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.

The strong high pressure ridge will keep record setting warm weather over the region again today. Temperatures remained in the upper 40's overnight again last night bringing the number of days without temperatures dropping below freezing to eight in a row. Temperatures should climb into the 60's again today above 7000 ft. As the day warms up some cumulus clouds could start to develop. By this afternoon the forecast calls for a 30% chance of scattered thunderstorms over the area. Starting tonight cooler weather should begin to push into the region with overnight lows forecasted to drop into the upper 30's and low 40's above 7000 ft. By tomorrow daytime highs should only reach into the 50's in the mountains.

Recent Observations:

The thin frozen crust on the snow surface had already melted in many areas below 8000 ft by 8:30 am yesterday morning. On northerly aspects between 6700 ft and 8000 ft on Donner Peak (photos, more info), melt water had penetrated 8-24 inches deep into the snowpack over the last week (photo). The deeper layers remained cold and dry. Stability tests on the top 3-4 ft of the snowpack showed the potential for continued wet snow instabilities as a result of the melting weakening the snowpack. Ski cuts on steep slopes triggered small wet snow sluffs on NE-N-NW aspects between 8:30 am and 11 am (photo). At 10 am 2-4 inches of corn snow with a supportable base existed on the NW-N aspects yesterday. 

Even though air temperatures remained warm last night, the clear skies allowed the snowpack to radiate enough heat out into space for a thin surface refreeze to occur. The combination of very warm air temperatures and the intense April sunshine will cause this thin weak refreeze to melt as soon as the sun hits the slopes today. The melting will add more free water to an already wet snowpack causing the snowpack to lose strength. As the snowpack weakens, wet snow instability will form quickly today on all aspects and at all elevations. Roller balls, wet loose snow avalanches, and wet slab avalanches all remain possible.

Like previous days, human-triggered wet snow instabilities should represent most of today's avalanche activity; however, natural avalanche events will remain possible in isolated areas. Due to uncertainty surrounding the precise timing of wet snow avalanches and the difficulty associated with predicting these instabilities, conservative decision making remains a prudent choice under these conditions. Surface instabilities such as roller balls and pinwheels or simple observations such as stepping off of one's skis, snowboard, or snowmobile and sinking up to boot-top depth or more in wet snow can indicate potential instability on a slope.

Other things to think about: The Persistent Weak Layer

Due to the depth of the persistent weak layer and the strength of the snow above it, triggering a deep slab avalanche on NW-N-NE aspects remains unlikely. At this time data indicates collapsing this layer would require an extremely large trigger or significant weakening of the snowpack. Spring melting could eventually cause enough snowpack weakening for a deep wet slab avalanche cycle to occur. Even though data indicates conditions that could create this scenario have not yet arrived, they keep getting closer as more and more melting occurs each day and free water percolates deeper into the snowpack.

The bottom line:

Widespread MODERATE avalanche danger will form quickly today on all aspects at all elevations on slopes 35 degrees and steeper in response to daytime warming. Pockets of CONSIDERABLE danger may form this afternoon on slopes 37 degrees and steeper. The majority of instability that occurs today is likely to be human triggered, but natural avalanches are possible in isolated areas.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature:	46-49 deg. F.
Max. temperature in the last 24 hours:	57-64 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	15-20 mph
Maximum wind gust in the last 24 hours:	46 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	25-76 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly sunny becoming mostly cloudy with a 30% chance for scattered thunderstorms in the afternoon	Partly cloudy with a 20% chance of isolated thunderstorms in the evening becoming clear overnight	Mostly cloudy with a 20% chance of thunderstorms in the afternoon
Temperatures:	61-67 deg. F.	36-43 deg. F.	52-58 deg. F.
Wind direction:	South	Southwest	South
Wind speed:	light increasing to 10-20 mph with gusts to 30 mph in the afternoon	15-20 mph with gusts to 35 mph	Light increasing to 10-15 mph with gusts to 25 mph in the afternoon
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly sunny becoming mostly cloudy with a 30% chance for scattered thunderstorms in the afternoon	Partly cloudy with a 20% chance of isolated thunderstorms in the evening becoming clear overnight	Mostly cloudy with a 20% chance of thunderstorms in the afternoon
Temperatures:	55-61 deg. F.	34-41 deg. F.	46-52 deg. F.
Wind direction:	South	Southwest	South
Wind speed:	20-30 mph with gusts to 40 mph	25-35 mph with gusts to 40 mph	15-25 mph
Expected snowfall:	O in.	O in.	O in.