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.

Two low pressure systems should converge over the forecast area during the next 24 hours, one from the southwest and the other from the northwest. These systems should bring rain and snow, increased winds, and much cooler temperatures. The southwest winds have already started to increase and should continue to do so through tomorrow. Temperatures this morning measured a few degrees cooler than yesterday morning. Daytime highs should only reach into the upper 40's today. Tonight the lows should finally sink into the freezing range and tomorrow's highs should remain in the low to mid 30's. As the clouds increase, rain may start to fall across the region this afternoon. Most of the precipitation associated with these low pressures should fall tonight and tomorrow. The forecast calls for snow levels to start around 9000 ft today and most of tonight then drop to 5500 ft Thursday and Thursday night. Higher elevation areas along the Sierra Crest could see up to 6 inches of new snow with 2-4 inches as a more likely scenario once the snow levels drop. At the lower elevations and in areas east of the Lake, the forecast calls for less accumulation. 

Recent Observations:

Yesterday on Red Lake Peak, 4-6 inches of soft wet snow existed on the snow surface on all aspects up to 9000 ft. by 11am. Ski cuts on steep test slopes did cause small wet loose sluffs that only involved this top layer of wet snow. If a refreeze did occur in this area, it had melted on all aspects by 10am. Snowpit data on an ENE aspect in this area showed that free water has percolated all the way through the snowpack (photo), and that the old persistent weak layer has changed into melt-freeze snow at that location. Tests on that layer indicated that fracture propagation no longer remains likely at this location (snowpit).

Even though temperatures did fall closer to freezing last night, they still remained a few degrees above that mark. These warm temperatures combined with some cloud cover overnight and this morning should have prevented much if any refreezing from occurring. More warm temperatures today with some sun this morning and some rain this afternoon will again cause enough melting for wet snow instabilities to form on all aspects and at all elevations. Roller balls, wet loose snow avalanches, and wet slab avalanches all remain possible.

Cooler temperatures and more cloud cover today should keep natural wet snow avalanches unlikely until more rain impacts the area tonight. Unlikely does not mean impossible and some of these natural avalanches could still occur on very isolated terrain features. Human-triggered wet snow instabilities will remain possible today. If more rain falls than forecasted today, wet snow avalanches could become more widespread.  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

The depth of the persistent weak layer and the strength of the snow above it have made triggering a deep slab avalanche on NW-N-NE aspects 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 on all aspects at all elevations on slopes 30 degrees and steeper due to daytime warming and rain on the snowpack today. If more rain impacts the area than forecasted, the danger of wet avalanches will increase.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	35-43 deg. F.
Max. temperature in the last 24 hours:	50-57 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	15-30 mph
Maximum wind gust in the last 24 hours:	68 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	19-73 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Cloudy with scattered rain showers in the afternoon	Cloudy with rain showers	Cloudy with snow showers through the day
Temperatures:	49-55 deg. F.	33-37 deg. F.	37-42 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	15-25 mph with gusts to 40 mph	20-30 mph with gusts to 45 mph	30-45 mph with gusts to 65 mph
Expected snowfall:	O in.	O in.	up to 2 in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Cloudy with scattered rain showers in the afternoon	Cloudy with snow showers	Cloudy with snow showers through the day
Temperatures:	42-47 deg. F.	27-33 deg. F.	31-36 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	20-30 mph with gusts to 50 mph	35-45 mph with gusts to 60 mph	50-60 mph with gusts to 90 mph
Expected snowfall:	O in.	up to 2 in.	up to 2 in.