THIS AVALANCHE ADVISORY EXPIRED ON December 20, 2013 @ 7:00 am
Avalanche Advisory published on December 19, 2013 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

MODERATE avalanche danger will continue on the near and below treeline NW-N-NE aspects on slopes 32 degrees and steeper due to lingering persistent slabs.

Human triggered avalanches remain possible. Due to the shallow snowpack any avalanches that do occur would drag people through and into obstacles like rocks, stumps, and other hard immobile obstacles.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Persistent Slab
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Persistent slabs still exist on the near and below treeline NW-N-NE aspects. A widespread persistent weak layer (the Dec. 7th facets) with a small layer of cohesive snow above it provides the ingredients needed to keep this problem active: a weak layer and a slab. The additional snowfall expected today should not add enough weight to the snowpack to cause natural avalanche activity. However, it will keep this already unstable snowpack teetering on the edge just waiting for some kind of trigger to break it. Human triggered avalanche activity remains possible on these slopes. People can trigger these kind of persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain because fractures can travel through the persistent weak layer. 

Any of these avalanches that do occur would drag a person through rocks, stumps, trees, and other obstacles because of the shallow snowpack and the fact that the near and below treeline terrain holds the most unstable snow.

Avalanche Problem 2: Wind Slab
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The small amount of new snow and the strong NE winds could form some small, isolated, and mostly insignificant wind slabs on the above treeline wind-loaded W-SW-S aspects and a few of the cross loaded NW and SE aspects. In most areas these slabs will build on top of bare ground. Some cracking or other signs of instability may occur where these small slabs exist, but significant avalanche activity is not expected from these wind slabs. Still it only takes a small amount of moving snow to knock a person over. Falling onto the rocks and other hard obstacles that remain exposed on these aspects could take a toll on people and equipment.   

recent observations

Yesterday on Castle Peak (snowpit, more info), observations and data showed a well consolidated layer of snow resting on top of a thin melt freeze crust above the weak sugary facets. No human triggered whumpfing or collapsing occurred on the NW aspects in the areas I traveled through. Snowpit data still indicated that the sugary snow grains that comprise the the Dec. 7th facets remain weak, and that they are easy to break. However, those tests also indicated that the resulting fractures did not travel very far through the snowpack. On every other day since 12/7 some significant sign of instability has occurred (whumpfing, collapsing, small avalanches, unstable snowpit test results, etc). Yesterday's data represents the first and only instance of a day without some of those significant signs of instability since 12/7. Until more data supporting a strengthening snowpack exists, consider this an outlier. I for one am not willing to put much faith in this data due to the overwhelming number observations pointing towards persistent instability.

On the more exposed above treeline northerly aspects, large areas of bare ground remain exposed due to wind scouring by strong E-NE winds from earlier this fall. Clear sunny weather and warmer temperatures have melted most of the snow off of the aspects facing the southerly half of the compass and only few small areas of snow remain on some of them.

Barely covered rocks, stumps, logs, and other hard, immobile, season-ending, equipment-breaking objects still exist on all slopes. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

Weather and CURRENT CONDITIONS
weather summary

The winds shifted to the N and NE around 4am heralding the arrival of a cold front. This front should bring colder temperatures, continued strong NE winds, and some snow. The forecast calls for 2-5 inches of accumulation above 7000 ft. Snowfall should start to taper off by noon with only snow showers expected this afternoon. With colder air over the region today's highs should remain in the upper teens and low 20's in the mountains. By tomorrow expect sunny skies, warmer temperatures (highs in the low 30's above 7000 ft.), and continued NE winds as the high pressure ridge returns to the area. For more details and up to date weather information check in with the Reno NWS.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 18 to 27 deg. F.
Max. temperature in the last 24 hours: 37 to 47 deg. F.
Average wind direction during the last 24 hours: SW shifting to the NE at 4am
Average wind speed during the last 24 hours: 25 to 30 mph
Maximum wind gust in the last 24 hours: 71 mph
New snowfall in the last 24 hours: 0 to 1 inches
Total snow depth: 7 to 16 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Snow in the morning. Snow showers tapering off in the afternoon. Partly cloudy with a few scattered snow showers. Sunny in the morning with some clouds developing in the afternoon.
Temperatures: 18-25 deg. F. 11-17 deg. F. 30-37 deg. F.
Wind Direction: NE NE NE
Wind Speed: 20-30 mph with gusts to 40 mph 20-30 mph with gusts to 40 mph 15-25 mph with gusts to 40 mph
Expected snowfall: 2-4 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Snow in the morning. Snow showers tapering off in the afternoon. Partly cloudy with a few scattered snow showers. Sunny in the morning with some clouds developing in the afternoon.
Temperatures: 18-24 deg. F. 9-16 deg. F. 29-36 deg. F.
Wind Direction: N shifting to the NE NE NE
Wind Speed: 20-25 mph with gusts to 40 mph increasing to 35-40 mph with gusts to 60 mph 45-50 mph with gusts to 70 mph 35-40 mph with gusts to 60 mph
Expected snowfall: 2-4 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.