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, December 28, 2006 at 12:30 pm

Afternoon Update: The windslabs deposited by the northeast winds are stiff and dense. They will are sitting on top of a mix of unconsolidated snow and crusts left after the storm ended yesterday. The interface between these layers fails easily when a small area is isolated. However, the slab is so cohesive and strong that it spreads out the force of a skier or snowboarder well and makes it hard for them to transmit force to down to the weak interface. A larger trigger such as a snowmobiler, large cornice failure, or several people on the same slope could be able to transmit force through the slab.

Near and above treeline, avalanche danger is MODERATE. Below treeline, avalanche danger is MODERATE. Avalanche activity will be most likely on recently wind loaded NW-N-NE-E aspects 35 degrees and steeper.

Previous advisory posted at 7:00 am today:

A cold high pressure system has moved in following this storm. Our storm snow totals range from 15-18 inches along the Sierra crest with the areas on the north end of the forecast area receiving the most snow. The Mt. Rose area got about 14 inches of new snow. Yesterday afternoon the winds shifted to the northeast. Those northeast winds are forecasted to remain strong through today and should start to moderate late tonight. The next few days should be clear and cold while the high pressure remains over our area.

Yesterday afternoon ridgetop winds shifted to the northeast and have been blowing at about 70 mph ever since. These winds are loading the NW-W-SW slopes. The windslabs deposited on the NW aspects will be the most reactive. They are sitting on crusts left from the southwest winds during the storm. Since it has been cold ever since the wind shifted, these new windslabs will not be able to bond to those crusts very quickly. The windslabs will also be stiffer and more likely to propagate fractures than than yesterday's windslabs, because this is the second time the wind has transported this snow. This time it will be able to pack the snow more densely and create a more cohesive, stiffer slab. The N-NE-E aspects that were loaded yesterday will be scoured today. The storm snow was able to bond very well to the existing snowpack due to the initial warmth of the storm. This bonding will help keep some of that initial storm snow in place on those aspects as the wind blows today. All of the wind slabs that were deposited yesterday on the N-NE-E aspects will not be moved today. Due to the high wind speeds during the storm, these windslabs made their way much farther downslope and into more typically sheltered areas than usual. They will still exist in those areas today where they are sheltered from the strongest NE winds. Yesterday numerous test slopes failed and a few small avalanches occurred due to skier triggering at mid to lower elevations. Observers also reported shooting cracks propagating from their ski tips along the crest and whumphing in the Rose area. The snowpack is still tender and in many places a skier, snowboarder, or snowmobiler could be all that is needed to trigger an avalanche.

Today human triggered avalanches will be probable on any steep windloaded slopes. At mid to high elevations the NW aspects will be most suspect. At mid to low elevations the N-NE-E aspects will still be fragile. Any avalanches that are triggered will likely be those windslabs failing on weaknesses in the storm snow and stepping down to the crust layers. Watch out for any areas that look windloaded. Look for ripples, drifts, and other wind features to clue you into whether or not a slope is wind loaded today.

Near and above treeline, avalanche danger is CONSIDERABLE. Below treeline, avalanche danger is CONSIDERABLE. Avalanche activity will be most likely on recently wind loaded NW-N-NE-E aspects 33 degrees and steeper.

Andy Anderson , Avalanche Forecaster
Today's Central Sierra Weather Observations:
0600 temperature at Sierra Crest (8,700 feet):
6 deg. F
Max. temperature at Sierra Crest past 24 hours:
24 deg. F
Average wind direction at Sierra Crest past 24 hours:
Northeast
Average wind speed at Sierra Crest past 24 hours:
70 mph
Maximum wind gust at Sierra Crest past 24 hours:
108 mph
New snow fall at 8,200 feet past 24 hours:
11 inches
Total snow depth at 8,200 feet:
37 inches
Mountain Weather Forecast For Today:
Partly cloudy
Temperature forecast for 8,000 to 9,000 feet:
31 deg. F
Ridgetop winds forecast for the Sierra Crest:
northeast at 40-60 mph with gusts to 105 diminishing to 90 this afternoon.
Snowfall expected in the next 24 hours:
0 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet
For today, partly cloudy with winds out of the the northeast at 30-40 mph gusting to 45. Daytime high around 29 degrees F. Overnight clear with lows 8 to 18 degrees F. East winds at 20-30 mph with gusts to 35 weakening slightly after midnight. Wednesday, sunny with daytime highs around 42 degrees F. Northeast winds 10 to 15 mph.

Above 8000 Feet
For today, partly cloudy with winds out of the the northeast at 40-60 mph gusting to 105. Daytime high around 31 degrees F. Overnight clear with lows 14 to 22 degrees F. Northeast winds at 30-45 mph with gusts to 70 weakening slightly after midnight. Wednesday, sunny with daytime highs around 43 degrees F. Northeast winds 15 to 20 mph.

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.