected areas this has meant the continual development of a weak sugary layer underneath firm wind slabs.  This dense wind slab over persistent weak facet combination is where our limited avalanche concern has existed for the past 2 weeks.  Well above normal temperatures have been leading to melt-freeze crust formations even on north facing slopes up to 9,500’.  Southern slopes are a patchwork of thin snow at best, if not bare.  Below 8,500’ around Mammoth there is not enough snow to ride on, and this snow line is  higher north, and dramatically higher further south.  With the final breakdown of the Ridiculously Resilient High Pressure Ridge that has begun, when significant new snow load occurs the widespread facet formation that has been forming could most likely become a much more active weak layer for larger avalanches to fail on. 

Weather and CURRENT CONDITIONS
weather summary

The high pressure ridge appears to have broken.  While this doesn’t automatically bring in Atmospheric Rivers, at least it is a start!  For Thursday expect cloudy conditions with scattered snow flurries, with chances of up to an inch of accumulation, slightly cooler temperatures with highs expected in the mid 30s at 10,000’, and moderate winds from the south.  Another chance of slightly greater amounts of precipitation exists for Friday night into Saturday, with 1-5” possible at higher elevations.

Long term: Models are showing greater probability of a low pressure system moving in from off shore during the period of Monday thru Tuesday evening, bringing the possibility of 0.6-0.75” of water into the Sierra, which could mean between 5-10” of snow at higher elevations.  Snow levels are expected to start high above 7500’, and drop down to 6000-7000’ by Tuesday night. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Chance of snow and rain in the morning. Isolated snow showers in the afternoon. Partly cloudy then becoming mostly cloudy. Mostly cloudy. Isolated snow showers.
Temperatures: 40 to 50 deg. F. 28 to 33 deg. F. 40 to 48 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 45 mph 10 to 15 mph. Gusts up to 35 mph increasing to 50 mph after midnight 10 to 15 mph. Gusts up to 55 mph decreasing to 45 mph in the afternoon
Expected snowfall: up to 1 in. 0 in. up to 1 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Chance of snow in the morning, then isolated snow showers in the afternoon Partly cloudy then becoming mostly cloudy. Mostly cloudy. Isolated snow showers.
Temperatures: 31 to 37 deg. F. 23 to 29 deg. F. 32 to 38 deg. F.
Wind Direction: South South Southwest
Wind Speed: 20 to 35 mph. Gusts up to 60 mph decreasing to 50 mph in the afternoon 20 to 30 mph with gusts to 45 mph increasing to southwest 30 to 40 mph with gusts to 65 mph after midnight 30 to 45 mph with gusts to 70 mph decreasing to 25 to 35 mph with gusts to 60 mph in the afternoon.
Expected snowfall: up to 1 in. 0 in. up to 1 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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