Eastern Sierra Avalanche Advisory - 4/8/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 9, 2017 @ 6:33 am
Avalanche Advisory published on April 8, 2017 @ 6:33 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

Between 1.5 and 2+ feet of new snow since Thursday, and strong, gusty southwest winds will create very dangerous avalanche conditions today. Natural wind slab and storm slab avalanches will be likely and human triggered avalanches very likely. Due to the strong winds and local variability, wind slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Storm slab avalanches will be likely in steep, sheltered terrain. Potentially large, destructive avalanches could run into the lower elevations. Travel in and under avalanche terrain is not recommended.

How to read the advisory

Bottom Line

Between 1.5 and 2+ feet of new snow since Thursday, and strong, gusty southwest winds will create very dangerous avalanche conditions today. Natural wind slab and storm slab avalanches will be likely and human triggered avalanches very likely. Due to the strong winds and local variability, wind slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Storm slab avalanches will be likely in steep, sheltered terrain. Potentially large, destructive avalanches could run into the lower elevations. Travel in and under avalanche terrain is not recommended.

Avalanche Problem 1: Wind Slab
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Winds across the region have been strong and gusty overnight and wind slabs have been building on many aspect and all elevations. Terrain features that usually promote wind loading, such as below ridgelines, the side walls of gullies, and the leeward side of convexities are all places to avoid. Even openings in the trees down slope may be suspect. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Better yet, watch for blowing snow and cornice formation to rule out hazardous terrain before you get too close.

Avalanche Problem 2: Storm Slab
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The winter storm warning will remain in effect until 5pm today. We received between 18 and 29 inches of dense, wet snowfall since Thursday and storm slabs will certainly be on my mind as I leave the office this morning. These slabs will be found in more sheltered terrain on slopes of about 35 degrees or steeper. With colder temperatures last night and today bringing snow levels down to valley floors, storm slabs may be found at all elevations that had good pre-storm snow coverage: above about 8000’ on more southerly aspects, and down to about 7000’ on northerly aspects.

advisory discussion

Another Atmospheric River event rolled into the region Thursday night disrupting our recently sunny spring weather with an unusual amount of moisture for this time of year. How unusual? As of 5 AM this morning, the eastern Sierra has received between 1 and 5 inches of liquid water equivalent since Thursday afternoon. According to the Reno National Weather Service, we’ve only been hit with similar precipitation events 3 times in April since 2000. In terms of actual snowfall, it’s deep. And thick and heavy. Snow-water contend ratios are between 12% and 17%. That’s wet!

Storm Totals as of 5 AM today:

Virginia Lakes- 18”

Agnew Pass- 24”

June Mtn Weather Plot- 25”

Sesame Site, Mammoth Mtn- 29”

Rock Creek- 23”

Sawmill near Big Pine- 22”

And as is typical of Atmospheric River storms this season, it’s been windy. Winds gusted up to 98 mph at the top of Chair 22 on Lincoln Mountain in Mammoth overnight. Due to these strong winds and local channeling, wind slabs may be encountered in unusual locations today and in normally sheltered areas, possibly lower in elevation than usual.

New snow and continuous winds will keep sensitive storm and wind slabs forming throughout the day today. Avalanches could be large and destructive and traveling near terrain where they could run is not recommended. Stay away from slopes steeper than about 35 degrees. Stay out from under ridgelines and off of cross loaded slopes.

weather

...WINTER STORM WARNING IN EFFECT UNTIL 5 PM PDT THIS AFTERNOON...

Gusty winds will continue with heavy mountain snow into this morning. The main moisture plume for this storm is shifting southward and it is expected that heavier snow will displace snow showers in Mono County by late morning. Instability will be sufficient to support a slight chance of thunderstorms by this afternoon. Conditions will remain breezy today with gusts up to 100 mph over ridge tops. Temperatures will be colder with highs in the low 30s for the Sierra. Low pressure begins to exit the region later today removing forcing for showers and thunderstorms by the early evening hours. Overnight lows will be rather cold with lows in the single digits to low 20s.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Cloudy. Snow showers. Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the evening. Sunny then becoming partly cloudy.
Temperatures: 25 to 31 deg. F. 8 to 16 deg. F. 34 to 40 deg. F.
Wind direction: SW W
Wind speed: 20 to 30 mph with gusts to 70 mph. 15 to 20 mph with gusts to 45 mph in the evening becoming light. Light
Expected snowfall: 80% probability...4 to 8 in. 20% probability...8 to 14 in. 90% probability...up to 1 in. 10% probability...1 to 2 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: Cloudy. Snow showers through the day. Slight chance of thunderstorms in the afternoon. Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the evening. Partly cloudy.
Temperatures: 16 to 24 deg. F. 5 to 11 deg. F. 27 to 33 deg. F.
Wind direction: SW W SW
Wind speed: 40 to 60 mph with gusts to 100 mph decreasing to 30 to 50 mph with gusts to 90 mph in the afternoon. 20 to 30 mph with gusts to 55 mph decreasing to 15 to 20 mph with gusts to 35 mph after midnight. 0 to 15 mph with gusts to 30 mph.
Expected snowfall: 80% probability...4 to 8 in. 20% probability...8 to 15 in. 90% probability...up to 1 in. 10% probability...1 to 2 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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