THIS AVALANCHE ADVISORY EXPIRED ON February 14, 2014 @ 6:44 am
Avalanche Advisory published on February 13, 2014 @ 6:44 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger is MODERATE with pockets of CONSIDERABLE avalanche danger near and above treeline on NW-N-NE-E-SE aspects due to unstable wind slabs on slopes 35 degrees and steeper. In wind protected areas below treeline and in wind scoured areas near and above treeline, avalanche danger is LOW.

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 to gale force ridgetop winds over the past 24 hours have redistributed recent new snow on the ground. Wind loading and subsequent wind slab formation has been observed near and above treeline on NW-N-NE-E-SE aspects. Areas above 8,400' that did not receive any rain on Feb 9 had significantly more snow available for wind transport. Wind slabs up to 3 feet thick are know to exist in lee areas above 9,000'. Human triggered avalanches are possible to likely today. Natural avalanches remain possible.

Avalanche Problem 2: Deep Slab
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Weak layers of well developed facets and depth hoar remain buried deep in the snowpack on near and below treeline NW-N-NE aspects above 9,000 ft. The strong thick layer of new snow above these weak layers has made collapsing these weak layers difficult. Deep persistent slab avalanche activity is now in the unlikely but not impossible category. Areas where a shallower slab exists and around trigger points such as exposed rocks are the most likely location to trigger this type of avalanche. Since near treeline and below treeline areas above 9,000 ft. represent only a small portion of the forecast area, this problem remains isolated in distribution.

recent observations

Observations made yesterday revealed unstable wind slabs in near and above treeline terrain. On Stevens Peak (Carson Pass area), a natural avalanche was observed around 11:30 am in above treeline terrain on an E aspect at about 9,500' with an estimated slope angle of 45 degrees. Approximate measurements of the avalanche were 100' wide, 400' vertical run, and a 3' crown. See below for photos and more info.

On Lincoln Ridge (Yuba Pass area) smaller unstable wind slabs were observed on NE aspects in near and above treeline terrain above 7,700'. Small wind slabs up to 9 inches thick cracked easily and small cornice features easily collapsed in response to the weight of a skier. Shooting cracks up to 20' long were observed as wind slabs and small cornices failed. Dense wind slabs were noted to fail on the top of a 2 inch layer of low density recent storm snow sandwiched between the bottom of the wind slab and the top of the Feb 9 rain crust. See below for photos and more info.

Weather and CURRENT CONDITIONS
weather summary

A weather system passing mainly to the north of the forecast area is bringing warm air and a few light showers to the northern portion of the forecast area this morning. Snow level is around 9,500'. Little to no accumulation is expected today. In areas where precipitation occurs, expect a rain/snow mix around 8,500' to 9,000'. The more significant element of the passing system is strong to gale force ridgetop wind. Winds over the past 24 hours have been out of the southwest to west with ridgetop gusts recorded to 102 mph. Strong southwest winds are forecast to continue through tomorrow with ridgetop gusts 90 to 100 mph. Maximum daytime air temperatures will reach the low 40s to low 50s today for areas between 7,000' and 9,000'.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 35 to 40 deg. F.
Max. temperature in the last 24 hours: 38 to 45 deg. F.
Average wind direction during the last 24 hours: West Southwest
Average wind speed during the last 24 hours: 44 mph
Maximum wind gust in the last 24 hours: 102 mph
New snowfall in the last 24 hours: 0 to trace inches
Total snow depth: 45 to 56 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy skies with a slight chance of rain. Mostly cloudy skies with a slight chance of rain. Partly cloudy skies.
Temperatures: 42 to 52 deg. F. 32 to 42 deg. F. 45 to 55 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 15 to 25 mph with gust to 40 mph. 20 to 30 mph with gusts to 45 mph. 20 to 30 mph with gusts to 45 mph.
Expected snowfall: 0 in. 0 to trace in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy skies with a slight chance of rain and snow. Mostly cloudy skies with a slight chance of rain and snow. Partly cloudy skies.
Temperatures: 39 to 49 deg. F. 29 to 39 deg. F. 39 to 49 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 50 to 60 mph with gusts to 90 mph. 55 to 70 mph with gusts to 100 mph. 50 to 65 mph with gusts to 95 mph.
Expected snowfall: 0 to trace in. 0 to trace in. 0 in.
Disclaimer

This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.