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 advisory was posted on December 18, 2007 7:05 AM
Click here for a detailed map of the SAC forecast area

Low avalanche danger
Today's Advisory:
The avalanche danger is LOW this morning at all elevations and aspects. By the end of the day on slopes steeper than 35 degrees, it will increase to MODERATE on the most heavily wind loaded N-NE-E aspects near and above treeline and on open NW-N-NE aspects that have been sheltered from the wind and had snow cover previous to this storm below treeline. Any avalanche activity that does occur today is likely to be limited in size and extent. However with so many barely covered objects out there a small slide or careless riding could cause a collision with several rocks, logs, stumps, or trees.

Storm totals from the first small system only reached 3 to 5 inches across the Sierra Crest. Most of that snow fell Sunday night and during the day yesterday in the northern half of the forecast area. In the southern half of the forecast area most of the snow fell late yesterday and last night. The Mount Rose area missed most of this initial wave. The next system has already pushed into central California. It has started snowing along the Sierra Crest south of Highway 50 and in Colfax, CA. The forecast is for the snow to spread across the Sierra and continue through today. Snowfall rates are forecast to reach 2-3 inches an hour for much of the day. Another 10 to 18 inches of snow is forecast above 7000' by the end of today. The snow should taper off tonight before another storm moves into the forecast area late Wednesday night. The southwest winds accompanying this series of storms should remain consistently strong throughout most of this week.

As these southwest winds have more snow to transport, they will begin forming wind slabs on open slopes and gully features near and above treeline on N-NE-E aspects. These slabs should start out as small, disconnected pillow drifts. Sufficient anchoring will help keep widespread instability from forming on many of these slopes. However, as more snow accumulates the anchors will have less and less influence. These anchors also have a negative side. Right now they are only buried under a few inches of unconsolidated snow making them hard to see but easy to hit. By the end of today many of these anchors could be buried enough to allow some small avalanche activity on the most heavily wind loaded slopes near and above treeline that are steeper than 35 degrees.

In the areas that still held snow cover from the December 6-7th storm, the combination of a shallow snowpack and consistently cold temperatures have caused the snow grains to weaken. In those areas the snowpack consists of a mix of decomposing new snow and faceted grains (weak, sugary snow). This stratigraphy is overlain by a variety of stiff crusts in many areas. Surface hoar has been seen on top of these crusts in the Mount Rose area, the Carson Pass area, and in the Mount Judah area. Bonding between the new snow that falls today and the old snow surfaces that are a mix of slippery crusts and surface hoar will be suspect. By the end of today some small avalanche activity could result due to new snow overloading this interface. These slides would also be small in size and should be limited to open slopes below treeline steeper than 35 degrees that had snow cover on them before this storm. The below treeline NW-N-NE aspects that are sheltered from the wind will be the most suspect because they are the slopes that would be the most likely to preserve the surface hoar and allow it to be buried by the new snow that falls today.

The biggest hazard is still going to be a shallow snowpack that barely covers many rocks, logs, and stumps. Crashing into these objects could easily ruin the winter. The avalanche hazard will increase as more snow falls today. Remember to stay observant as you travel, look for indications of the current snowpack stability such as recent avalanche activity, wind loading, collapse, audible whumpfing sounds, shooting cracks, and/or test slope failure. Any of these signs are indications of current snowpack instability.

The bottom line: The avalanche danger is LOW this morning at all elevations and aspects. By the end of the day on slopes steeper than 35 degrees, it will increase to MODERATE on the most heavily wind loaded N-NE-E aspects near and above treeline and on open NW-N-NE aspects that have been sheltered from the wind and had snow cover previous to this storm below treeline. Any avalanche activity that does occur today is likely to be limited in size and extent. However with so many barely covered objects out there a small slide or careless riding could cause a collision with several rocks, logs, stumps, or trees.

Please send us your snow, weather, and avalanche observations by clicking the submit observations link on our contact page.

Andy Anderson, Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet): 21 deg. F
Max. temperature at Sierra Crest past 24 hours: 23 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 45 mph
Maximum wind gust at Sierra Crest past 24 hours: 80 mph
New snow fall at 8,200 feet past 24 hours: 6 inches
Total snow depth at 8,200 feet: 15 inches
Mountain Weather Forecast For Today:
Snow with accumulation of 12-18 inches.
Temperature forecast for 8,000 to 9,000 feet: 26 to 30 deg. F.
Ridgetop winds forecast for the Sierra Crest: Southwest at 30 to 45 mph with gusts to 90 mph.
Snowfall expected in the next 24 hours: 10-18 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today 10 to 18 inches of snow with daytime highs around 32 deg. F. Winds out of the southwest at 20 to 30 mph with gusts to 45 mph expected.

Tonight 2 to 4 inches of snow with overnight lows around 25 deg. F. Winds out of the southwest at 15 to 25 mph with gusts to 35 mph expected.

For Wednesday cloudy with a chance of snow. Daytime highs around 34 deg. F. Winds out of the southwest at 20 to 30 mph with gusts to 50 mph expected by the afternoon.

Above 8000 Feet:
Today 12 to 18 inches of snow with daytime highs around 29 deg. F. Winds out of the southwest at 30 to 45 mph with gusts to 90 mph expected.

Tonight 2 to 4 inches of snow with overnight lows around 20 deg. F. Winds out of the southwest at 25 to 35 mph with gusts to 60 mph expected.

For Wednesday cloudy with a chance of snow. Daytime highs around 31 deg. F. Winds out of the southwest at 20 to 35 mph with gusts to 80 mph expected by the afternoon.

The bottom line:

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	deg. F.
Max. temperature in the last 24 hours:	deg. F.
Average wind direction during the last 24 hours:
Average wind speed during the last 24 hours:	mph
Maximum wind gust in the last 24 hours:	mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	inches

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

For 7000-8000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.