Avalanche Advisory: Sunday - Dec 16, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 17, 2018 @ 6:59 am
Avalanche Advisory published on December 16, 2018 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

VERY STRONG S-SW winds are on the rise again today making small human triggered avalanches possible in isolated areas.  While exposed areas have very little loose snow left for transport, steep lower elevation areas that are typically somewhat sheltered will actually be more concerning due to the strength of today’s winds.  Early season obstacles exist! 

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Avalanche Problem 1: Wind Slab
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The winds are on the increase again today with gusts recorded into the 80s already this morning ontop of Mammoth Mtn.  My house in mammoth is shaking from them as I write this.  Strong winds have been blowing on and off for most of this past week.  Areas at all elevations with any kind of exposure are a wind-affected mix of stripped surfaces, hard wind-board and breakable crust.  With ever-dwindling amounts of loose snow available for transport, wind slabs sensitive to human triggering are becoming much fewer and farther between.  That being said, the strength of today’s winds means that wind slabs are likely to creep into more sheltered lower elevation areas.  Watch for areas of denser snow especially on steep convex slopes that end in a terrain trap.

advisory discussion

Winds have wreaked havoc for the Eastern Sierra over the last several days.  Strong N-NE winds on Wednesday, then VERY strong SW winds on Friday, and today’s very strong S-SW winds have stripped snow from exposed areas at all elevations and aspects leaving surfaces a mix of firm wind-board, breakable crust, and even stripped bare ground.  Sensitive wind slabs have formed, stabilized, and re-formed, and re-stabilized?  With so little loose snow left for transport in these mid to upper elevation exposed areas, avalanche concern will actually be greater for lower elevation, somewhat more sheltered areas where today’s strong winds will find loose snow to transport. Recent observations have found that the loose snow that does exist out there is faceting, and that some recent thin wind slabs are sitting ontop of weak facets.  These facets will be of greater concern on Monday when we have fresh snow deposits sitting ontop of them.   

recent observations
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12/15 - Mammoth - Sherwins:  Thin windslabs ontop of faceted snow.

12/15 - June - Behind June:  Thin coverage and wind affect

12/14 - Convict - Mini Morrison:  Natural and human triggered wind slabs

Weather and CURRENT CONDITIONS
weather summary

Another windy day is on-tap as a low pressure system make its way into our area tonight into tomorrow morning bringing a small dose of new snow to the mountains.  Along with increasingly strong SW winds gusting into the 70s at upper elevations, expect cloudy skies and hi temperatures in the low 30s around 10,000’ today.  Flurries may begin just before dusk, but the “heavier” snow and accumulation of up to 6” is expected between late tonight and tomorrow morning.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy. Cloudy. Chance of snow in the evening, then snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 70%. Mostly cloudy then becoming partly cloudy. Snow showers likely in the morning. Snow levels below 7000 feet. Chance of precipitation is 60%.
Temperatures: 32 to 42. deg. F. 23 to 31. deg. F. 26 to 36. deg. F.
Mid Slope Winds: South 15 to 25 mph with gusts to 50 mph. South 15 to 30 mph with gusts to 60 mph. Southwest 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 0 in. 80% probability of 2 to 5 inches. 20% probability of 5 to 10 inches | SWE = 0.15-0.30 inch. in. 90% probability of 1 to 2 inches. 10% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in.
Over 10,000 ft.
Sunday Sunday Night Monday
Weather: Partly cloudy. Cloudy. Chance of snow in the evening, then snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 90%. Mostly cloudy then becoming partly cloudy. Snow showers likely in the morning. Snow levels below 7000 feet. Chance of precipitation is 60%.
Temperatures: 24 to 34. deg. F. 13 to 23. deg. F. 25 to 30. deg. F.
Ridge Top Winds: Southwest 25 to 35 mph with gusts to 55 mph increasing to south 30 to 50 mph with gusts to 75 mph in the afternoon. South 35 to 55 mph with gusts to 90 mph. Southwest 30 to 45 mph with gusts to 75 mph becoming west 20 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: 0 in. 70% probability of 2 to 6 inches. 30% probability of 6 to 10 inches. | SWE = up to 0.30 inch. in. 80% probability of 1 to 3 inches. 20% probability of 3 to 5 inches. | SWE = up to 0.15 inch. 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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