THIS AVALANCHE ADVISORY EXPIRED ON February 11, 2014 @ 7:00 am
Avalanche Advisory published on February 10, 2014 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

CONSIDERABLE avalanche danger exists on slopes 32 degrees and steeper on all aspects above 7000 ft. due to wind slabs, storm slabs, and deep persistent slabs resting on a variety of weak layers. Below 7000 ft. where more of the storm's precipitation fell as rain, less avalanche danger exists. Even though the best window for natural avalanches has passed, human triggered avalanches remain likely today. Large, deep, and destructive avalanches could still occur today.

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.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Large wind slabs exist above a variety of weak layers on the wind loaded NW-N-NE-E-SE aspects in near and above treeline terrain. Observations have shown these wind slabs at 3-6+ ft in depth. The weak layers under the slabs have not had time to adjust to this large heavy load and human triggered avalanches will remain likely where these wind slabs exist. Large destructive avalanches would result from the failure of these wind slabs.

Avalanche Problem 2: Storm Slab
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This storm started out cold and and grew progressively warmer until last night. This warming trend created an upside down snowpack with lighter softer snow resting below heavier more dense snow layers. Below all of that storm a variety of weak layers ranging from facets above a rain crust to even softer colder snow from earlier storms exists. This combination of heavy snow sitting above weak snow layers means that human triggered avalanches remain likely even in non-wind affected areas. These storm slabs avalanches could also be large and destructive. 

Avalanche Problem 3: Deep Slab
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In areas above 9,500' deep persistent slabs linger near treeline and below treeline on NW-N-NE aspects. Weak layers of well developed facets and depth hoar remain buried in the snowpack in these areas. With the load above these old weak layers more than quadrupling since last week, human triggered deep slab avalanches remain possible today. Locations that are both above 9,500' and near treeline or below treeline terrain represent only a small portion of the forecast area, keeping this problem isolated in distribution.

recent observations

Yesterday on Tamarack Peak and on Andesite Ridge, the remains of several natural avalanches existed on N-NE aspects. The crown depths on these avalanches measured 2 to 4 ft and the slides all entrained enough snow to deeply bury people. The failure layer for these avalanches appeared to be a layer of light soft snow. Some of avalanches stepped down to lower older weak layers and the ground. Tests and snowpit data in both of these areas yesterday showed that the weaknesses still remained active. Skier triggered whumphing, collapsing, and shooting cracks occurred on all aspects in the Tamarack Peak area yesterday. A piece of cornice dropped onto a wind loaded test slope also triggered a small wind slab failure on that test slope in the Tamarack Peak area yesterday.

On Andesite Ridge, rising snow levels allowed rain to fall on snow up to 8200 ft. Ski cuts on wet test slopes did release some small wet loose snow instabilities like pinwheels and roller balls.

Weather and CURRENT CONDITIONS
weather summary

The forecast area received another 1.5 to 2.5 inches of water in the last 24 hours. Since snow level hovered between 8000 to 8400 ft. until noon yesterday, much of the precipitation fell as rain below this elevation. Snow levels started to fall yesterday afternoon and dropped down to 7000 ft in some areas. For the rest of the forecast area snow levels dropped overnight and sit around 6000 ft. this morning. Above 8200 ft. the mountains received another 7 to 12 inches of heavy wet snow in the last 24 hours. Snow showers should continue this morning with up to 2 more inches of snow expected. By mid morning snow showers should start to taper off. The winds started to decrease early this morning and should remain light out of the west for today and tomorrow. As the snow showers taper off skies should begin to clear and temperatures above 7000 ft. should climb into the mid 30's for today and upper 30's to low 40's for tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 26 to 31 deg. F.
Max. temperature in the last 24 hours: 32 to 35 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30 to 40 mph
Maximum wind gust in the last 24 hours: 77 mph
New snowfall in the last 24 hours: 7 to 12 inches
Total snow depth: 50 to 63 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Snow decreasing to isolated showers by 8 am. Skies becoming mostly to partly cloudy by the afternoon Mostly cloudy Mostly cloudy to partly sunny
Temperatures: 32 to 38 deg. F. 23 to 29 deg. F. 35 to 43 deg. F.
Wind Direction: West Variable Southwest
Wind Speed: 10 to 15 mph Light 10 to 15 mph with gusts to 25 mph
Expected snowfall: up to 2 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Snow decreasing to isolated showers by 8 am. Skies becoming mostly to partly cloudy by the afternoon Mostly cloudy Mostly cloudy to partly sunny
Temperatures: 28 to 34 deg. F. 22 to 27 deg. F. 30 to 38 deg. F.
Wind Direction: West West West
Wind Speed: 15 to 25 mph with gusts to 40 mph 15 to 20 mph with gusts to 30 mph 15 to 25 with gusts to 40 mph
Expected snowfall: up to 2 in. 0 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.