Avalanche Advisory: Tuesday - Dec 11, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 12, 2018 @ 6:45 am
Avalanche Advisory published on December 11, 2018 @ 6:45 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

With decreased winds and sunny skies today, stubborn small wind slabs that formed over the past few days with lingering sensitivity will be the greatest avalanche concern on upper elevation SE-E-NE-N-NW facing slopes.  Sunny facing slopes, especially SE facing, that have been wind loaded will have the additional concern of small wet slabs as they warm in the late morning from the sun.  While natural avalanches will be unlikely, human triggered avalanches will be possible.   

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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SW winds over the last 3 days have resulted in skier triggered and naturally triggered wind slab avalanches at upper elevation SE-E-NE-N-NW facing slopes.  While these have been small (4-10” crown heights), they have been the entire width of gullies and chutes.  With much lighter winds today out of the NE, these older wind slabs with lingering sensitivity will trump the concern over fresh new wind slab formation.  Be wary of firm hollow sounding snow, and use clues such as cornice formation and ridges in the snow surface to indicate where these slabs may be. Realize that today they will be more stubborn and harder to trigger.  While they may not be large enough to bury a person unless a terrain trap is involved, they could result in a bad fall.  Read the discussion below for talk about the potential weak layers these slabs may be sitting on, and do your own localized assessments to help determine how well they are bonding before committing to potentially wind loaded steep terrain. 

Avalanche Problem 2: Wet Slab
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Light NE winds and sunny skies will result in slopes facing the sun to heat up today, especially near rock bands.  Small loose wet slides will become increasingly likely as slopes become more directly exposed to the sun, from the E to SE to S to SW as the day progresses.  Where fresh wind deposits exist, a larger wet slab avalanche could result from this warming.  The combination of the past day's fresh wind-loading and where sun will warm slopes for the first time today mean that SE facing slopes will be the greatest concern for a wet slab in the late morning.  Watch for warning signs such as the snow becoming wet and less supportable and small roller balls to indicate slopes are warming and becoming less stable, and should be avoided.    

advisory discussion

The last 3 days have been consistently filled with new reports of small skier triggered and natural wind slab avalanches occurring at upper elevations.  Up to 8” of light snow fell last Tuesday and Wednesday with calm winds, then SW winds optimal for snow transport began Friday night and gradually increased through the weekend and into the day yesterday with gusts into the 50mph range.  Even though these winds didn’t create the dramatic snow banners over ridgelines that we often see in the Sierra, they resulted in widespread wind slab development at upper elevations.  As lighter NE winds are expected thru this morning before dying down even more this afternoon, new wind slab development will become less likely. While the wind slabs that developed over the past few days will become harder and less likely to trigger, they will remain a concern at least thru today, especially with the unknown that some may be sitting on a weaker faceted snow layer or possibly even isolated areas of fragile surface hoar.  Take the time to do many small test pits to see for yourself if there are underlying weak layers and to see how these surface wind slabs are bonding. Remember that variability across slopes can be great!  Just because one spot looks good, doesn’t mean everywhere on the slope is good.  When in doubt, play it safe and choose less consequential terrain.  

Weather and CURRENT CONDITIONS
weather summary

Sunny skies, lighter winds out of the NE and warmer temperatures into the mid 30s above 10,000’ are expected today as a weak low-pressure system to our north moves away. Winds and clouds will increase again tonight thru Wednesday as another weak low-pressure system moves inland to our north, with no new snow expected for us.  However another weak low-pressure system Friday could bring some light snowfall to our area. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Partly cloudy Partly cloudy
Temperatures: 37 to 42 deg. F. 16 to 21 deg. F. 32 to 37 deg. F.
Mid Slope Winds: Light winds becoming southwest Northwest
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Partly cloudy Partly cloudy
Temperatures: 34 to 39 deg. F. 14 to 19 deg. F. 27 to 32 deg. F.
Ridge Top Winds: Northeast West Northwest
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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