Avalanche Advisory: Monday - Jan 7, 2019

THIS AVALANCHE ADVISORY EXPIRED ON January 8, 2019 @ 6:59 am
Avalanche Advisory published on January 7, 2019 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

HIGH avalanche danger exists this morning where 1-2ft of new snow fell overnight accompanied by the most EXTREME SW winds of the season.  Large Wind Slab avalanches will be very likely at all elevations where slopes were not stripped by the wind, even in typically sheltered locations.  Deeper and larger Persistent Slab avalanches are likely as well.  Avoid traveling on or under slopes recently loaded by snow.  

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Near Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Below Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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Large to very large human triggered wind slab avalanches are very likely today and natural avalanches are possible.  Avoid traveling on or under slopes recently loaded by snow.  1-2ft of snow with over 2” of water content fell overnight with intensities reaching 2-3” an hour at times, on top of 1.5-2.5ft of snow that fell in the previous 24 hours.  EXTREME SW winds gusted up to 125mph over ridge-tops, and over 100mph even at lower mountain elevations.  No areas were spared by the wind last night, so expect to find wind slabs in even the most sheltered terrain today at all elevations where winds did not strip the snow away.  Continued strong SW winds today could continue to transport and deposit snow. SE-E-N-NW facing terrain will be the greatest concern, but swirling winds at mid to low elevations could have formed slabs on all aspects where terrain topography decreases wind-speeds resulting in deposition.  Even small typically sheltered slopes that end in a terrain trap could be dangerous today.  

Avalanche Problem 2: Persistent Slab
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Before these latest storms widespread weak faceted snow has been found at the surface of the old snow or near the surface just underneath crusts.  With the additional recent snow load it is now possible that avalanches will fail on these now-deeper buried weak layers, resulting in much larger avalanches than might be otherwise expected.  Avalanches that fail on these deeper layers could propagate great distances across slopes, run much farther downslope, and be much more deadly.  Avalanches could fail directly on this deeper layer, or smaller wind slab avalanches could act as the trigger.  This problem and concern could last for some time after the recent wind slabs stabilize.  Avoid being on or under terrain today where recent wind deposited snow could be burrying this weak layer.   

advisory discussion

Several factors exist making the snowpack especially dangerous today: 1-2ft of new dense snow overnight, EXTREME SW winds even at low elevations, 1.5-2.5ft of new snow in the previous 24hours, persistent weak faceted layers in the old snow, and the new snow last night falling “upside down”.  The snowfall last night started falling cold, and temperatures rose consistently throughout the snowfall period, leading to more dense snow falling ontop of less dense snow.  This is a much weaker structure than if colder snow fell ontop of warmer snow.  This effect will be far less concerning today with the dramatic wind transport being the leading cause of snow density change, but it is still something to consider.     

Weather and CURRENT CONDITIONS
weather summary

The latest intense overnight storm dropped over 2” of water and over 1.5ft of snow in favored mountain locations with EXTREME SW winds gusting over 125mph over ridge-tops, and over 100mph even at lower mountain elevations.   

Lingering snow showers will taper off later this morning, leading to cloudy skies and a slight chance of flurries for the afternoon with decreasing SW winds gusting “merely” in the 70mph range over ridge-tops.  Highs are expected near 30 degrees around 10,000’.  

For Tuesday expect a break in storm systems with cloudy skies, moderate southerly winds, and warmer temperatures reaching above freezing near 10,000’.  The next small storm is expected to bring a few more inches of snow Tuesday night into Wednesday. This active storm pattern will continue for 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. Snow showers likely in the morning. Slight chance of snow showers in the afternoon. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 35%. Cloudy. Mostly cloudy. Slight chance of snow showers in the afternoon. Snow levels 8000 feet. Chance of precipitation is 15%.
Temperatures: 30 to 35. deg. F. 25 to 31. deg. F. 34 to 39. deg. F.
Mid Slope Winds: Southwest 30 to 45 mph with gusts to 85 mph decreasing to 15 to 30 mph with gusts to 60 mph in the afternoon. South 10 to 15 mph. Gusts up to 45 mph decreasing to 30 mph after midnight. South 10 to 15 mph. Gusts up to 30 mph increasing to 50 mph in the afternoon.
Expected snowfall: 80% probability up to 2 inches. 20% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in. 0 in. trace in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy. Snow showers likely through the day. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 65%. Cloudy. Mostly cloudy. Slight chance of snow showers in the afternoon. Snow levels 8000 feet. Chance of precipitation is 15%.
Temperatures: 24 to 29. deg. F. 20 to 25. deg. F. 29 to 34. deg. F.
Ridge Top Winds: Southwest 45 to 65 mph with gusts to 110 mph decreasing to 30 to 45 mph with gusts to 70 mph in the afternoon. South 15 to 30 mph. Gusts up to 55 mph decreasing to 40 mph after midnight. South 15 to 25 mph with gusts to 40 mph increasing to 25 to 40 mph with gusts to 60 mph in the afternoon.
Expected snowfall: 80% probability up to 2 inches. 20% probability of 2 to 4 inches. | SWE = less than 0.10 inch. in. 0 in. trace 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 Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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