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 25, 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 is MODERATE with pockets of CONSIDERABLE on heavily wind loaded slopes, 35 degrees and steeper. NW-N-NE-E aspects should experience the most wind loading. These pockets of CONSIDERABLE danger should become more widespread as more wind loading forms larger wind slabs today. Below treeline, the avalanche danger is LOW with pockets of MODERATE avalanche danger in open areas 40 degrees and steeper.

A strong low pressure system off the west coast should spin some more moisture into the Sierra. The forecast calls for another 4 to 8 inches of snow above 7000 feet today. Following this storm there should be a break in the weather and dry air from the south should move into the forecast area. As this change in the weather happens the winds should also start to increase dramatically. Today the forecast calls for the winds to shift to the south and increase to a consistent 30 to 45 mph above 8000 feet. These winds should continue to increase through tonight before starting to calm down some tomorrow. The more southerly flow should also bring slightly warm temperatures and slightly more dense snow to the forecast area.

Since the start of this series of storms on Monday wind loading and wind slab formation seems to have been limited to only the most exposed areas along the Sierra Crest (i.e. places like the Judah ridgeline and Donner Peak). The winds have been calmer just a short distance to the east of the crest and much calmer in the Mount Rose area. Observations from the Hidden Peak area and the Mount Rose area show that the wind transport of new snow started to increase yesterday afternoon. Wind loading should continue to increase today due to a higher wind speeds and more consistent winds across the forecast area. The new snow that falls today and the light fluffy snow that has fallen since Monday should provide perfect material to form windslabs on wind loaded slopes near and above treeline. These windslabs should become more extensive both in the horizontal and downslope direction toady as the winds move snow farther today than they have all week.Observations continue to show the new snow gaining cohesion and starting to look more like a potential slab layer even where it is unaffected by the wind. Today's winds should help this snow to gain more cohesion as they transport it forming human triggerable slabs in heavily wind loaded areas. The south winds will form the windslabs mentioned above on slightly different slopes than usual and they will crossload other slopes. Use clues like drifting, wind textures on the snow surface, blowing snow, and cornices to help determine which slopes are wind loaded and be cautious in dealing with those slopes.

These newly forming slabs seem to be gaining strength faster than the weak layers that they sit on top of. Observers continue to report several weak layers in the new snow and a weak interface at the boundary between the old and new snow. Graupel layers, rimed crystal layers, and density inversions (heavy snow on top of light snow) seem to be widespread across the forecast area in the new snow. Test slopes and quick test pits indicate that both the weaknesses in the new snow and the interface between the new snow layer and the old snow surfaces (crusts and old near surface facets, a thin layer of weak sugary snow) are weak and sensitive to human triggering. Observers reported some very small natural avalanche activity on steep, north facing wind loaded slopes on Donner Peak yesterday. These slides could be the precursor to larger activity as more wind loading formslarger and more widespread wind slabs over the next few days.

Today human triggering of these newly formed windslabs on wind loaded NW-N-NE-E slopes steeper than 35 degrees is possible. Possible avalanche activity on these slopes could fail on weaknesses in the new snow and step down to the old/new snow interface or even fail at the old/new interface if the trigger loads the slope in the right spot. These slides would initially involve the newly formed windslabs but could entrain some of the softer new snow as they start to move downslope. In the most heavily wind loaded areas there may be steep pockets where some natural activity becomes possible today. Loose snow avalanche activity (sluffs) is also possible on slopes steeper then 40 degrees where wind loading has not played as big a role. Below treeline today, slab activity is unlikely but not impossible. There may be areas where the new snow has gained enough cohesion to behave as a slab and the bond between the old/new snow is very weak. These areas should be rare, very isolated in their distribution, and very small. Loose snow avalanche activity (sluffs) remains possible in steep open areas due to the poor bonding at the old/new snow interface.
The bottom line: Near and above treeline the avalanche danger is MODERATE with pockets of CONSIDERABLE on heavily wind loaded slopes, 35 degrees and steeper. NW-N-NE-E aspects should experience the most wind loading. These pockets of CONSIDERABLE danger should become more widespread as more wind loading forms larger wind slabs today. Below treeline, the avalanche danger is LOW with pockets of MODERATE avalanche danger in open areas 40 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): 15 deg. F
Max. temperature at Sierra Crest past 24 hours: 17 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest shifting to the south and increasing
Average wind speed at Sierra Crest past 24 hours: 28 mph
Maximum wind gust at Sierra Crest past 24 hours: 70 mph
New snow fall at 8,200 feet past 24 hours: 4 to 6 inches
Total snow depth at 8,200 feet: 74 inches
Mountain Weather Forecast For Today:
Slightly warmer and windier. More snow showers with accumulation ranging from 4 to 8 inches.
Temperature forecast for 8,000 to 9,000 feet: 16 to 26 deg. F
Ridgetop winds forecast for the Sierra Crest: South 25 to 35 mph increasing to 30 to 45 mph with gusts to 75 mph in the afternoon.
Snowfall expected in the next 24 hours: 7 to 13 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, snow with accumulations of 4 to 8 inches. Daytime highs around 26 to 31 deg. F. Winds out of the south at 5 to 15 mph increasing to 15 to 25 mph with gusts to 35 mph in the afternoon.

Tonight, snow with 3 to 5 inches of accumulation. Overnight lows 22 to 27 deg. F. Southeast winds at 15 to 25 mph with gusts to 35 mph.

For Saturday, snow showers likely with up to 2 inches of accumulation. Daytime highs 29 to 34 deg. F. South winds at 15 to 25 mph with gusts to 35 mph.

Above 8000 Feet:
Today, snow with accumulations of 4 to 8 inches. Daytime highs 16 to 26 deg. F. Winds out of the south at 25 to 35 mph increasing to 30 to 45 mph with gusts to 75 mph in the afternoon.

Tonight, snow with 3 to 5 inches of accumulation. Overnight lows 16 to 26 deg. F. Winds out of the southeast at 30 to 45 mph with gusts to 70 mph increasing and shifting to the south at 45 to 60 mph with gusts to 85 mph.

For Saturday, snow showers likely with up to 2 inches of accumulation. Daytime highs 29 to 35 deg. F. Winds out of the southwest at 40 to 60 mph with gusts to 85 mph decreasing to gusts to 75 mph in 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:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Friday:	Friday Night:	Saturday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.