THIS AVALANCHE ADVISORY EXPIRED ON February 6, 2014 @ 6:58 am
Avalanche Advisory published on February 5, 2014 @ 6:58 am
Issued by Andy Anderson - Tahoe National Forest

The avalanche danger remains LOW at all elevations and on all aspects. Even though avalanches have become unlikely on a regional scale, some unstable snow could still lurk on isolated terrain features or in extreme terrain.

1. Low

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

1. Low

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

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
Avalanche Problem 1: Normal Caution
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Across the forecast area a layer of soft, weak snow sits on top of a strong refrozen snowpack in most places. Since no slabs exists on top of the soft, weak snow this weakness does not currently pose a problem, and avalanches remain unlikely in most places today. However, two potential problems could arise concerning this layer of soft snow once more snow accumulates above it:

  1. A buried surface hoar layer exists in the upper few inches of the snowpack in some areas. Surface hoar often forms during the calm, clear, cold nights here in Tahoe, but it rarely gets buried. Last Sunday's light snowfall buried a surface hoar layer under 2-4 inches of light, soft snow in some areas.
  2. The snow above the rain crust has started to weaken and facet and take on the characteristics of a persistent weak layer.

For now keeping track of where these layers exist and how they are changing will help determine which slopes may pose future problems. While traveling in the backcountry, stop and dig into the snow from time to time. Sift soft snow above the rain crust through your hands. If it feels sugary and you can't make a snowball out of it, it is probably facets. If you see any triangular shaped, striated snow grains in the top few inches of the snowpack, a shallowly buried surface hoar layer exists in that area.

Avalanche Problem 2: Persistent Slab
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In areas above 9,500 to 9,800 ft, rain did not percolate all the way through the old facetted snow layers on near treeline and below treeline NW-N-NE aspects. In these upper elevation areas, human triggered persistent slab avalanches are not impossible. Weak layers of well developed facets and depth hoar exist below the overlying slab of recent snow in these areas. Since there are very few areas in the forecast area with terrain above this elevation where these conditions exist, this problem remains isolated in distribution.

recent observations

Yesterday on Tamarack Peak, 8 to 12 inches of soft, cold, unconsolidated snow existed above a 4 to 6 inch refrozen rain crust on the N-NE aspects below 9500 ft. On other aspects much less of the soft snow remained. Ski cuts on test slopes, snowpit data, and other observations did not reveal signs of instability in the upper soft snow layers in most places. Data did indicate that this soft snow layer has begun to facet and weaken. One wind loaded NE facing test slope at 9200 ft did release a 3 to 4 in deep wind slab in response to a ski cut (photos). This small slab was about 15 ft wide and ran downhill for about 20 to 30 ft.  Below the rain crust an unconsolidated layer of large snow grains (what used to be the Dec. 7th facets) still remained. Tests and observations indicated that it is difficult to get force through the thick rain crust.

Above 9500 ft. the rain crust thinned along with the thickness of the old snow layers below it. On many of the near and above treeline slopes in this area the recent soft snow sat on top of the ground with no old snow below it (see photo from the top of the Hourglass).

Unlike Silver Peak and Rubicon Peak earlier this week, no signs of buried surface hoar existed in this area.

Weather and CURRENT CONDITIONS
weather summary

After another cold, dry, and partly cloudy day today, the first of a series of storms should move into the region tonight. The west and southwest winds should increase some this afternoon and evening ahead of this storm. Clouds and winds should conintue to increase through the night and some scattered snow showers could occur. The forecast for tomorrow calls for 2 to 5 inches of new snow above 7000 ft. and 2 to 4 inches at lake level tomorrow. Temperatures should remain cold with daytime highs above 8000 ft in the 20's both today and tomorrow. The forecast calls for a stronger, warmer, and much wetter storm to move into the area this weekend. For more information and current forecasts check in with the Reno NWS

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 10 to 21 deg. F.
Max. temperature in the last 24 hours: 22 to 30 deg. F.
Average wind direction during the last 24 hours: West Southwest
Average wind speed during the last 24 hours: 10 to 20 mph
Maximum wind gust in the last 24 hours: 45 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 14 to 30 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy becoming mostly cloudy Mostly cloudy with a 30% chance of scattered snow showers after midnight Snow likely
Temperatures: 28 to 35 deg. F. 11 to 20 deg. F. 28 to 35 deg. F.
Wind Direction: West Southwest Southwest
Wind Speed: 15 to 20 mph with gusts to 30 mph in the afternoon 15 to 20 mph with gusts to 30 mph 15 to 25 mph with gusts to 30 mph increasing to 40 mph in the afternoon
Expected snowfall: 0 in. trace in. 1 to 4 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy becoming mostly cloudy Mostly cloudy with a 30% chance of scattered snow showers after midnight Snow likely
Temperatures: 21 to 28 deg. F. 11 to 19 deg. F. 19 to 28 deg. F.
Wind Direction: West West Southwest
Wind Speed: 10 to 15 mph with gusts to 25 mph increasing to 25 to 30 mph with gusts to 45 mph in the afternoon 20 to 30 mph with gusts to 45 mph 25 to 35 mph with gusts to 40 mph increasing to 50 mph in the afternoon
Expected snowfall: 0 in. trace in. 2 to 5 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.