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.

Welcome to the heat wave. Maximum daytime air temperatures are running 15 to 20 degrees above average for this time of year. This morning is the sixth morning in a row with air temperatures having remained above freezing overnight. Remote sensors are reporting air temperatures above 8,000' mainly in the upper 40s this morning across the forecast area, the warmest reported this spring. Maximum daytime air temperatures above 7,000' are forecast to reach the upper 50s to near 70 degrees today. Ridgetop winds are out of the southwest this morning and are light in speed. Light winds are expected to last through most of today with moderate speed gusts by this evening. Similar weather is forecast for tomorrow with afternoon cloud build up leading to a slight chance of afternoon thunderstorms south of Hwy 50 over Alpine County.

Recent Observations:

Observations made yesterday on Rubicon Peak (West Shore Tahoe area) and on Round Top Peak (Carson Pass area) both revealed that a snow surface refreeze had occurred the night before. Despite above freezing air temperatures around the clock, clear skies and radiational cooling allowed the top 4 to 6 inches of the snowpack to refreeze. Above freezing air temperatures and strong incoming solar radiation quickly returned the snow surface to melt phase. A melt freeze crust remained below the top 4 to 6 inches of the snow surface and remained supportable until the mid morning hours and until around noon on northerly aspects. Once this crust melted, the snow surface became unsupportable, with boot penetration up to 2 feet deep noted on an E aspect near the summit of Rubicon Peak at 9,050' at noon (photos, more info). In areas observed over the past few days, free water has remained limited to the top 1 to 2 feet of the snowpack.

Today's Primary Avalanche Concern: Wet snow

Clear skies are expected to have once again allowed for some amount of snow surface refreeze to have occurred last night in many locations despite above freezing air temperatures in all areas. The warmest morning air temperatures of this past week and clear skies this morning will quickly melt the refreeze that occurred last night. Areas of 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.

The vast majority of avalanche activity that occurs today is likely to be human triggered, but an isolated natural avalanche event is not out of the question. Wet snow avalanches are much more difficult to accurately predict than dry snow avalanches. Uncertainty surrounding the precise timing of wet snow avalanches makes conservative decision making a prudent choice under these conditions. Surface instabilities like roller balls and pinwheels or simple observations like 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 vast majority of instability that occurs today is likely to be human triggered, but an isolated natural avalanche event is not out of the question.

Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	39 to 47 deg. F.
Max. temperature in the last 24 hours:	54 to 62 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	14 mph
Maximum wind gust in the last 24 hours:	25 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	30 to 82 inches

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

For 7000-8000 ft:
	Saturday:	Saturday Night:	Sunday:
Weather:	Sunny skies, becoming partly cloudy.	Partly cloudy skies becoming clear.	Sunny skies, becoming partly cloudy. A slight chance of thunderstorms in the afternoon over Alpine County.
Temperatures:	64 to 70 deg. F.	38 to 45 deg. F.	66 to 72 deg. F.
Wind direction:	S	SW	S
Wind speed:	Light winds becoming 10 to 15 mph in the afternoon.	10 to 15 mph with gusts to 25 mph in the evening, becoming light.	15 to 20 mph with gusts to 30 mph.
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Saturday:	Saturday Night:	Sunday:
Weather:	Sunny skies, becoming partly cloudy.	Partly cloudy skies becoming clear.	Sunny skies, becoming partly cloudy. A slight chance of thunderstorms in the afternoon over Alpine County.
Temperatures:	58 to 64 deg. F.	39 to 46 deg. F.	60 to 66 deg. F.
Wind direction:	S	SW	S
Wind speed:	10 to 15 mph with gusts to 25 mph in the afternoon.	15 to 20 mph with gusts to 30 mph.	15 to 25 mph with gusts to 40 mph.
Expected snowfall:	O in.	O in.	O in.