Avalanche Advisory: Saturday - Dec 29, 2018

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THIS AVALANCHE ADVISORY EXPIRED ON December 30, 2018 @ 6:32 am
Avalanche Advisory published on December 29, 2018 @ 6:32 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

Natural and human triggered avalanches will be unlikely today. An important fact to consider is the strong to EXTREME prevailing N-NE winds the past few days that have been transporting recent low density (2-4”) snowfall.  Be wary of fresh wind slabs in extreme terrain (NW-W-SW-S-SE) that are deposited from these N-NE winds. Follow solid backcountry travel practices and always be on the lookout for unstable snow during your outing.   Utilize small, benign test slopes to investigate how newly loaded snow is reacting.  Be aware of  thin early season snowpack conditions.

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Below Treeline
Avalanche Problem 1: Wind Slab
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110mph winds from the NE were recorded on the evening of 12/27 on the top of Mammoth Mtn and continued without relent in the 60-70mph range through Friday stretching into this morning with decreasing velocities.  The N-NE aspects are unfortunately getting stripped from the current wind direction, and the blowing snow is either being sublimated in the atmosphere, or being deposited on NW-W-S-E aspects. Recent low density snow (2-4”) combined with the Christmas Eve (5-8”) snowfall has provided available snow for transport, and visible bannering of snow from peaks has been observed in the forecast area . Sunny (W-SW-S-SE) aspects have had minimal to no snow present this season, but there has been a few opportunities for skiing in areas such as the south side of Dunderberg Peak in Virgina Lakes. Skiers that find themselves so inclined to venture on to (W-S-SE) terrain should be on the lookout for recent wind pillows, ribbons, and freshly loaded looking panels of snow. Although there is likely not enough deposited snow to develop significant avalanche formation, even a small slide in committing terrain could be hazardous with all the exposure to rock present right now.  Topography such as gullys adjacent to arêtes/ridges and channels existing in the S and W facing terrain could be building fresh wind slab with the recent Northerly wind patterns.

 

advisory discussion

Despite thin conditions out there, the recent addition of small amounts of cold low density snow has made for good conservative turns where there is enough shelter and coverage to ski. Controlling the desire to cut loose, and ski with abandon is the hardest part, as a lot of terrain looks good, but in reality there are a significant amount of lurking snow snakes just under the surface.  Yesterday there was a large amount of 3-5mm surface hoar residing in the sheltered areas of Mammoth Bowl (see 12/28 Mammoth Bowl observation).  It will be good to track these feathery crystals to see if they hang around or get destroyed in the near future.  Surface Hoar can be one of the biggest drivers of avalanche formation when it remains undisturbed and is buried by an overlying slab.

The 9300-10000ft elevations in the Sherwins have around 60-70cm of snow at their maximum, whereas areas around the Mammoth crest 10000-11000ft hold the deepest snow from 120-160cm.  Snow depths drastically decrease as you descend below 9000ft and 15-30cm snow depth is standard in the low lying areas.  Lack of much snow in the southern part of the forecast area (Bishop Creek) has led to not much skiing nor observations. Snow depth totals in the northern end of the zone (Virginia Lakes)  area are still holding on average 60-90cm in depth and are a mixed bag of faceting layers and hard thin wind slabs (see 12/26 Virginia Lks observation). Snowpack depths in general are quite thin, and this combined with residing cold temperatures are driving strong temperature gradients across the snow (see 12/28 Sherwins temp profile observation). These two conditions are leading to more faceting and metamorphism of the residing snow on the ground. The faceting process is essentially making the snowpack much weaker than it was at the beginning of the month in some areas, but it also has had the benefit on other slopes of starting to break down and decompose some of the existing spatially variable wind slabs that developed during December. We will have to see how it all sorts out when we get the next big load of snow—something were all hoping will happen soon.

 

Weather and CURRENT CONDITIONS
weather summary

Milder conditions are on tap today with warmer temperatures (low-mid 30sF), a bit less wind, and sunny skies. The north wind will continue today, albeit at slower speeds (20-30’s gusting to 50mph in the upper elevations).  It’s been chilly lately!  It was 12F yesterday at 1230pm, 9380ft and the wind on top of the Sherwin ridge was cutter. Temperatures remained cold Friday and the northerly wind blew continually all day in the upper elevations. In the short term, another cold front will be moving our way Sunday night with slight chance of snow showers and again cold temperatures and hazardous wind chill values for New Year’s Eve. Weather models also continue to show potential for more active weather moving into the region by next weekend.

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: Sunny Clear Partly cloudy
Temperatures: 35 to 40. deg. F. 19 to 24. deg. F. 38 to 44. deg. F.
Mid Slope Winds: North 10 to 15 mph with gusts to 40 mph in the morning becoming light. Light winds becoming west 10 to 15 mph after midnight. Gusts up to 35 mph. Northwest 10 to 15 mph with gusts to 40 mph.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Sunny Clear Partly cloudy
Temperatures: 31 to 36. deg. F. 16 to 21. deg. F. 31 to 36. deg. F.
Ridge Top Winds: Northeast 20 to 35 mph with gusts to 50 mph. North 15 to 30 mph with gusts to 45 mph. Northwest 20 to 35 mph with gusts to 50 mph.
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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