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 April 18, 2012:

April 18, 2012 at 7:01 am

Widespread MODERATE avalanche danger will form today on all aspects on slopes 35 degrees and steeper at all elevations in response to daytime warming.

Forecast Discussion:

Most remote sensors across the forecast area recorded above freezing temperatures again last night. A few located between 8000 ft. and 9000 ft. showed temperatures around 30-31 degrees F. this morning. Temperatures have now remained above freezing in many areas for 3 days. The forecast calls for continued warm weather with temperatures today similar to those seen yesterday. Like the last two days cloud cover should persist over the mountains due to a low pressure moving through the Pacific Northwest. Temperatures should remain mild again tonight and climb another 5-10 degrees above today's highs tomorrow as the high pressure ridge over the region grows stronger. The forecast calls for the moderate southwest winds to continue through tonight before shifting to the northwest tomorrow. 

Recent Observations:

Yesterday sticky, wet snow existed on all aspects on Castle Peak and on Relay Peak by midday. On most of the sun-exposed southerly aspects in these areas, the snow became unsupportable by that time. Numerous wet snow instabilities like roller balls, pinwheels, and wet loose snow sluffs had occurred since Sunday in these areas (photo from Relay). On Castle Peak ski cuts on steep test slopes caused pinwheels up to 3 ft in diameter and some small wet loose snow sluffs (photo). At the higher elevations on Relay Peak, these skier-triggered instabilities also occurred; however, they remained smaller in size. Previously wind scoured slopes above 9600 ft on Relay Peak and above 8400 ft on Castle Peak did hold some corn conditions. Below these elevations the snow had not transitioned to corn snow, and it remained wet and sticky. On the NW-N-NE aspects in these areas (video from Castle, video from Relay) and in Horse Canyon near Bear Valley, snowpit data indicated a strong and stable upper snowpack. Free water had only percolated 4-8 inches deep on these aspects. On Castle Peak this wet surface snow did result in some smaller wet snow instabilities on these northerly aspects. Tests on the old persistent weak layer on Relay Peak still indicated that it could become reactive again if the snow above it weakens enough (video).

Today's Primary Avalanche Concern: Wet snow

With some cloud cover and temperatures near or above freezing last night, only a thin weak refreeze should have occurred if a refreeze occurred at all. Warm temperatures and April sunshine should quickly melt this thin refreeze and allow the layer of wet snow that already exists from the last three days of melting to become deeper. High sun angles allowing sunshine to reach the northerly aspects and clouds that will help distribute today's warmth across all aspects will mean that  widespread wet snow instabilities could exist on all aspects at all elevations. Roller balls, wet loose snow avalanches, and wet slab avalanches will all remain possible today. The largest wet snow instabilities will likely occur on the most sun-exposed E-SE-S-SW-W aspects and in areas with more of the recent storm snow remaining. Surface instabilities like roller balls and pinwheels and simple observations like stepping off one's skis, board, or snowmobile and sinking up to boot-top depth in wet snow can indicate potential instability on a slope. Use these clues to help determine when to move to a different 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 has become 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 remain some time away, they keep getting closer as more and more melting occurs each day and free water percolates farther into the snowpack.

The bottom line:

Widespread MODERATE avalanche danger will form today on all aspects on slopes 35 degrees and steeper at all elevations in response to daytime warming.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature: 30-37 deg. F.
Max. temperature in the last 24 hours: 43-50 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 20-30 mph
Maximum wind gust in the last 24 hours: 48 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 38-90 inches

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

For 7000-8000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Mostly cloudy Mostly cloudy Partly cloudy
Temperatures: 48-54 deg. F. 30-37 deg. F. 53-60 deg. F.
Wind direction: Southwest Southwest West shifting to the north in the afternoon
Wind speed: 15-20 mph with gusts to 35 mph 10-20 mph with gusts to 30 mph 10-20 mph with gusts to 30 mph
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Wednesday: Wednesday Night: Thursday:
Weather: Mostly cloudy Mostly cloudy Partly cloudy
Temperatures: 40-50 deg. F. 28-36 deg. F. 45-55 deg. F.
Wind direction: Southwest Southwest Northwest
Wind speed: 20-35 mph with gusts to 55 mph 15-25 mph with gusts to 45 mph 20-30 mph with gusts to 45 mph
Expected snowfall: O in. O in. O in.