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 29, 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, avalanche danger is CONSIDERABLE with isolated pockets of HIGH danger on heavily wind loaded N-NE-E aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE on open slopes 40 degrees and steeper.

Another low pressure system is moving through the Pacific Northwest today. Combined with a high pressure area off the west coast should cause the winds to remain intense today.Remote sensors and direct observations showed the the winds decreased into the the 35 mph range yesterday and last night. The winds should increase again and shift towards the west and northwest today and tomorrow. Show showers should continue today and tonight before a small break in the weather tomorrow. Temperatures should get even colder today as the cold front impacts the forecast area. The forecast calls for afternoon temperatures to fall into the single digits and teens with wind chills well below zero. Another storm should move into the area on Thursday.

Even though the winds slowed down some last night and yesterday, they remained strong enough to transport snow. The slower speeds of 30-35 mph caused sensitive wind slabs and cornices to start forming along the ridge tops and in the upper portions of avalanche start zones. This wind loading combined with the wind loading that occurred lower in the avalanche start zones due to the strong Ridgetop winds associated with the start of this storm should create large wind slabs that extend well below the ridgelines. The snow that forms these wind slabs has been subjected to several episodes of high winds and wind transport. The slabs that formed yesterday and the slabs that continue to form today should be a mix of stiff, hard slabs and some softer wind slabs. Alpine Meadows Ski Patrol reported "stubborn hard slabs" had formed on heavily wind loaded areas near and above treeline. Hard slab formation was also reported in the Echo Summit area and in other areas along the Sierra Crest. 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. Be wary of wind loaded areas where there is a stiff slab on the surface with softer snow below. It is easy to find out if this is the case by probing with a ski pole or digging a quick hand pit. This configuration sometimes sounds hollow as well. Significant wind loading and soft slab formation was observed during the day yesterday by the Mt Rose Ski Patrol and along Andesite Ridge near Castle Peak. In both areas these wind slabs were sensitive to human triggering. Tender cornices started to form along Andesite Ridge yesterday as well. These cornices could be triggered with the tap of a ski pole. Most of the slab formation observed early yesterday afternoon had not connected to the previous areas of wind loading. However, since there is more than enough snow available for transport and the winds continued to blow all night, these slabs should have grown enough to reach those wind loaded areas farther downslope.

Weak layers and interfaces exist below these newly formed windslabs. Higher density new snow deposited over lower density snow, created upside down layer conditions within the top 4 feet of the snowpack. The near crust facets at the base of the storm snow from last week also could be a problem. The threat of large human triggered avalanches will remain throughout the day. Backcountry travelers should not be surprised by a human triggered avalanche that is large enough to bury, injure, or kill a person. 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 creating large, deep, destructive avalanches. Due to the variability of distribution of both these weak layers and the wind slabs these areas will be hard to predict without digging into the snow to test the weak layers. Near and above treeline, human triggered avalanches are likely today on N-NE-E aspects in steep wind loaded areas. Natural avalanches are possible in the most heavily wind loaded areas. Below treeline some soft slab activity and loose snow sluffs could fail due to human triggering on slopes steeper than 40 degrees. The weaknesses at these elevations are the same as the ones at higher elevations but the slabs are not as cohesive or heavy; therefore, avalanche activity is less of a threat.
The bottom line: Near and above treeline, avalanche danger is CONSIDERABLE with isolated pockets of HIGH danger on heavily wind loaded N-NE-E aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE on open slopes 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): 6 deg. F
Max. temperature at Sierra Crest past 24 hours: 15 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 30-35 mph
Maximum wind gust at Sierra Crest past 24 hours: 81 mph
New snow fall at 8,200 feet past 24 hours: trace
Total snow depth at 8,200 feet: 77 inches
Mountain Weather Forecast For Today:
Mostly cloudy with snow showers.
Temperature forecast for 8,000 to 9,000 feet: 18 to 23 deg. F falling to 8 to 13 deg. F
Ridgetop winds forecast for the Sierra Crest: Southwest 25 to 35 mph with gusts to 55 mph, shifting to the west and increasing to 55 to 70 mph with gusts to 85 mph in the afternoon.
Snowfall expected in the next 24 hours: 6 to 12 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, mostly cloudy skies with snow showers. Snow accumulation 3 to 6 inches. Daytime highs 18 to 23 deg. F falling to 11 to 16 deg. F. Southwest winds at 25 to 30 mph with gusts to 45 mph.

Tonight, mostly cloudy skies with up to 6 inches of snow. Overnight lows 5 to 10 deg. F. West winds at 10 to 15 mph with gusts to 30 mph.

For Wednesday, mostly cloudy skies with daytime highs 18 to 23 deg. F. West winds at 10 to 15 mph.

Above 8000 Feet:
Today, mostly cloudy skies with snow showers. Snow accumulation 3 to 6 inches. Daytime highs 18 to 23 deg. F falling to 8 to 13 deg. F. Southwest winds at 25 to 35 mph with gusts to 55 mph, shifting to the west and increasing to 55 to 70 mph with gusts to 85 mph in the afternoon.

Tonight, mostly cloudy skies with up to 6 inches of snow. Overnight lows 0 to 5 deg. F. West winds at 25 to 35 mph with gusts to 65 mph.

For Wednesday, mostly cloudy skies with daytime highs 14 to 19 deg. F. West winds at 25 to 35 mph with gusts to 60 mph, shifting to the northwest and decreasing to 15 to 25 mph with gusts to 45 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:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.