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 January 31, 2008 7:00 AM
Click here for a detailed map of the SAC forecast area

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Today's Advisory:

Near and above treeline, the avalanche danger will remain CONSIDERABLE on NW-N-NE-E aspects, 35 degrees and steeper due to new snowfall and high winds today. Pockets of HIGH danger could develop on the most heavily wind loaded slopes by later today. Below treeline, avalanche danger is LOW with more areas of MODERATE danger developing on open wind loaded slopes 37 degrees and steeper.

After a small break in the weather, another storm system should move into the Sierra today. This pattern should continue through the weekend as more storms line up behind this one. Another break between storms should occur tomorrow. Today's storm will move quickly through the area. Snowfall should begin this morning and increase through today and tonight before tapering off tomorrow. The forecast area could see as much as 12 to 18 inches of new snow by Friday morning. The winds that accompany this storm should be from the southwest and strong. Sustained wind speeds should increase to 65 and 75 mph today. Maximum wind gusts are forecasted to reach 130 mph and could be even higher along the ridgelines.

Yesterday the winds calmed much more than forecasted; therefore, little snow transport occurred. This break in loading allowed the new snow to start to settle and consolidate causing the avalanche danger to be less widespread. Today forecasted new snow and high winds at all elevations should lead to an increase in avalanche danger as more wind slabs form throughout the forecast area. The widespread strong winds should cause these wind slabs to form farther down slope from ridgelines, on open slopes below treeline, and even in some of the more protected areas. These new wind slabs will combine with the wind slabs that have formed in the lower, upper, and side regions of avalanche start zones during this week. These high winds will also help to create larger, stiffer wind slabs in some areas.

Old weaknesses in the snow pack have started to settle and gain strength in some areas and not in others. The rain crust from 2 weeks ago has developed weak near crust facets below it in several areas and continues to show weak bonding in snow pit tests in the Kirkwood area, on NW-NE aspects between 7000-7500 ft in the Silver peak area and on some of Mount Judah's NW-NE aspects between 7000 and 8000 ft. Reports from Waterhouse Peak, Jake's Peak and the Mount Rose area indicate that this layer is not prevalent in those areas. Moving up from the old crust the next weak layer of concern is the density inversion (heavy snow on top of light snow) that formed earlier this week. In some areas like Jake's Peak this interface seems to have consolidated and disappeared. Observations on Waterhouse Peak and Mount Judah indicate that this upside down layering is still present. Above 7500' on Silver Peak yesterday this density inversion was present in some areas and not in others. Moving even farther up in the snowpack the shifting winds have deposited a variety of hard and soft wind slabs on top of these layers. Again the distribution and thickness of these slabs varies from peak to peak.

The take home meassage is that this variability in weak layer distribution and strength as well as location, depth, and type of slab creates much more uncertainty the avalanche potential and in snow stability assessments. The only way to determine exactly what is going on in a specific area will be to go there and look into the snow pack by probing with a pole, digging a pit, or using x-ray vision. The uncertainty that this variability adds to assessments means that decisions should be more conservative on a day like today.

Today human triggered avalanche activity and cornice failure will again be probable near and above treeline as new snow accumulates and strong winds form new wind slabs. On the most heavily wind loaded and corniced N-NE-E slopes steeper than 35 degrees human triggering of avalanches may become likely and some natural activity may occur by the end of today. Be wary of wind loaded areas where there is a stiff slab on the surface with softer snow below. These hard slabs should be more difficult to trigger; however, if they do break they are more likely to break well above the person who triggers them making escape extremely hard and burial much more likely. In some isolated areas where wind loading has been the heaviest and the old weak layers are the weakest, these avalanches have the potential to start in the newest storm snow and step down to the crust layers and facets that existed on the snow surface before it started snowing last week. This would result in large, deep, destructive avalanches. Below treeline human triggered avalanche activity is less likely due to less slab formation and stronger bonding in the upper snowpack. As the winds begin to load slopes below treeline today it may be possible for human triggered avalanches to occur on steep, open, heavily wind loaded slopes.
The bottom line: Near and above treeline, the avalanche danger will remain CONSIDERABLE on NW-N-NE-E aspects, 35 degrees and steeper due to new snowfall and high winds today. Pockets of HIGH danger could develop on the most heavily wind loaded slopes by later today. Below treeline, avalanche danger is LOW with more areas of MODERATE danger developing on open wind loaded slopes 37 degrees and steeper.

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): 12 deg. F
Max. temperature at Sierra Crest past 24 hours: 19 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 5-15 mph until this morning when wind speeds increased to 40 mph
Maximum wind gust at Sierra Crest past 24 hours: 54 mph
New snow fall at 8,200 feet past 24 hours: 0 inches
Total snow depth at 8,200 feet: 81-87 inches
Mountain Weather Forecast For Today:
Increasing snow throughout today.
Temperature forecast for 8,000 to 9,000 feet: 20 to 25 deg. F
Ridgetop winds forecast for the Sierra Crest: West 50 to 60 mph with gusts to 115 mph shifting to the southwest and increasing to 65 to 75 mph with gusts to 130 mph in the afternoon.
Snowfall expected in the next 24 hours: 10 to 20 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, snow increasing in intensity throughout the day. Snow accumulation 3 to 7 inches. Daytime highs 25 to 30 deg. F. West winds at 20 to 30 mph with gusts to 45 mph shifting to the southwest and increasing to 30 to 45 mph with gusts to 55 mph in the afternoon.

Tonight, snow with 6 to 12 inches of accumulation. Overnight lows 16 to 21 deg. F. Southwest winds at 25 to 40 mph with gusts to 60 mph decreasing to 20 to 30 mph with gust to 45 mph after midnight.

For Friday, light scattered snow showers with accumulations of up to 1 inch. Daytime highs 19 to 24 deg. F. Southwest winds at 15 to 25 mph.

Above 8000 Feet:
Today, snow increasing in intensity throughout the day. Snow accumulation 4 to 8 inches. Daytime highs 20 to 25 deg. F. West winds at 50 to 60 mph with gusts to 115 mph shifting to the southwest and increasing to 65 to 75 mph with gusts to 130 mph in the afternoon.

Tonight, snow with 6 to 12 inches of accumulation. Overnight lows 6 to 16 deg. F. Southwest winds at 55 to 75 mph with gusts to 125 mph decreasing to 50 to 60 mph with gusts to 100 mph after midnight.

For Friday, light scattered snow showers with accumulations of up to 1 inch. Daytime highs 15 to 20 deg. F. Southwest winds at 20 to 30 mph with gusts to 40 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.