Eastern Sierra Avalanche Advisory - 1/5/18

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THIS AVALANCHE ADVISORY EXPIRED ON January 6, 2018 @ 7:00 am
Avalanche Advisory published on January 5, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Natural and human triggered avalanches remain unlikely, but not impossible for today (Friday).  The primary avalanche concern is for isolated small new wind slabs at upper elevations on E-NE-N slopes that could have formed in the most favorable of locations from up to 1 inch of new snow and strong SW winds.  The secondary avalanche concern is the patch-work of old hard stubborn shallow wind slabs 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. 

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Avalanche Problem 1: Wind Slab
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Up to an inch of new snow fell since early yesterday morning at elevations above 9500’, with strong SW winds.  Loose snow was seen blowing over high ridges yesterday afternoon over 11,000’.  With up to an additional inch of snow forecasted today, It is possible that in limited isolated areas, this meager amount of new snow could be deposited by the wind into some small wind slabs that could be sensitive to human triggering.  These could possibly be found on E-NE-N facing slopes below ridgetops, the leeward side of gullies, or around other features that promote drifting.  If you are venturing into high alpine terrain, be on the lookout for small smooth dense patches of new snow, check how deep the deposit is, and use your own stability assessments before venturing onto them in steep terrain.

Avalanche Problem 2: 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.  This problem could likely become much more active in the near future with a significant additional snow load. 

advisory discussion

The Sierra snowpack remains thin a variable.  Our last meager snowfall before the trace amounts that fell over the past day 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.  As the pattern change brings in the potential for significant snowfall, these facet layers both below the surface and at the surface could likely become active weak layers that sizeable avalanches could fail on.  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. 

Weather and CURRENT CONDITIONS
weather summary

Change is upon us, the high and dry spell has been broken!  Unsettled weather is expected into the weekend, and then early next week potential is increasing for a storm that could actually be measured in feet! 

For today (Friday), expect cloudy skies, warm temperatures, breezy SW winds, and scattered precipitation as a low-pressure system slides down toward the Sierra from off the coast of the Pacific Northwest.  This low pressure should bring some moisture to the area for Friday night into Saturday, with forecasts calling for 2-5” of snow possible above 8500’. 

Long-Term:  The exciting news is the storm which forecasters are gaining confidence with that is looking likely to drop 1-2” of liquid (=1-2ft+ of snow at higher elevations) over our area Monday night thru Tuesday.  This storm has a much more organized appearance, and will track further south than the previous small storms we have had this season, and will be centered more over Mono County.  It looks to start warm, but snowline has the potential to drop below 7000’ by Tuesday evening. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy then becoming mostly cloudy. Isolated snow showers. Mostly cloudy. Scattered showers. Mostly cloudy. Scattered showers in the morning, then chance of showers in the afternoon.
Temperatures: 41 to 51 deg. F. 28 to 34 deg. F. 38 to 46 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 15 mph with gusts to 50 mph 15 to 25 mph. Gusts up to 50 mph increasing to 60 mph after midnight 15 to 25 mph. Gusts up to 65 mph decreasing to 50 mph in the afternoon
Expected snowfall: up to 1 in. up to 2 in. 1-2 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy then becoming mostly cloudy. Isolated snow showers. Mostly cloudy. Scattered showers. Mostly cloudy. Scattered showers in the morning, then chance of showers in the afternoon.
Temperatures: 32 to 38 deg. F. 24 to 29 deg. F. 31 to 36 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 25 to 35 mph with gusts to 60 mph 30 to 40 mph with gusts to 65 mph increasing to 30 to 50 mph with gusts to 75 mph after midnight 35 to 55 mph with gusts to 75 mph decreasing to 30 to 45 mph with gusts to 65 mph in the afternoon
Expected snowfall: up to 1 in. up to 3 in. 1-2 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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