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 first of two storms should reach the forecast area today bringing colder temperatures, increased winds, and some snow to the region. The forecast calls for up to 6 inches of new snow in the next 24 hrs. The winds have already shifted to the south-southwest ahead of this storm and increased. Tonight the winds should shift to the southwest and temperatures should drop even more due to a second stronger, colder storm approaching the area. As this second system impacts the region tomorrow, daytime highs above 7000' should only reach into the low 20's and snow levels should drop to around 4000'. The forecast calls for 4-9 inches of snow and strong southwest winds above 7000' during the day tomorrow due this second storm system.

Observations:

Near Carson Pass yesterday, a snowboarder-triggered avalanche occurred at 8000 ft. on a 35 degree, NE facing, convex slope above a cliff. The snowboarder triggered this 2ft deep slide near a shallow rock (the trigger point). This slide propagated for about 60 ft across the slope above the cliff and ran about 150 ft downslope. Luckily no one was injured. This slide continues the trend of human-triggered avalanches occurring in complex terrain near trigger points. Observations on the north side of Cup Lake Peak (Echo Summit area) showed some lingering weaknesses near the Feb. 19th interface. Layer bonding tests on these weaknesses showed that even though it requires more force to break these weaknesses, if they do break, the resulting fracture can still propagate through the snowpack along this interface. Observers made these observations near a natural avalanche that most likely occurred during the night on Feb. 26th or during the early morning hours on Feb. 27th. Observers also reported that most of the slide paths in the Echo summit area have either slid naturally or released due to human-triggering since Feb. 26th. Observations on Castle and Basin Peaks (Donner Summit area) showed that the Feb 19th interface has gained some strength in this area. Layer bonding tests in these areas still showed variability in the type of layers at that interface and some variability in the strength of those layers. However, these tests indicated significant force is required to break the bonds between the Feb 19th surfaces and the layers above them. These tests also show that fracture propagation along these layers is becoming less likely. Snowmobile cuts on test slopes and 35-38 degree, NW-N-NE facing, wind-loaded slopes did not cause any failures.

Avalanche Concern #1: Deep slab instability

Human-triggered avalanche activity due to poor bonding between the Feb 19th snow surfaces and the snow above will remain possible today. Observations indicate that weak bonding between the Feb. 19th snow surfaces and the snow above still exists on some northerly aspects. Areas of complex terrain near and above treeline hold the most potential for triggering these large, deep avalanches. However, the right trigger in the right spot (click here for a description of likely trigger points) could still overload the Feb 19th interface on other steep, NW-N-NE facing slopes as well. The type of snow layer at this interface and its location, strength, and distribution varies greatly across the forecast area. Digging snowpits down to the older snow and testing the bonding at the Feb 19th interface can help determine whether or not worrisome weaknesses exist at this interface. As more snow accumulates over the next 2 days these types of avalanches could become easier to trigger once again.

 

Avalanche Concern #2: Wind Slabs

Today, new snow and strong winds will start to build new wind slabs on wind-loaded slopes. These slabs should remain small for most of the day due to a lack of new snow accumulation. Small, human-triggered avalanches involving these new wind slabs may become possible later in the day in areas that receive the high end of forecasted snow today. These avalanches could step down to the Feb. 19th interface resulting in large destructive slides. The wind-loaded NW-N-NE-E aspects and cross-loaded SE aspects near and above treeline are the most likely locations for fragile wind slabs and cornices to form today.

The bottom line:

Near and above treeline, avalanche danger is MODERATE on NW-N-NE-E aspects with pockets of MODERATE danger on SE aspects, 35 degrees and steeper due to deep slab instability and new wind loading. Below treeline, the avalanche danger is LOW with pockets of MODERATE danger on NW-N-NE-E-SE aspects on slopes 35 degrees and steeper. Large destructive avalanches remain possible.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	26 deg. F.
Max. temperature in the last 24 hours:	37 deg. F.
Average wind direction during the last 24 hours:	Southwest shifting to south southwest after 1 am
Average wind speed during the last 24 hours:	25-35 mph
Maximum wind gust in the last 24 hours:	64 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	82-117 inches

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

For 7000-8000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:	Chance of snow in the morning with snow becoming likely as the day progresses.	Cloudy with scattered snow showers.	Snow
Temperatures:	23-29 deg. F.	16-23 deg. F.	19-23 deg. F.
Wind direction:	South	Southwest	Southwest
Wind speed:	20-30 mph with gusts to 40 mph increasing to 55 mph in the afternoon	25-35 mph with gusts to 60 mph	15-25 mph with gusts to 45 mph
Expected snowfall:	2-3 in.	up to 2 in.	4-8 in.
For 8000-9000 ft:
	Tuesday:	Tuesday Night:	Wednesday:
Weather:	Chance of snow in the morning with snow becoming likely as the day progresses.	Cloudy with scattered snow showers.	Snow
Temperatures:	20-25 deg. F.	12-19 deg. F.	17-20 deg. F.
Wind direction:	South	Southwest	Southwest
Wind speed:	30-40 mph with gusts to 75 mph	30-40 mph with gusts to 75 mph decreasing to 60 mph after midnight	20-30 mph with gusts to 50 mph increasing to 70 mph in the afternoon
Expected snowfall:	2-4 in.	up to 2 in.	6-9 in.