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 Avalanche Advisory was published on December 30, 2007:


December 30, 2007 at 1:00 am

Forecast Discussion:


This advisory was posted on December 30, 2007 7:00 AM
Click here for a detailed map of the SAC forecast area

low avalanche danger
Today's Advisory:

Avalanche danger is LOW for all elevations and aspects. Normal caution is advised.

The combination of a high pressure system off the CA coast and a low pressure system to the north of the forecast area should allow the southwest winds to continue today. As the low pressure system moves east a weak, dry cold front should move over the forecast area. This front has very little moisture associated with it. The main effect of this front should be some cooler temperatures today. As the front passes to the east, cold northeasterly flow should follow. The forecast calls for the winds to shift to the east and increase overnight. The high pressure system following this front should bring warmer drier air to the forecast area for early next week.

Yesterday's snow showers brought up to 4 inches of accumulation in some places along the Sierra Crest and only about 1 inch in the Mount Rose area. The strong southwest winds transported this snow and began forming small cornices and pillows along exposed ridgelines above treeline. Some small wind slabs started to form on the most heavily wind loaded N-NE-E aspects. These slabs are small and only extend a short distance downslope from the ridgelines. The size and extent of these slabs should increase some today as the southwest winds continue. However, due to a decrease in winds and the amount of snow available for transport these slabs should not grow large enough to pose a serious or widespread threat to backcountry travelers. As winds shift to the NE and E tonight these areas will be scoured and the newly formed wind slabs should decrease in size.

Observers reported the thin, fragile rain crust that formed up to 9000' is limited to areas along the Sierra Crest and west. The freezing rain turned to snow as it moved east across the mountains. On north aspects in the Mount Rose area there was another thin, weak surface crust that the new snow fell on top of. The interface between both of these crusts and the layers below failed easily during layer bonding tests yesterday. Luckily, only 1-3 inches of dry, unconsolidated snow sits on top of these crusts, therefore one of the key avalanche ingredients is missing: the slab. Along the crest and in the Mount Rose area observations and snow pit data indicate that facets continue to form in the lower layers of the the snowpack. However layer bonding tests continue to show the snowpack gaining strength within layers and between layers in the lower part of the current snowpack.

A mix of snow surface conditions exists on various aspects and at different elevations throughout the area. Along the Sierra Crest 1-4 inches of new snow sits top of the thin, zipper crust formed by the rain.Further east on the Sierra Crest and in the Mount Rose area the new snow fell on top of a mix of unconsolidated snow on the sheltered north aspects, melt-freeze/sun crusts on the S-SW-W aspects, and wind scoured surfaces on most aspects above treeline. On the windward slopes expect heavy scouring and a shallow snowpack. Sheltered north and east aspects still have unconsolidated snow and still provide fun riding.

The bottom line: Avalanche danger is LOW for all elevations and aspects. Normal caution is advised.

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): 24 deg. F
Max. temperature at Sierra Crest past 24 hours: 27 deg. F
Average wind direction at Sierra Crest past 24 hours: Southwest
Average wind speed at Sierra Crest past 24 hours: 40 mph
Maximum wind gust at Sierra Crest past 24 hours: 72 mph
New snow fall at 8,200 feet past 24 hours: 2 to 4 inches
Total snow depth at 8,200 feet: 33 inches
Mountain Weather Forecast For Today:
Partly cloudy skies with some isolated snow showers in the morning and clearing in the afternoon.
Temperature forecast for 8,000 to 9,000 feet: 27 to 32 deg. F
Ridgetop winds forecast for the Sierra Crest: Southwest at 20 to 35 mph with gusts to 60 mph.
Snowfall expected in the next 24 hours: 0 inches
2 Day Mountain Weather Forecast:

7000 to 8000 Feet:
Today, partly cloudy skies with daytime highs 30 to 35 deg. F. Southwest winds at 15 to 25 mph with gusts to 35 mph.

Tonight, partly cloudy skies in the evening then clearing. Overnight lows 15 to 20 deg. F. North winds shifting to the east after midnight at 15 to 20 mph with gusts to 25 mph.

For Monday, sunny skies with daytime highs 34 to 39 deg. F. East winds at 10 to 15 mph with gusts to 25 mph.

Above 8000 Feet:
Today, partly cloudy skies with daytime highs 27 to 32 deg. F. Southwest winds at 20 to 35 mph with gusts to 60 mph.

Tonight, partly cloudy skies in the evening then clearing. Overnight lows 18 to 23 deg. F. Northwest winds shifting to the northeast after midnight at 10 to 20 mph with gusts to 35 mph.

For Monday, sunny skies with daytime highs 35 to 40 deg. F. East winds at 10 to 20 mph with gusts to 30 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:

  Sunday: Sunday Night: Monday:
Weather:
Temperatures: deg. F. deg. F. deg. F.
Wind direction:
Wind speed:
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Sunday: Sunday Night: Monday:
Weather:
Temperatures: deg. F. deg. F. deg. F.
Wind direction:
Wind speed:
Expected snowfall: O in. O in. O in.