Eastern Sierra Avalanche Advisory - 12/19/17

THIS AVALANCHE ADVISORY EXPIRED ON December 21, 2017 @ 6:42 am
Avalanche Advisory published on December 19, 2017 @ 6:42 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Tuesday:

Older wind-deposited slabs from last weekend that lie on top of weak sugary snow will be the primary avalanche concern until early Wednesday morning. Natural and human triggered avalanches will be unlikely, but that does not mean there is no danger. Watch for unstable snow on isolated terrain features. You can often trigger these slabs where the snowpack is shallower or where the overlying slab is thinner.

Wednesday:

A fast moving storm will bring short periods of intense snowfall and strong winds to the mountains late Tuesday night. Winds will start from the Southwest and shift to the North as the front passes leaving wind slabs on all aspects at middle and upper elevations. Some new wind deposits will sit on top of weak, sugary facet snow. Favored locations include the downwind side of ridges and the sides of chutes and gullies. Natural avalanches may be possible; human triggered avalanches likely. Careful snowpack evaluation, cautious route-finding and conservative decision-making will be essential.

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Above Treeline

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Near Treeline

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Below Treeline
Avalanche Problem 1: Wind Slab
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Beginning Tuesday afternoon winds will increase as a fast moving cold front approaches the area with High Wind warnings in Mono and Inyo counties. By Wednesday afternoon, 1 to 6” of new snow may fall in short, intense showers along the Sierra crest. Southwest winds accompanying the storm will deposit fresh wind slabs on leeward slopes and in some cases on top of weak faceted snow grains that could fail under the new load. Avalanches could range from small and widespread to large on favored slopes. Winds will shift to the North with frontal passage moving new precipitation onto opposite aspects. With gusty, turbulent conditions, watching for blowing snow is often the best  way to determine where wind slabs are building. Avoid round, pillow shaped drifts. Shooting cracks from you feet and hollow sounding snow are indicators that you may already be in harm’s way.

Avalanche Problem 2: Persistent Slab
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The secondary concern, and especially on Tuesday prior to the next storm passage, is old wind slabs that were deposited last weekend at middle and upper elevations on the leeward sides of ridges, the sidewalls of chutes, and in depressions near and above treeline. Ordinarily these wind slabs would have bonded to the underlying snow by now. However, weak layers of sugary faceted snow lie underneath old wind deposits in more sheltered locations. Some recent observations have shown that this sugary snow can still be reactive under stress. Steep and shaded slopes where the snowpack is shallower are often places you could trigger even stubborn persistent slab avalanches. Be wary of hollow sounding snow where wind deposits are thinner. Firm persistent slabs can break uphill of a skier providing little chance of escape.

advisory discussion

Dense early season storms in November deposited enough snow for good coverage, however generally dry conditions since then have left the snowpack in the Sierra thin and snowline is around ~9,000 to 9,500’ around Mammoth, and higher elsewhere. Recent high pressure and mild daytime temperatures have slowly edged snowline higher with only patchy coverage below. Early season conditions exist with many obstacles hiding just under the snow surface.

A mild disturbance to the Northeast brought snow and wintry conditions to the Sierra on December 16th depositing small wind slabs on West-South-East aspects. Prior to that last shot of weather, moderate to strong temperature gradients in the snowpack formed during the high pressure period (roughly the first two weeks of December) weakening the upper snowpack. Layers of in-cohesive, sugary snow have been observed between denser layers of old wind deposit or in combination with the melt-freeze crust that was buried on November 27th. Most of the upper snowpack consists of these facet grains in one form or another. The snowpack near and above treeline alternates between soft facets in sheltered areas, wind stripped, wind deposited areas, and melt/freeze patches. Near and below treeline, surface facets have provided some good skiing in sheltered areas, but warm temperatures have made things sticky and spring-like on sun exposed aspects up to ~10,000’. As the next storm passes through early Wednesday morning, the facets in the upper snowpack may form a weak layer once buried, and the melt-freeze crusts may act as a prime bed surface for future avalanches. Be wary of hollow sounding snow, and cracks shooting out from your feet. Observe wind loading patterns paying special attention to leeward slopes, gullies, and depressions. Below ~9,500’, natural and triggered releases are unlikely due to little or no snow.

Weather and CURRENT CONDITIONS
weather summary

...HIGH WIND WARNING IN EFFECT FROM 7 PM THIS EVENING TO 4 PM PST WEDNESDAY…

High pressure expected for one more day with near normal temperatures for the California interior. The next weather system arrives on Wednesday with precipitation, in the form of snow, across the Sierra Nevada. This fast moving storm is set to reach Northern California later today and exit the eastern Sierra by Thursday. The front will primarily bring colder temperatures and strong winds, but orographic lifting and a westerly shift to the storm track may produce short periods of intense snowfall rates up to 2-4 inches per hour. Southwest winds with gusts 50 to 60 mph in the valleys and gusts up to 80 mph in wind prone areas. Gusts of 125 mph along the Sierra Crest. Winds will become north Wednesday afternoon as they diminish.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy. Partly cloudy. Isolated snow showers after midnight. Mostly cloudy. Widespread snow showers in the morning, then snow showers likely in the afternoon.
Temperatures: 41 to 47 deg. F. 23 to 28 deg. F. 27 to 33 deg. F.
Wind Direction: Light winds becoming SW SW SW shifting to NW in the afternoon.
Wind Speed: Light becoming 15 to 25 mph with gusts to 55 mph in the afternoon. 20 to 30 mph increasing to 25 to 40 mph after midnight. Gusts up to 90 mph. 30 to 45 mph with gusts to 90 mph and 25 to 35 mph with gusts to 65 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 80% probability...of 1 to 3 in. 20% probability...of 4 to 6 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy. Partly cloudy. Mostly cloudy. Snow showers.
Temperatures: 36 to 41 deg. F. 17 to 22 deg. F. 21 to 27 deg. F.
Wind Direction: W shifting to SW in the afternoon. SW W becoming NW in the afternoon.
Wind Speed: 10 to 15 mph with gusts to 45 mph increasing to 25 to 40 mph with gusts to 70 mph in the afternoon. 40 to 60 mph with gusts to 90 mph increasing to 50 to 70 mph with gusts to 110 mph after midnight. 50 to 70 mph with gusts to 105 mph decreasing to 35 to 50 mph with gusts to 85 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 80% probability...of 1 to 4 in. 20% probability...of 4 to 6 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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