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 December 12, 2010:


December 12, 2010 at 7:51 am

LOW avalanche danger exists in most areas this morning. Pockets of MODERATE avalanche danger will quickly develop on sun-exposed E-SE-S-SW-W aspects steeper than 37 degrees at all elevations due to daytime warming.


Forecast Discussion:


Most areas above 7000 ft have stayed well above freezing since yesterday morning. Below 7000 ft an inversion exists allowing colder temperatures at the lower elevations. Warm temperatures should continue today due to the high pressure ridge over the forecast area. The forecast calls for daytime highs in the mid to upper 40's above 8000 ft. and mid 50's below 8000 ft. Expect another sunny day today. By tomorrow a low pressure system moving towards the Pacific Northwest should push some clouds, increased southwest winds, and slightly cooler temperatures into the Central Sierra.

Observations:

Rain crusts and melt-freeze crusts exist on the snow surface across all aspects in the forecast area. Observations from Mt. Rose and Carson Pass over the last few days have found a thin sometimes-breakable, sometimes-supportable rain crust on northerly aspects up to 9500 ft. A 2-4 inch layer of wet snow exists below this rain crust in these areas. Snowpit data and layer bonding tests show a mostly stable snowpack below the wet snow (pit profiles). On the more sun-exposed, southerly aspects on Donner Summit and in the Mt. Rose area yesterday, 6-12 inches of wet, sticky snow existed on the snow surface. In some areas below 7500 ft on Donner Summit unconsolidated, wet snow existed all the way to the ground. Some skier triggered roller-balls occurred in both of these locations yesterday.

Primary Avalanche Concern: Wet-Snow Instabilities

On the sun-exposed southerly aspects, today's sun and warm temperatures will quickly soften whatever re-freeze occurred last night. This melting will introduce more water into the snowpack forming a layer of wet, unconsolidated surface snow in many areas. Human-triggered, wet-snow instabilities will remain possible today. Roller balls, point releases, and sluffs should comprise the majority of these wet-snow instabilities. Even though most of these slow-moving, wet, loose snow avalanches may not be enough to bury a person, they could push a person into an area with serious consequences like over a cliff, into a creek or gully, or into some other LOW danger does not mean no danger. Areas of unstable snow may still exist on isolated terrain features and in extreme terrain (steep couloirs, unsupported snowfields, complex terrain, etc).


The bottom line:

LOW avalanche danger exists in most areas this morning. Pockets of MODERATE avalanche danger will quickly develop on sun-exposed E-SE-S-SW-W aspects steeper than 37 degrees at all elevations due to daytime warming.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 39-43 deg. F.
Max. temperature in the last 24 hours: 44-48 deg. F.
Average wind direction during the last 24 hours: West southwest
Average wind speed during the last 24 hours: 10-15 mph
Maximum wind gust in the last 24 hours: 39 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 33-49 inches

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

For 7000-8000 ft:

  Sunday: Sunday Night: Monday:
Weather: Sunny Clear Partly cloudy
Temperatures: 45-55 deg. F. 28-38 deg. F. 40-50 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: around 10 mph around 10 mph 10-20 mph with gusts to 35 mph
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Sunday: Sunday Night: Monday:
Weather: Sunny Clear Partly cloudy
Temperatures: 38-48 deg. F. 32-42 deg. F. 34-44 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 10-15 mph with gusts to 30 mph 10-15 mph with gusts to 30 mph increasing to 15-25 mph with gusts to 45 mph after midnight 25-35 mph with gusts to 55 mph increasing to 35-45 mph with gusts to 70 mph in the afternoon
Expected snowfall: O in. O in. O in.

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