Avalanche Advisory: Saturday - Mar 3, 2018

THIS AVALANCHE ADVISORY EXPIRED ON March 4, 2018 @ 6:38 am
Avalanche Advisory published on March 3, 2018 @ 6:38 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

The avalanche danger this morning will remain HIGH, especially near and above treeline where wind slab and storm slab avalanches are likely. As the winds subside and snowfall trends towards showers later in the day the avalanche danger may dip to CONSIDERABLE, but human triggered avalanches will still be likely. Travel in or under slopes of 30 degrees or more is still not recommended today.

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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SW winds continued to redistribute snow throughout yesterday and last night, even down to lower elevations. Blowing snow, as well as cornice and wind slab failures were observed yesterday afternoon across the range. As winds subside throughout today remember that wind slabs can take hours to days to stabilize and they could remain sensitive to human triggering for a while. Colder temperatures may make these slabs more brittle and thus more touchy. Though the predominant flow has been from the SW, which will build slabs on NW-N-NE-E-SE aspects, local channeling has been observed cross-loading S and W facing slopes. Exposed areas with smooth drifts are areas to avoid, especially under cornices or near the sidewalls of chutes and gullies and even larger openings in the trees further down slope.

Avalanche Problem 2: Storm Slab
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The snow during this storm as been increasing in density. An “upside down” storm is one where heavier snow sits atop lighter snow, creating storm slabs. The heavier weight on top can overpower the loose snow underneath. A large natural storm slab avalanche was observed yesterday at 8800’ on the Sherwins in Mammoth. Sheltered slopes steeper than about 30 degrees where denser snow sits on top of softer snow are the places you can expect storm slabs today. Falling through the surface snow into softer snow below, or worse, long cracks shooting from your feet, are indications that you are in storm slab territory.

Avalanche Problem 3: Deep Slab
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Weak layers of sugary facet snow are buried between 75 an 120cm down under all the recent storm snow. These layers have been observed on northerly aspects from the Mammoth area and north, between 9000’ and 10,000’. The new load on top of them could overpower what little strength they have and the resulting avalanche could be large and destructive. Sudden collapsing of these layers, heard and felt as a “whumph,” are direct signs of instability in the snowpack and should not be ignored.

advisory discussion

Snow and weather conditions continue to be hazardous today with plenty of recent snowfall and wind, and enough forecast throughout today to keep the snowpack unstable. However, the danger that worries me the most today is the human factor. As the storm works its way south and east today and snowfall becomes more showery, winds lighter, people will naturally want to venture out into the mountains. But remember that most avalanche problems take hours to days to subside and while the snowpack will slowly trend towards stability the risk of playing in avalanche terrain is still high. You will need expert route-finding skills and the discipline to make very conservative decisions in order to stay safe today. Rule out avalanche terrain steeper that 30 degrees, which includes the slopes near or under avalanche terrain. Terrain traps like gullies or above cliffs are also good places to avoid. Even lower elevations may continue to see avalanche activity today. Obstacles may have been covered over just enough to be invisible, but still hit-able. Winds at middle and upper elevations will still be breezy. Wind chill and poor visibility at times may complicate your travels. Another round of heavy snow showers may complicate things locally by this evening.

recent observations
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3/2-  Trailhead Snow Survey: June, Tioga, Lundy, Virginia Lakes, Aspendell

3/2- Natural D1.5 Avalanche Low on the Sherwins

24hr New Snow/New Water (4AM)

Agnew Pass                         6”/--
June Mtn Weather Plot          5”/0.61”
Sesame Snow Study Site     5.8”/0.63”

Weather and CURRENT CONDITIONS
weather summary

...WINTER STORM WARNING IN EFFECT UNTIL 10 AM PST THIS MORNING…
While the worst overall conditions have passed, this storm is still not quite finished with our region. Snow will continue across the Sierra and parts of western Nevada through this morning, with another round of snow showers arriving later today through tonight. Additional snow accumulations will be more spotty compared to recent days, but locally heavy snowfall remains possible.

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. Widespread snow showers in the morning, then scattered snow showers in the afternoon. Mostly cloudy. Scattered snow showers. Sunny.
Temperatures: 18 to 24 deg. F. 0 to 6 deg. F. 23 to 28 deg. F.
Mid Slope Winds: SW W W
Expected snowfall: 90% probability...of 2 to 6 inches. 10% probability...of 6 to 8 in. 80% probability...of 1 to 4 inches. 20% probability...of 4 to 8 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy. Widespread snow showers. Mostly cloudy. Scattered snow showers. Partly cloudy then becoming sunny.
Temperatures: 10 to 17 deg. F. -4 to 1 deg. F. 17 to 22 deg. F.
Ridge Top Winds: SW SW NW
Expected snowfall: 90% probability...of 3 to 7 inches. 10% probability...of 7 to 9 in. 80% probability...of 1 to 4 inches. 20% probability...of 4 to 8 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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