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 5, 2008 6:57 AM
Click here for a detailed map of the SAC forecast area

4_high.jpg
Today's Advisory:
Near and above treeline the avalanche danger is HIGH on NW-N-NE-E-SE aspects, 32 degrees and steeper. Below treeline, avalanche danger is CONSIDERABLE with pockets of HIGH danger in open areas 35 degrees and steeper.

Yesterday's storm was the first in a series of systems expected to move through the forecast area. Air temperatures stayed warmer longer than expected yesterday and snow fall rates did not reach the forecast intensity in the mountains. Therefore, the forecast area received less snow. The temperatures finally fell late yesterday and snowfall rates picked up to 3 inches per hour for a few hours in the evening. Snowfall continued overnight bringing another 18 to 20 inches of snow to the forecast area. This accumulation brings storm totals to 40 to 45 inches in the Mount Rose area and 28 to 36 inches along the Sierra Crest. The forecast calls for a small break in the storm this morning as the next system arrives bringing more snowfall with it. 10 to 18 inches of snow should fall today. Another system follows tonight and should add 15 to 30 more inches of snow to the storm totals. Yesterday the southwest wind averaged 75 mph from 6am until 4pm. During each hour the maximum gust recorded was over 112 mph. Strong southwest winds should continue today, albeit with less intensity. Wind speeds along the ridges should be in the 35 to 55 mph range with gusts only reaching into the 85 mph range.

There are several weak layers and interfaces in the current snowpack. During this storm the shifting winds and changing snow levels have created at least three new weaknesses in the storm snow. A density inversion, a rain crust, and layers of rimed snow crystals. The density inversion is present across most of the forecast area at the bottom of the new snow. Most of the snow that fell yesterday was dense and heavy. This new snow fell on top of 4 to 6 inches of lighter snow from Thursday night. The new rain crust formed below 7500' as yesterday's rain froze in the snowpack. New snow fell on top of that layer yesterday afternoon and through the night. Rimed snow crystals (graupel- snow's equivalent of ball bearings) also fell during the storm and will form weakness within the newest storm snow. Observations from Alpine Meadows this morning and from north of Castle Peak late yesterday show weak bonding between these layers and the new dense snow on top of them. Weak layers are also lurking deeper in the snowpack in the form of old crusts and basal facets (weak sugary grains near the bottom of the snowpack).
The winds and dense new snow also formed stiff, heavy slabs on top of these weak layers. Yesterday's extremely strong winds transported snow farther downslope and into protected areas. This transport extended windslabs into areas that they do not usually exist. Today the less intense wind should be the perfect speed to form windslabs and cornices in the traditional starting zones. The new snow expected today will provide more material for the wind to build more extensive windslabs. Even in the more protected areas the layer of dense heavy snow should be able to behave as a slab. The combination of these slabs sitting on the weak layers mentioned above should be a good recipe for avalanche activity today.

The majority of avalanche activity should occur within the new snow or near the old/new snow interface where a layer melt-freeze crust, wind crust, or near surface faceted snow crystals exists, depending on location. Snowpack failure into the basal faceted layer at ground level is a possibility, especially once a large avalanche is already in motion.

Near and above treeline natural and human triggered avalanches are likely today on all wind loaded slopes steeper than 32 degrees. NW-N-NE-E-SE aspects will be the most likely to wind load from the southwest wind. Below treeline, natural avalanches are possible on all aspects and likely on any open slopes steeper than 35 degrees. Human triggered avalanche are probable to likely in these areas as well.

The bottom line: Near and above treeline the avalanche danger is HIGH on NW-N-NE-E-SE aspects, 32 degrees and steeper. Below treeline, avalanche danger is CONSIDERABLE with pockets of HIGH danger in open areas 35 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: 31 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 45 mph
Maximum wind gust at Sierra Crest past 24 hours: 165 mph
New snow fall at 8,200 feet past 24 hours: 21 to 26 inches
Total snow depth at 8,200 feet: 56 inches
Mountain Weather Forecast For Today:
Snow showers in the morning becoming more continuous this afternoon.
Temperature forecast for 8,000 to 9,000 feet: 20 to 24 deg. F
Ridgetop winds forecast for the Sierra Crest: Southwest at 35 to 55 mph with gusts to 85 mph.
Snowfall expected in the next 24 hours: 10 to 18 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, cloudy skies with snow showers in the morning becoming more continuous this afternoon. Snow accumulation 8 to 14 inches. Daytime highs 22 to 26 deg. F. Southwest winds at 25 to 35 mph, with gusts to 50 mph.

Tonight, cloudy skies with heavy snowfall. Snow accumulation 12 to 24 inches. Overnight lows 12 to 16 deg. F. Southwest winds at 20 to 30 mph with gusts to 45 mph.

For Sunday, cloudy skies with snow showers. Snow accumulation 6 to 12 inches. Daytime highs 20 to 26 deg. F. Southwest winds at 15 to 25 mph with gusts to 40 mph.

Above 8000 Feet:
Today, cloudy skies with snow showers in the morning becoming more continuous this afternoon. Snow accumulation 10 to 18 inches. Daytime highs 20 to 24 deg. F. Southwest winds at 35 to 55 mph, with gusts to 85 mph.

Tonight, cloudy skies with heavy snowfall. Snow accumulation 15 to 30 inches. Overnight lows 10 to 16 deg. F. Southwest winds at 35 to 55 mph with gusts to 105 mph.

For Sunday, cloudy skies with snow showers. Snow accumulation 6 to 12 inches. Daytime highs 16 to 22 deg. F. Southwest winds at 30 to 45 mph with gusts to 80 mph decreasing to 20 to 35 mph with gusts to 70 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.