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 19, 2007 7:00 AM
Click here for a detailed map of the SAC forecast area

Low avalanche danger
Today's Advisory:
The avalanche danger is LOW today at all elevations and aspects. Low danger does not mean no danger and some small, disconnected instabilities could still be hiding in the snowpack. These would be limited to slopes steeper than 35 degrees 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. With so many barely covered objects out there, even a small slide or careless riding could cause a collision with several rocks, logs, stumps, or trees.

Storm totals have reached 15 to 18 inches across the Sierra Crest. The areas south of Highway 50 received a little more snow from this system than the the areas north of Highway 50. The Mount Rose area only received 3-5 inches out of this storm. For most of the day, the forecast area should be in between storms. The forecast calls for the next system to arrive this evening. Snow should be heavy overnight and through tomorrow morning before tapering off in the afternoon and evening. This next system should be colder with stronger winds and more snowfall. Today the southwest winds should diminish some before increasing again tonight.

Most of the snow that fell yesterday was relatively heavy and wet. Near and above treeline the southwest winds loaded the N-NE-E aspects that were exposed ground before this storm. The rocks, shrubs, and small trees that are only barely buried (or even still exposed) should provide sufficient anchoring and should help keep widespread instability from forming on many of these slopes. As the wind continues to transport snow today and the snow continues to accumulate tonight and tomorrow, these anchors will be buried and their stabilizing influence will decrease. The wind slabs that started forming on open slopes and gully features near and above treeline on N-NE-E aspects will continue to grow through today and tomorrow. Avalanche activity involving these wind slabs is unlikely but not impossible today and would be small in size and limited to the most heavily windloaded areas if it occurs.

Below treeline this new, more dense snow fell on a combination stiff crusts and unconsolidated snow left over from the Dec 6-7 storm. Surface hoar was seen on the snowpack in the Mount Rose area, the Carson Pass area, and in the Mount Judah area prior to this storm. Bonding between the new snow and the old snow surfaces that are a mix of slippery crusts and surface hoar is suspect. The variable distribution of the surface hoar and slippery crusts makes it hard to predict what will happen on individual slopes without testing to see if that layer is present. Dig into the snowpack and check out the interface between the old and new snow before jumping onto any slope. Instability due to the old/new snow interface is unlikely to cause any large slides today. It should be more isolated to small pockets in areas with the perfect combination of ingredients. Avalanche activity resulting from this instability is unlikely but not impossible. It should 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.

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 throughout the week. 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 today at all elevations and aspects. Low danger does not mean no danger and some small, disconnected instabilities could still be hiding in the snowpack. These would be limited to slopes steeper than 35 degrees 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. With so many barely covered objects out there, even 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: 27 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: 81 mph
New snow fall at 8,200 feet past 24 hours: 12 inches
Total snow depth at 8,200 feet: 25 inches
Mountain Weather Forecast For Today:
Cloudy with scattered snow showers.
Temperature forecast for 8,000 to 9,000 feet: 25 to 31 deg. F.
Ridgetop winds forecast for the Sierra Crest: Southwest at 25 to 35 mph with gusts to 60 mph.
Snowfall expected in the next 24 hours: 13-17 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today cloudy with scattered snow showers. Daytime highs 31 to 36 deg. F. Winds out of the southwest at 15 to 20 mph with gusts to 35 mph expected.

Tonight 7 to 12 inches of snow with overnight lows 23 to 29 deg. F. Winds out of the southwest at 20 to 30 mph with gusts to 50 mph expected.

For Thursday 5 to 10 inches of snow. Daytime highs should reach the mid 20's in the morning before falling into the teens in the afternoon. Winds out of the southwest at 20 to 35 mph with gusts to 50 mph shifting to the northwest at 15 to 25 mph by the afternoon.

Above 8000 Feet:
Today cloudy with scattered snow showers and up to 1 inch of accumulation. Daytime highs 25 to 31 deg. F. Winds out of the southwest at 25 to 35 mph with gusts to 60 mph expected.

Tonight 12 to 16 inches of snow with overnight lows 21 to 26 deg. F. Winds out of the southwest at 30 to 45 mph with gusts to 100 mph expected.

For Thursday 6 to 10 inches of snow. Daytime highs should reach the low 20's in the morning before falling into the low teens in the afternoon. Winds out of the southwest at 40 to 55 mph with gusts to 100 mph in the morning shifting to the northwest at 25 to 35 mph with gusts to 80 mph 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:
	Wednesday:	Wednesday Night:	Thursday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.