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.


This Avalanche Advisory was published on April 4, 2010:


April 4, 2010 at 6:50 am

Near and above treeline, avalanche danger is LOW with pockets of MODERATE danger on NW-N-NE aspects, 37 degrees and steeper. Below treeline, avalanche danger is LOW with pockets of MODERATE danger in open wind affected areas on NW-N-NE aspects, 37 degrees and steeper. Destructive avalanches are possible today in very isolated areas. A significant and widespread increase in avalanche danger is expected to occur during the evening and overnight hours.


Forecast Discussion:


An approaching storm system will bring increasing southwest winds and light snowfall to the forecast area today. Snowfall rates are expected to increase significantly after 5 pm. Up to 2 inches of new snow is expected today with a additional 6 to 16 inches of new snow possible overnight. Moderate speed southwest ridgetop winds this morning are expected to continue through Monday. Remote sensors above 8,000' are reporting air temperatures in the mid teens to low 20s this morning. Air temperatures across the forecast area are expected to warm into the 20s and low 30s today.

Observations:

Observations made yesterday on Angora Peak (Desolation Wilderness/Echo Summit area) revealed dense wind slabs on top of lower density snow from the March 30 storm cycle. No obvious signs of instability were observed while on tour in the area between 6,400' and 8,400' including no failures from ski cuts made on steep convex test slopes. Snowpit data collected below treeline at 8,200' on a N aspect revealed repeatable test results indicating that fracture propagation was possible on the layer density change at the bottom of the wind slab (video, more info). On Hidden Peak (West Shore Tahoe area), no evidence of instability was observed during informal observations on N-NE aspects between 6,700' and 9,100' in below treeline and near treeline terrain. Settlement cones were noted around trees in several areas. Snowpit data collected along the summit ridge at 9,050' in near treeline terrain on a NE aspect 38 degree slope revealed no evidence of instability. A similar but less abrupt change from a higher density surface wind slab to the lower density March 30 snow layer below was observed (pit profile).

Avalanche concerns: Wind Slabs

Avalanche concerns for today focus on lingering wind slabs that formed during the April 2 snowfall. With very little evidence of instability observed yesterday, any avalanche activity that occurs today will be human triggered and will occur within larger areas of seemingly very stable snow. The most likely areas for lingering instability are in steep terrain both above and below treeline on NW-N-NE aspects where the higher density surface wind slab sits on top of significantly lower density snow. With a higher density surface slab in place, it is very possible that snowpack failure may occur after one or more people has traveled a slope and after a person is well out onto the slope. With the existing slabs composed of high density snow, destructive avalanches are possible today in very isolated areas. Minimal old snow is available for wind transport today and new snow amounts are expected to be less than 2 inches, keeping concerns for new slab formation to a minimum.

 


The bottom line:

Near and above treeline, avalanche danger is LOW with pockets of MODERATE danger on NW-N-NE aspects, 37 degrees and steeper. Below treeline, avalanche danger is LOW with pockets of MODERATE danger in open wind affected areas on NW-N-NE aspects, 37 degrees and steeper. Destructive avalanches are possible today in very isolated areas. A significant and widespread increase in avalanche danger is expected to occur during the evening and overnight hours.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 15 to 21 deg. F.
Max. temperature in the last 24 hours: 25 to 30 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 64 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 102 to 158 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:

  Sunday: Sunday Night: Monday:
Weather: Cloudy skies with isolated snow showers in the morning. Snow likely in the afternoon. Cloudy skies with snow. Cloudy skies with snow likely in the morning. Cloudy skies with a chance of snow showers in the afternoon.
Temperatures: 23 to 33 deg. F. 12 to 16 deg. F. 18 to 28 deg. F.
Wind direction: SW SW SW
Wind speed: 20 to 30 mph with gusts to 50 mph. 20 to 30 with gusts to 55 mph. 15 to 25 mph with gusts up to 50 mph. Gusts decreasing to 35 mph in the afternoon.
Expected snowfall: Up to 2 in. 6 to 12 in. 2 to 5 in.

For 8000-9000 ft:

  Sunday: Sunday Night: Monday:
Weather: Cloudy skies with isolated snow showers in the morning. Snow likely in the afternoon. Cloudy skies with snow. Cloudy skies with snow likely in the morning. Cloudy skies with a chance of snow showers in the afternoon.
Temperatures: 20 to 25 deg. F. 8 to 12 deg. F. 15 to 20 deg. F.
Wind direction: SW SW W
Wind speed: 20 to 40 mph with gusts to 50 mph. Gusts increasing to 65 mph in the afternoon. 25 to 45 mph with gusts to 70 mph. 20 to 30 mph with gusts to 50 mph.
Expected snowfall: Up to 2 in. 10 to 16 in. 2 to 5 in.

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