Avalanche Advisory: Friday - Dec 21, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2018 @ 5:53 am
Avalanche Advisory published on December 21, 2018 @ 5:53 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

Human triggered wind slab avalanches will be possible today in isolated areas and extreme terrain on SE-E-N-NW facing slopes. This morning’s new snowfall (1-2") combined with SW winds 40-60mph and associated gale force gusts (potentially upwards of 95mph) will cause sporadic loading patterns at all elevations. Although avalanches may be small in size, that doesn’t mean they are not a danger when traveling in exposed terrain. Evaluate snow and terrain carefully, especially convex rolls and unsupported slopes.  Early season obstacles exist!

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Avalanche Problem 1: Wind Slab
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Winds were consistently in the mid 40 to 50mph range from the S on Thursday and were visibly moving snow throughout the day. S/SW winds will blow today and are forecasted to gust up to 95mph. These high winds will redistribute the new snowfall that started this morning on lee slopes SE-E-N-NW where topography is conducive to capturing or slowing down blowing snow. Seek out low-consequence test slopes to see if newly loaded snow is sensitive and pay attention to hollow sounding snow or shooting cracks when you’re traveling. Identify features of concern today especially terrain traps, or convex loaded slopes that are above exposed rock, cliff or trees.

advisory discussion

Overall snow depths have been decreasing with our continued high winds, warmer temperatures and settlement during our prolonged dry period. The paramount danger out there right now is thin and variable conditions that will be just barely covered with todays light accumulations. Areas around the Mammoth crest and specific sheltered NE high alpine couloir features hold the deepest snow from 120-160cm while lower terrain such as the Sherwins have around 60-70cm of snow at their maximum.  Snow depths drastically decrease as you descend and 15-30cm snow depth is standard in the low lying areas.  Although I found softer non-wind affected snow up high on a NE aspect Wednesday (see 12/19 June Area observation) it doesn’t mean the  winds Thursday and Friday will not start wind packing transportable snow and forming wind slab in isolated areas. Overall stability is good right now and we have not observed any persistent weak layers, but there are thinner shields of faceting alpine snow, spatially variable deposits of wind board, and some weakening of basal snow. These traits all could lead to future dynamic or sensitive behavior when we get that next bigger snow event or loading.

Weather and CURRENT CONDITIONS
weather summary

Partly cloudy conditions will exist today and S-SW winds will continue to blow in the 40 to 60 mph range with gusts possibly in the mid 90s. Several inches of new snow have fallen already this early morning with stormy conditions. Friday's day-time temperatures are expected to remain mild and will be above freezing, 37-42F at mid elevations and 32-37F at upper elevations.  Snowfall is expected to cease and winds are forecasted to abate a bit this afternoon to the 20-30mph range with a calmer day and drier conditions expected for Saturday.  The storm for Christmas Eve is still looking promising with possible 6-15” of snow on the crest and 5-6” in the valley bottoms. The overall amount of water in the storm is not expected to be that great, hence forecasters are not calling for a true Sierra dump at this point.

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. Slight chance of snow showers. Snow levels below 7000 feet increasing to 7500 feet in the afternoon. Chance of precipitation is 10%. Partly cloudy Partly cloudy
Temperatures: 37 to 42. deg. F. 20 to 25. deg. F. 37 to 42. deg. F.
Mid Slope Winds: Southwest 15 to 25 mph. Gusts up to 65 mph decreasing to 40 mph in the afternoon. Light winds. Gusts up to 25 mph in the evening. Light winds. Gusts up to 25 mph in the morning.
Expected snowfall: Up to 1 inch. in. 0 in. 0 in.
Over 10,000 ft.
Friday Friday Night Saturday
Weather: Partly cloudy. Slight chance of snow showers. Snow levels below 7000 feet increasing to 7500 feet in the afternoon. Chance of precipitation is 20%. Partly cloudy Partly cloudy
Temperatures: 32 to 37. deg. F. 20 to 25. deg. F. 31 to 36. deg. F.
Ridge Top Winds: Southwest 40 to 60 mph with gusts to 95 mph decreasing to 20 to 35 mph with gusts to 50 mph in the afternoon. West 15 to 20 mph with gusts to 30 mph. Southwest 15 to 20 mph with gusts to 30 mph.
Expected snowfall: Up to 1 inch 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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