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 23, 2008 7:00 AM
Click here for a detailed map of the SAC forecast area

1_low.jpg
Today's Advisory:

In most areas, avalanche danger is LOW for all elevations and aspects. Isolated pockets of MODERATE avalanche danger may exist near and above treeline in recently wind loaded areas, 35 degrees and steeper. Below treeline, very isolated pockets of MODERATE avalanche danger exist in open areas 40 degrees and steeper where loose snow avalanche activity remains possible.

Cold weather and scattered snow showers should continue today. The showers and accumulation should increase this afternoon and evening before tapering off tomorrow. Another series of storms should follow this one later in the week. The winds shifted to the northeast yesterday along the Sierra crest and to the southeast in the Mount Rose area. These wind patterns should prevail today and start to shift back to the southwest tomorrow as the next series of storms approaches the forecast area. Temperatures should remain cold.

This storm's snow showers have been inconsistent across the forecast area. They have behaved more like summer squalls than a winter storm. Areas in close proximity to each other have been blessed or cursed with widely different snow amounts, wind speeds, and wind directions creating new snow depths and windslabs that vary greatly in geographic distribution. In general the winds have been calmer and snow accumulations greatest in the Mount Rose area where observers report storm totals of 18 to 24 inches. Along the Sierra Crest the winds have been stronger and snow depths variable. North of Tahoe City storm totals range from 12 to 20 inches, and in the southern part of the forecast area storm totals range from 10 to 16 inches. Most of this snow fell prior to yesterday with 24 hour snowfall totals only in the 4 to 6 inch range across the forecast area.

Observations from the Mount Rose area, north of Castle Peak, and the Echo summit area indicate that a graupel layer exists in the new snow. This graupel layer seems to be widespread across the forecast area. Observations from these areas yesterday also indicate that the new snow has not bonded well to the old crust surfaces above and below treeline. Test slopes and quick test pits indicate that both the graupel layer and the interface between the new snow layer and the old crusts are weak and sensitive to human triggering. North of Castle Peak the graupel layer reacted by producing shooting cracks in small wind loaded pillows due to human triggering. These mini windslabs were only 1 to 6 inches deep and 3 to 10 feet wide. The wind loading did not extend more than a few feet down slope. Below treeline in this area and in the Mount Rose area small sluffs occurred on the interface between the new snow and the crust layers.

Luckily the new snow is very low density (5-8%) with almost no cohesion, and the winds associated with this storm have been moderate. Therefore, slab formation has been minimal. Even with those two weaknesses in the snowpack there are very few areas where a cohesive slab sits on top of these weaknesses. Some pockets of small, shallow, and human triggerable slabs could exist today on recently wind loaded slopes steeper than 35 degrees. Today's NE winds should crossload some NW-N aspects as well as loading the W-SW-S aspects. As the winds increase and shift over the next few days they will transport and re-transport the light, new snow. This redistributed snow will start to form cohesive windslabs on these weaknesses and avalanche danger should increase. Below treeline today, the possibility of loose snow avalanche activity (sluffs) remains possible in steep open areas.
The bottom line: In most areas, avalanche danger is LOW for all elevations and aspects. Isolated pockets of MODERATE avalanche danger may exist near and above treeline in recently wind loaded areas, 35 degrees and steeper. Below treeline, very isolated pockets of MODERATE avalanche danger exist in open areas 40 degrees and steeper where loose snow avalanche activity remains possible.

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): 13 deg. F
Max. temperature at Sierra Crest past 24 hours: 20 deg. F
Average wind direction at Sierra Crest past 24 hours: East northeast
Average wind speed at Sierra Crest past 24 hours: 30 mph
Maximum wind gust at Sierra Crest past 24 hours: 56 mph
New snow fall at 8,200 feet past 24 hours: 2 to 4 inches
Total snow depth at 8,200 feet: 68 inches
Mountain Weather Forecast For Today:
Scattered snow showers becoming more widespread this afternoon.
Temperature forecast for 8,000 to 9,000 feet: around 22 deg. F
Ridgetop winds forecast for the Sierra Crest: Southeast to Northeast at 15 to 25 mph with gusts to 45 mph.
Snowfall expected in the next 24 hours: 6 to 12 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, scattered snow showers becoming more widespread this afternoon with accumulations up to 3 inches. Daytime highs around 25 deg. F. East winds at 10 to 15 mph with gusts to 25 mph.

Tonight, snow with 2 to 4 inches of accumulation. Overnight lows around 16 deg. F. East winds at 10 to 15 mph with gusts to 25 mph shifting to the south after midnight.

For Thursday, cloudy skies with scattered snow showers. Snow accumulation up to 1 inch. Daytime highs around 24 deg. F. Southwest winds at 10 to 20 mph with gusts to 35 mph.

Above 8000 Feet:
Today, scattered snow showers becoming more widespread this afternoon with accumulations up to 6 inches. Southeast to northeast winds at 15 to 25 mph with gusts to 45 mph. Daytime highs around 22 deg. F.

Tonight, snow with 3 to 6 inches of accumulation. Overnight lows 10 to 16 deg. F. East winds at 10 to 20 mph shifting to the south at 10 to 15 mph with gusts to 30 mph after midnight.

For Thursday, cloudy skies with scattered snow showers. Snow accumulation up to 2 inches. Daytime highs 15 to 21 deg. F. Southwest winds at 10 to 25 mph with gusts to 30 mph increasing to 20 to 30 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:
	Wednesday:	Wednesday Night:	Thursday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:
Temperatures:	deg. F.	deg. F.	deg. F.
Wind direction:
Wind speed:
Expected snowfall:	O in.	O in.	O in.