Avalanche Advisory

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Avalanche Advisory published on January 19, 2018 @ 6:48 am
This Avalanche Advisory expires in 21 hours, 34 minutes
This advisory is valid for 24 hours
Issued by Clancy Nelson - Eastern Sierra Avalanche Center

Avalanche danger today will be CONSIDERABLE near and above treeline. Newly formed wind slabs could produce many small and even some large avalanches on leeward slopes. In isolated areas near treeline, wind slab avalanches could activate the remains of our persistent slab problem and potentially create avalanches that are quite large. Natural avalanches are possible; human triggered avalanches are likely.

Below ~9,000’, even with small amounts of new snow, coverage is thin and avalanches are unlikely.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Strong and consistent SW winds have been rapidly redistributing new snow onto steep slopes near and above treeline. These slabs will most likely continue to build throughout today on NW-N-E facing slopes. Cornice formation, blowing snow, drifting are clues that point to the formation of new wind slabs. Avoid steep open slopes, the down-wind side of ridges, and the sidewalls of gullies. Beware of hollow sounding snow and shooting cracks from your feet.

Avalanche Problem 2: Persistent Slab
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Stability tests from near and above treeline in the Mammoth area have shown that layers of sugary snow under denser slabs have become less reactive and prone to propagation in recent days. However, new and rapid loading by wind-drifted snow, or even a small wind slab avalanche running down on top of a buried weak layer could stress it to the point of failure. Because these facets tend to be buried between 40 and 70cm down in the snowpack, resulting avalanches could be very large. Though less likely, avalanches that affect buried weaknesses should not be ignored. Investigating the snowpack, looking for stronger slabs over weaker sugary snow, will tell you if these larger avalanches are possible in your area.

advisory discussion

Southwest winds overnight have been in the 30 to 80 mph range on Mammoth Mountain, and they are forecast to continue into the late afternoon. Snow amounts have been relatively unimpressive as of 4:00am, however showers will persist into the evening with up to 8” in areas north of Bishop. Wind can deposit snow at 3 to 5 times the rate at which it can fall from the sky which means that even modest amounts of precipitation can create deep wind slabs in favored areas. Expect small wind slab avalanches in many areas on steep, leeward slopes. Large wind slab avalanches may occur where winds have the most snow to transport – just under ridgelines, and on cross-loaded features.

Snowpack tests resulting in failures and propagation of fractures in our persistent weak layers have become fewer and farther between recently. There have been no reports of recent avalanche activity, no test slope failures, and there have been no reports of slope-wide collapsing in these layers since before last weekend. Although unlikely, an avalanche failing on one of these layers has the potential to be large and destructive. Especially worrying is the possibility that a smaller wind slab avalanche could suddenly overload one of these deeper instabilities. In isolated areas, these reactive layers have been observed between 40 to 70cm down and in areas where the snowpack is generally less than ~1m deep. Extra detective work with a shovel is required to see what these layers can do in your area.

Weather and CURRENT CONDITIONS
weather summary

...WINTER WEATHER ADVISORY IN EFFECT UNTIL 10 AM PST THIS MORNING...

Snow will diminish today as a winter storm system exits the region, but bands of upslope snow showers will develop through this evening. After a dry and cool Saturday, weak low pressure will bring light snow late Sunday into Monday, with a stronger cold storm possible by midweek.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Today Tonight Saturday
Weather: Mostly cloudy. Widespread snow showers. Mostly cloudy. Scattered snow showers. Partly cloudy.
Temperatures: 27 to 33 deg. F. 11 to 16 deg. F. 20 to 28 deg. F.
Wind Direction: SW N N
Wind Speed: 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon. 10 to 15 mph with gusts to 30 mph. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: 80% probability...of 1 to 5 inches. 20% probability...of 5 to 7 in. Up to 1 in. 0 in.
Over 10,000 ft.
Today Tonight Saturday
Weather: Mostly cloudy. Widespread snow showers. Mostly cloudy. Scattered snow showers. Partly cloudy.
Temperatures: 20 to 26 deg. F. 6 to 11 deg. F. 15 to 21 deg. F.
Wind Direction: SW N N
Wind Speed: 30 to 45 mph with gusts to 70 mph decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon. 15 to 25 mph with gusts to 35 mph. 15 to 20 mph with gusts to 35 mph.
Expected snowfall: 80% probability...of 2 to 6 inches. 20% probability...of 6 to 8 in. Up to 1 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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