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, February 22nd, 2007 at 7:05 am

The bottom line: Near and above treeline avalanche danger is MODERATE this morning and will increase to CONSIDERABLE on all wind loaded aspects with slopes steeper than 30 degrees by this afternoon. Below treeline avalanche danger is MODERATE. Remember that MODERATE avalanche danger means that human triggered avalanches are possible. Most avalanches should fail on weaknesses in the new snow formed during the storm or on the interface with the old snow surfaces. There is a small possibility of some avalanches stepping down to older weak layers in the Mt. Rose area. Avalanche activity will be most likely on wind loaded NW-N-NE-E-SE aspects today.

It started to snow across the forecast area early this morning. The forecast calls for the snow to continue all day before tapering off overnight. Snowfall rates are predicted to be about 1"/hr throughout the day today. The strong southwest winds that increased ahead of this storm will continue until the cold front passes across the area later this afternoon. After frontal passage the winds are forecasted to shift to the west and decrease slightly. The temperatures should also drop this afternoon. Today's high temperatures should occur this morning. Tomorrow some clearing is expected as the cold front settles over the area. Temperatures should remain low and the winds should be moderate out of the west.

As the snow falls today, avalanche danger will increase rapidly. The air temperatures dropped below freezing do last night a few hours before it started snowing. The cold temperatures will have translated to cold snow surfaces on all aspects for the new snow to fall on. The new snow is falling on a mix of crusts, wind scoured surfaces, unconsolidated snow (with weak grains already in it), and some hard wind slabs. This mix of surface conditions can be summed up as a mix of weak grains and hard, slippery surfaces. These surface conditions combined with a cold storm do not bode well for allowing any new snow that falls to bond well to the old snow surfaces. The cold temperatures will also cause the snow to be much less dense than "normal" Sierra snow with snow densities forecasted between 7 and 10%. This lighter snow means the the winds will be able to transport more of it across longer distances. Wind loading on the leeward aspects will create windslabs farther downslope from ridgelines today. Some of the windslabs may even extend into more commonly sheltered areas like sparse trees. On crossloaded slopes the windslabs will extend farther out into the slope. With the winds forecasted to shift to the west, windslabs should also form across more aspects than normal and on slopes that are normally protected from significant wind loading. These windslabs have already started forming and will only get bigger as the day goes on. By the end of the day some natural avalanche activity could occur on some of the wind loaded slopes. Human triggering of these windslabs will become more and more likely as the day progresses. Most avalanches should fail on weaknesses in the new snow formed as conditions change during the storm or on the interface with the old snow surfaces. Since this storm is forecasted to have changing temperatures, wind speeds, wind directions, etc., there will be plenty of opportunities to create these weaknesses in the storm snow. There is also a small possibility of some avalanches stepping down to older weak layers in the Mt. Rose area.

Today is a day to be cautious and conservative in terrain choices. The avalanche conditions will be changing by the minute throughout the day as we get more snow and wind. Last weekend 6 avalanche fatalities and countless close calls occurred in the western USA. Even though the winter has been sparse, don't let "powder fever" overrule good judgment.

No avalanche activity was reported yesterday.For a list of avalanche activity reported to the Sierra Avalanche Center this season, click here.

The bottom line: Near and above treeline avalanche danger is MODERATE this morning and will increase to CONSIDERABLE on all wind loaded aspects with slopes steeper than 30 degrees by this afternoon. Below treeline avalanche danger is MODERATE. Remember that MODERATE avalanche danger means that human triggered avalanches are possible. Most avalanches should fail on weaknesses in the new snow formed during the storm or on the interface with the old snow surfaces. There is a small possibility of some avalanches stepping down to older weak layers in the Mt. Rose area. Avalanche activity will be most likely on wind loaded NW-N-NE-E-SE aspects today.

At this time, we are still short $8,000 in funding for our operating budget this winter. If you use this service with any regularity and would like to continue to do so, please show your financial support for the Sierra Avalanche Center. You may make a tax deductible donation directly to us via Paypal or by sending a check to our address listed above, c/o the Truckee Ranger Station. A financial graph showing the amount of fundraising completed this season and our overall budget is available by clicking here. The financial support already received this winter from a variety of users is greatly appreciated.

Andy Anderson, Avalanche Forecaster
Today's Central Sierra Weather Observations:
0400 temperature at Sierra Crest (8,700 feet):
22 deg. F
Max. temperature at Sierra Crest past 24 hours:
32 deg. F
Average wind direction at Sierra Crest past 24 hours:
southwest
Average wind speed at Sierra Crest past 24 hours:
50-55 mph
Maximum wind gust at Sierra Crest past 24 hours:
92 mph
New snow fall at 8,200 feet past 24 hours:
1-2 inches
Total snow depth at 8,200 feet:
53 inches
Mountain Weather Forecast For Today:
Snow spreading over the area early this morning and continuing throughout the day. Accumulations of up to 14 inches.
Temperature forecast for 8,000 to 9,000 feet:
20's this morning and falling into the teens this afternoon.
Ridgetop winds forecast for the Sierra Crest:
Southwest 40 - 50 mph with gusts over 70 mph shifting to the west and decreasing slightly this afternoon.
Snowfall expected in the next 24 hours:
14-18 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet
Today snow spreading over the area early this morning and snow fall continuing through the day. Accumulations of 8-12 inches. Winds should be out of the southwest at 25-35 mph gusting to 60 mph this morning and are forecasted to shift to the west at 20-30 mph with gusts to 40 mph this afternoon. Daytime highs should start out this morning in the upper 20's and fall to around 20 degrees F this afternoon. Tonight, snow showers with 2 to 4 inches of accumulation. West winds 10 to 20 mph with gusts to 30 mph. Overnight lows 12 to 17 degrees F. Friday, mostly cloudy with a chance of snow. Winds out of the west at 10 to 20 mph. Daytime highs in the 18 to 23 degrees F.

Above 8000 Feet
Today snow spreading over the area early this morning and snow fall continuing through the day. Accumulations of 10-14 inches. Winds should be out of the southwest at 40-50 mph gusting to over 70 mph this morning and are forecasted to shift to the west at 25-40 mph with gusts to 50 mph this afternoon. Daytime highs should start out this morning in the 20's and fall into the teens this afternoon. Tonight, snow showers with 2 to 4 inches of accumulation. West winds 15 to 30 mph with gusts to 50 mph. Overnight lows 7 to 12 degrees F. Friday, mostly cloudy with a chance of snow. Winds out of the west at 15 to 25 mph. Daytime highs in the 14 to 19 degrees F.

Backcountry Travelers:
We want to hear from you! If you see avalanche activity, or want to share condition information for an area you've been in, please let us know. Call (530) 587-3558 or e-mail us at:

sac_avalanche@fs.fed.us

.

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.