Avalanche Advisory: Wednesday - Jan 16, 2019

THIS AVALANCHE ADVISORY EXPIRED ON January 17, 2019 @ 7:00 am
Avalanche Advisory published on January 16, 2019 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

With over a foot of new snow overnight and strong SW winds, HIGH avalanche danger will exist throughout the day in areas with fresh Wind Slabs.  Storm Slabs in sheltered areas and a scary widespread weak underlying snow pack will also be of concern.  Intense snowfall and winds will create Blizzard conditions overnight increasing danger dramatically and HIGH avalanche danger will quickly become widespread. Traveling on or under avalanche terrain will not be recommended. 

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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12” of dense snowfall overnight with strong SW winds have created dangerous fresh wind slabs. Continued strong winds throughout today (SW at upper elevations, S at lower elevations) will continue to transport new snow and build slabs.  Greatest concern will be for upper and mid elevation NW-N-E-SE facing slopes, but exposed lower elevations will also be of concern.  Natural avalanches are likely, and human triggered avalanches very likely.  Avoid being on or under wind-loaded terrain.  As snowfall and SW winds increase dramatically tonight, with a BLIZZARD WARNING in effect, dangerous wind slabs will become much more widespread at all elevations, with very good potential for large and deadly avalanches.  

Avalanche Problem 2: Storm Slab
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In areas sheltered from the wind, a foot of snowfall overnight on top of 2-3” from the night before is sitting on top of weak loose faceted snow, and potentially fragile buried surface hoar as well.  Observers have reported widespread surface formation over the last several days, including and most notably yesterday finding many patches where this surface hoar has become buried under a few inches of new snow. This is NOT TYPICAL for us in the Sierra!  These weak old surface layers will likely make fresh storm slabs very sensitive to a human trigger and lead to propagation farther than expected. Especially if surface hoar is involved, remote triggering a slope could be possible, including a slope above you. Carefully evaluate the snowpack in sheltered areas, and be more conservative than usual when dealing with this amount of new snow.  As snowfall intensity increases dramatically tonight, this concern will increase dramatically as well, and avalanche terrain should be avoided.

Avalanche Problem 3: Persistent Slab
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Layers of weak faceted sugary snow have been found deeper in our snowpack throughout the forecast area.  While no reports of avalanche activity have been made occurring on these deeper layers YET, plenty of recent snow pit tests show the potential exists.  The new snow load from last night, and much more notably the intense snow to come tonight, could very likely result in some very large avalanches failing on these deeper layers.  Even today a smaller wind slab or storm slab avalanche could be enough force to trigger one of these deeper larger destructive avalanches. Give slopes where this problem may exist a wide berth, especially during periods of intense loading.    

advisory discussion

Surface hoar formation is not that atypical for us in the Sierra.  However, in the Sierra, these fragile pretty feathers of snow usually get destroyed by either sun or wind before they get covered by new snow. The 2-3” of new snow that fell 2 days ago came in with low winds, and surface hoar that was not destroyed was found yesterday below this new snow.  Buried surface hoar is very concerning with a significant snow load on top, and should be treated with great respect.  Widely spaced trees and sheltered areas at mid to low elevations are the areas of greatest concern where buried surface hoar may be found. 

Weather and CURRENT CONDITIONS
weather summary

A Winter Storm warning is in effect thru 4am Friday morning.  12” of snow(1.5”SWE) fell last night near Main Lodge on Mammoth Mtn– at 9,000’ accompanied by strong SW winds blowing into the 80mph range over ridgetops.  Light snow is expected thru today accompanied by strong S to SW winds, with several inches of new snow possible.  A Blizzard Warning is in effect from 10pm tonight through 7am Thursday morning.  Snowfall and SW winds will pick up dramatically tonight thru tomorrow with 4ft of snow possible, before tapering off thru the night on Thursday.  

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Scattered snow showers. Snow levels below 7000 feet. Chance of precipitation is 50%. Cloudy. Snow. Snow levels 7500 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels 7000 feet decreasing to below 7000 feet in the afternoon. Chance of precipitation is 100%.
Temperatures: 31 to 37. deg. F. 23 to 28. deg. F. 30 to 38. deg. F.
Mid Slope Winds: Southwest 20 to 40 mph. Gusts up to 80 mph. South 30 to 45 mph with gusts to 85 mph. Southwest 25 to 40 mph with gusts to 75 mph.
Expected snowfall: 60% probability up to 3 inches. 40% probability of 3 to 5 inches. | SWE = up to 0.15 inch. in. 70% probability of 13 to 21 inches. 30% probability of 21 to 29 inches. | SWE = 0.95-1.40 inches. in. 70% probability of 20 to 27 inches. 30% probability of 16 to 20 inches. | SWE = 1.10-1.60 inches. in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Widespread snow showers in the morning, then scattered snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 75%. Cloudy. Snow. Snow levels 7500 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels 7000 feet decreasing to below 7000 feet in the afternoon. Chance of precipitation is 100%.
Temperatures: 24 to 30. deg. F. 17 to 22. deg. F. 25 to 30. deg. F.
Ridge Top Winds: Southwest 40 to 70 mph with gusts up to 95 mph. Southwest 50 to 85 mph with gusts up to 125 mph. Southwest 45 to 70 mph with gusts to 110 mph.
Expected snowfall: 60% probability up to 3 inches. 40% probability of 3 to 5 inches. | SWE = up to 0.15 inch. in. 70% probability of 17 to 25 inches. 30% probability of 25 to 34 inches. | SWE = 2.05-2.05 inches. in. 70% probability of 22 to 30 inches. 30% probability of 16 to 22 inches. | SWE = 1.20-2.20 inches. 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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