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 Thursday, January 4, 2007 at 7:00 am

The weather service has issued a winter storm warning for the forecast area through 4pm today. The freezing level was at 7200' feet this morning and will drop to down to the valleys by tonight. We have already received about 6" of 18-20% density snow above 8200' this morning. By the end of today we could see another 14" accumulate. With ridgetop winds blowing out of the southwest at 40-55mph, there will be significant windloading in the upper elevation starting zones today. The cold front will pass to the east of us by early tomorrow morning leaving strong north - east winds and cold temperatures in its wake.

The initial warmth of this storm will help the storm snow bond to the old snow. As the temperatures get colder, today the wet, heavy snow we received this morning will freeze to the old snow layers. There will also be density and crystal type changes in the storm snow as the temperatures drop. These changes will create layers in the storm snow that will serve as weakness for avalanches to fail on. Compounding these facts are the winds. The southwest winds are blowing at about the prefect speed to create significant windslabs on the upper elevation N-NE-E starting zones. These windslabs will be many times deeper than the overall accumulation from this storm. Deeper slabs mean more weight on the snowpack and a much greater chance for avalanches to be triggered on those slopes. Today will be tricky because the conditions will be changing throughout the day. Early in the day there will probably not be enough new weight on the snow pack to cause natural avalanches; however, the conditions will be prime for human triggering to occur. If the winds continue as forecasted and we receive 8-14" of new snow, we will probably start to see natural avalanches by this afternoon on windloaded slopes steeper than 35 degrees. The lower elevations will be slightly behind the upper elevations as it will take longer for them to cool down. There will also be less snow and the wind will have less of an effect on the slopes. Still there will be enough new snow for human triggered avalanches to occur on steep, open, windloaded, lower elevation slopes by this afternoon. Most of the avalanches that occur today will fail within the new snow. They could be large as we now have enough snow cover to create more uninterrupted starting zones.

Keep your eyes open today and watch out for any signs of windloading: drifting, cornices, ripples, blowing snow, etc. These windloaded slopes will be the most dangerous. Find some small, steep test slopes that would not harm you if they did slide and jump on them to see how they react. These miniature pieces of terrain can be great tools to indicate what could happen on similar larger terrain. Watch out for other obvious signs of instability like recent avalanche activity, cracks shooting out from your skis, and whumphing noises. On days like this when the snow is rapidly changing, you have to always be watching for indicators like these and be ready to move off of steeper terrain into safer areas like ridgelines and heavily treed slopes.

Above treeline the avalanche danger is CONSIDERABLE this morning and will increase to HIGH on wind loaded N-NE-E aspects, 35 degrees and steeper by this afternoon. Below treeline the avalanche danger is MODERATE will increase to CONSIDERABLE on steep, open, wind loaded slopes by this afternoon.

Andy Anderson , Avalanche Forecaster

Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet):
28 deg. F
Max. temperature at Sierra Crest past 24 hours:
48 deg. F
Average wind direction at Sierra Crest past 24 hours:
southwesterly
Average wind speed at Sierra Crest past 24 hours:
50-60 mph
Maximum wind gust at Sierra Crest past 24 hours:
104 mph
New snow fall at 8,200 feet past 24 hours:
7 inches
Total snow depth at 8,200 feet:
39 inches
Mountain Weather Forecast For Today:
Snow, heavy at times. Freezing levels at 7000' this morning dropping all the way to the valleys by this afternoon.
Temperature forecast for 8,000 to 9,000 feet:
18-24 deg. F this morning dropping to low teens by this afternoon
Ridgetop winds forecast for the Sierra Crest:
southwesterly 40 - 55 mph, G 80-100 mph
Snowfall expected in the next 24 hours:
10-16 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet
For today, snow with accumulations of 8-12". Freezing levels at 7000' this morning dropping all the way to the valleys by this afternoon. Highs 24 to 31 degrees F, decreasing to around 18 degrees F in the afternoon. Southwest winds 20 to 35 mph gusting to 50 mph in the afternoon. Overnight lows around 8 degrees F with isolated snow showers. Snow accumulation 1 inches. West winds at 20 to 30 mph shifting to the north after midnight. Friday will be partly cloudy. Daytime highs around 25 degrees F. North winds at 20-35 mph.

Above 8000 Feet
For today, snow with accumulations of 10-14". Freezing levels at 7000' this morning dropping all the way to the valleys by this afternoon. Highs 18 to 24 degrees F, decreasing to around 12-15 degrees F in the afternoon. Southwest winds 40 to 55 mph gusting between 80-100 mph. Overnight lows around 6 degrees F with isolated snow showers. Snow accumulation 2 inches. West winds at 30 to 45 mph shifting to the north after midnight with gusts to 85mph. Friday will be partly cloudy. Daytime highs around 17-22 degrees F. North winds at 30-40 mph with gust to 80.

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:
	Thursday:	Thursday Night:	Friday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Thursday:	Thursday Night:	Friday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.