Avalanche Advisory: Monday - Dec 24, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 26, 2018 @ 6:59 am
Avalanche Advisory published on December 24, 2018 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

Christmas Eve:  Avalanches will be unlikely through most of today until new snow begins accumulating this afternoon and into tonight.  Human triggered wind slab avalanches could become possible this afternoon.  As snow fall intensity picks up substantially tonight, with up to a foot of new snow possible along with strong SW winds, human triggered wind slab avalanches will become likely and natural wind slab avalanches possible.  In areas sheltered from the wind, human triggered storm slab avalanches will become possible late tonight as well.

Christmas Day:  Human triggered wind slab avalanches will be likely on all aspects as winds shift out of the north.  Human triggered storm slab avalanches will be possible in areas sheltered from the wind.

Early season obstacles exist!

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Avalanche Problem 1: Wind Slab
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Christmas Eve:  With up to 2” of new snow possible late this afternoon accompanied by strong SW winds, small wind slab avalanches will become possible on SE-E-N-NW facing slopes at mid to upper elevations.  As snowfall intensity increases substantially tonight with up to a foot of new snow possible in some higher elevation areas, continued SW winds will make these wind slabs much larger and more widespread.   Natural avalanches will become possible, and human triggered avalanches likely on leeward slopes, especially just below ridgelines and the sidewalls of gullies, and around other features that capture snow.  Avoid being on or under fresh dense wind deposited snow on steep slopes tonight.   

Christmas Day:  6-10” of new is expected to have fallen overnight (up to a foot in some higher elevation areas) accompanied by strong SW winds.  Sensitive wind slabs surely will have formed on leeward slopes and terrain features.  Today (Christmas), moderate north winds will blow snow creating sensitive wind slabs in completely different areas.  Exposed slopes facing all aspects at mid to upper elevations will be suspect for wind slabs that will likely be sensitive to human triggering. Avoid steep exposed slopes with denser wind deposited snow.  A resulting slide today could be large enough to bury, injure or kill someone.  

Avalanche Problem 2: Storm Slab
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6-10” of new snow is expected Christmas Eve night.  In areas sheltered from the wind, lower density snow deposits (storm slabs) could be sensitive to human triggering thru Christmas day.  These are likely to be found on all aspects at mid-elevations.  Be wary of steep sheltered slopes, especially convex rolls and slopes involving a terrain trap where sliding snow could accumulate deep enough to bury someone.  Do your own localized assessments of how well this new snow is bonding before committing to steep terrain, and if in doubt stick to lower angle slopes. 

advisory discussion

Thru most of Christmas Eve avalanche danger will be very minimal.  As snow begins to accumulate Christmas Eve afternoon, and especially Christmas Eve night when snowfall intensity is expected to increase substantially, avalanche danger will rise.  The greatest stability concerns will exist in the interface between the new snow and old snow.  Where the old snow surface is made of facets, or where facets exist just under an old wind slab, sensitivity to triggering is likely to be greater, the period of sensitivity will last longer, and a slide could be bigger.  It’s a good idea to take some time to investigate the old snow to get a better idea if faceted snow exists and to what extent.

Even more importantly, beware that the snowpack is thin and early obstacles exist!  A fresh layer of soft snow will make stumps, logs and rocks even harder to see. 

recent observations
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-12/23 - Mammoth Lakes Basin - Hammil Bowl: Snowpit tests, snow pack - no signs of instability.

-12/22 - Mammoth Lakes Basin - Crystal Bowl:  Faceted snow under wind-deposits, Propogating extended column test.

-12/22 - Rock Creek: Snow coverage, crusts and facets.

-12/22 - Mammoth Area - Minaret VIsta: New snow, small cornices, coverage

Weather and CURRENT CONDITIONS
weather summary

A moderate winter storm and cold front will move in today (Christmas eve) bringing light snow showers in the afternoon, increasing in intensity tonight, and becoming light after midnight.  6-10” of total new snow is possible, with up to a foot in some higher elevation areas.  SW winds will be on the increase this afternoon gusting into the 50s at mid elevations, before shifting out of the north for Christmas day.  Expect cloudy skies for Christmas day, lingering flurries, and cold temperatures reaching the mid 20s around 10,000’.

Cold temperatures are expected for the rest of the week, with another weak disturbance likely for Thursday, and then again possibly late in the weekend.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy. Chance of snow through the day. Cloudy. Snow in the evening, then widespread snow showers after midnight. Mostly cloudy. Chance of snow showers.
Temperatures: 36 to 42. deg. F. 16 to 22. deg. F. 24 to 29. deg. F.
Mid Slope Winds: Southwest 15 to 25 mph. Gusts up to 35 mph increasing to 50 mph in the afternoon. Southwest 15 to 30 mph. Gusts up to 55 mph decreasing to 45 mph after midnight. Northwest 15 to 25 mph with gusts to 45 mph.
Expected snowfall: 70% probability up to 2 inches. 30% probability no accumulation. in. 80% probability of 3 to 8 inches. 20% probability of 6 to 10 inches. in. Up to 1 in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy. Chance of snow through the day. Cloudy. Snow in the evening, then widespread snow showers after midnight. Mostly cloudy. Chance of snow showers.
Temperatures: 29 to 34. deg. F. 9 to 14. deg. F. 20 to 25. deg. F.
Ridge Top Winds: West 15 to 30 mph with gusts to 40 mph shifting to the southwest 25 to 35 mph with gusts to 60 mph in the afternoon. Southwest 30 to 45 mph shifting to the west 25 to 35 mph after midnight. Gusts up to 65 mph. North 20 to 35 mph with gusts to 55 mph.
Expected snowfall: 70% probability up to 2 inches. 30% probability no accumulation. in. 80% probability of 4 to 9 inches. 20% probability of 8 to 12 inches. in. Up to 1 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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