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 4, 2012:


December 4, 2012 at 8:00 am

Near and above treeline, avalanche danger remains MODERATE on NW-N-NE-E aspects on slopes steeper than 35 degrees due to lingering wind slabs and rain on snow today and tonight. For all other areas, avalanche danger is LOW with pockets of MODERATE danger on snow-covered slopes steeper than 35 degrees that receive rain today. Expect the avalanche danger to increase as more rain, snow, and wind impact the forecast area tonight and tomorrow.


Forecast Discussion:


Light rain and snow showers should begin over the mountains today as another warm low pressure system approaches the forecast area. The forecast only calls for about .1 inches of precip today. Most of this precipitation should fall as rain below 9000 ft. The main system should move into the region tonight bringing more precipitation, increased southwest winds, and continued warm temperatures. The winds should remain in the 30-40 mph range with gusts as high as 70 mph. The forecast calls for snow levels to remain around 9000 ft until tomorrow when they could drop to around 8500 ft. By tomorrow another 1-1.5 inches of precipitation could have fallen on the forecast area as a mix of rain and high elevation snow.

Recent Observations:

Data collected yesterday on Red Lake Peak, near Grouse Rocks, and on Tamarack Peak indicated a mostly stable snowpack with a few lingering areas of instability due to fragile wind slabs and poor bonding between the snow that fell at the end of the storm and a rain crust below it. On the eastern side of Tamarack Peak, a very small skier-triggered avalanche occurred resulting from the failure of one of these small wind slabs (more info). Tender cornices also still existed on Tamarack Peak. On Red Lake Peak, snowpit tests where the shallow wind slabs existed still showed that fractures could occur at the base of the winds slabs (snowpit). Ski cuts in steeper less wind affected terrain in the Grouse Rocks area resulted in ski width wide sluffs that entrained the 4-6 inches of snow that remained above a refreezing rain crust (photo). In all of these areas, the majority of observations showed settlement and consolidation in the snowpack (photos of settlement cracks near Grouse Rocks and on Red Lake Peak, stable snowpit results on Tamarack). Deeper in the snowpack snowpits on Red Lake Peak and in the Grouse Rocks area showed a mix of crusts, wet snow that continues to freeze, and consolidated snow.

Today's Primary Avalanche Concern: Rain on Snow

Light rain falling on the recent snow that sits on the surface will add weight to the snowpack and could weaken the strong surface slabs that exist in some areas. It will also add warmth to the snowpack and cause the refreezing process occurring within the snowpack to slow down or even stop in places. These effects will allow wet snow avalanches become possible today. Most of these types avalanches should manifest as loose snow sluffs, roller balls, and pinwheels; however, some wet slab avalanches could occur on the slopes that receive rain where more snow exists.

Today's Secondary Avalanche Concern: Wind Slabs

Wind slabs remaining on wind loaded near and above treeline slopes could still pose a problem today. Many of these winds slabs should remain stubborn and difficult to trigger. However, if rain falls on these wind slabs avalanches involving these wind slabs could become easier to trigger. The NW-N-NE-E aspects in near and above treeline terrain will represent the most likely places to find this instability. Human triggered avalanches large enough to bury or injure a person remain possible.


The bottom line:

Near and above treeline, avalanche danger remains MODERATE on NW-N-NE-E aspects on slopes steeper than 35 degrees due to lingering wind slabs and rain on snow today and tonight. For all other areas, avalanche danger is LOW with pockets of MODERATE danger on snow-covered slopes steeper than 35 degrees that receive rain today. Expect the avalanche danger to increase as more rain, snow, and wind impact the forecast area tonight and tomorrow.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 26-36 deg. F.
Max. temperature in the last 24 hours: 31-39 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 30-40 mph
Maximum wind gust in the last 24 hours: 60 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 39-58 inches

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

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Chance of rain and snow in the morning turning to rain in the afternoon. Snow level rising to near 9000 ft. Cloudy with rain especially after midnight. Rain
Temperatures: 39-46 deg. F. 30-37 deg. F. 35-42 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 25-35 mph with gusts up to 50 mph 25-35 mph with gusts to 50 mph 20-30 mph with gusts to 45 mph
Expected snowfall: rain up to .1 in. rain - around .4 in. rain - around .7 in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Chance of rain and snow through the day. Snow level rising to near 9000 ft. Rain and snow likely. Snow level near 9000 ft. Rain and snow. Snow level near 8500 ft.
Temperatures: 39-45 deg. F. 27-34 deg. F. 34-40 deg. F.
Wind direction: Southwest Southwest West
Wind speed: 35-45 mph with gusts between 65-70 mph decreasing to 55-60 mph in the afternoon 35-45 mph with gusts 60-70 mph 25-35 mph with gusts 45-55 mph
Expected snowfall: up to 1 in. up to 2 in. 3-7 in.

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