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.

Another 20-24 inches of snow has fallen since yesterday morning. This additional snow bring storm totals since Tuesday to 3-4 ft in most areas. The winds accompanying the snowfall over the last 24 hours have averaged 55-65 mph with gusts as high as 143 mph along the Sierra Crest. Below treeline strong winds also occurred with gusts in the 70-90mph range and average speeds in the 30-40mph range. The winds and snowfall started to decrease after midnight last night as this storm continued eastward. This morning the southwest winds have started to increase again ahead of the third storm in this series. Snow should continue today albeit with less intensity. By tomorrow snowfall should increase as this storm arrives over the region. The forecast calls for another 14-26 inches of snow by the end of the day tomorrow.

Observations:

By midday yesterday snowfall rates and wind speeds started to increase dramatically. The storm impacted the northern part of the forecast area first and progressed southward. In Negro Canyon widespread instabilities existed by mid morning. Long shooting cracks and natural and skier-triggered avalanches all occurred in this area by that time. These slides occurred on N-NE-E facing aspects steeper than 35 degrees. One avalanche occurred when a skier remotely triggered a cornice fracture from 40 ft away (photos, videos, more info). Farther south on Hidden Peak similar instabilities existed by 1pm in both above and below treeline terrain. Open wind-affected slopes below treeline would fracture as well as the wind-loaded slopes near and above treeline. Two natural avalanches similar to the ones in Negro Canyon occurred on NE facing, 38-40 degree, below treeline slopes on Hidden Peak (photos, videos, more info). In both the Negro Canyon area and on Hidden Peak the avalanches and fractures were about 2 ft deep. On Trimmer Peak and Powderhouse Peak south of Lake Tahoe, less instabilities existed at midday. On Trimmer Peak limited shooting cracks around 3 ft long started to occur on wind-loaded slopes by mid day. On Powderhouse snowpit data indicated that instabilities had just started forming by 2pm (pit profile). Observers reported an increase in storm intensity later in the day in the southern part of the forecast area.

Avalanche Concern #1: Wind Slabs

Even though the winds and snowfall have decreased, the winds remain strong enough to transport snow. This continued wind-loading and some added snowfall today should allow the current wind slabs to continue to grow larger and more widespread in leeward areas. These wind slabs rest on top of weaknesses that have formed during the storm and on top of older unstable wind slabs. This wind-loading will add weight to the snowpack faster than it can form new bonds that would hold the wind slabs in place. Less strong winds and less snowfall today should allow some of the more traditionally sheltered areas that the winds loaded during the last 24 hours to again become more sheltered. The leeward NW-N-NE-E-SE aspects near and above treeline will hold the largest and most fragile slabs. Natural and human-triggered avalanches will remain likely on these slopes especially if they have not already avalanched during the last 24 hours. Below treeline human-triggered avalanches will remain likely on wind-loaded slopes. Large, deep, and destructive avalanches that step down below the storm snow could result from the failure of these wind slabs.

Avalanche Concern #2: Storm Snow Weaknesses

During this storm changing temperatures, snowfall rates, and winds during the storm have created other weaknesses within the new snow. Human-triggered soft slab avalanches and loose snow sluffs will remain likely today due to failure of these weaknesses. Some natural avalanches may also still be possible due to failures of these weaknesses. These will remain smaller than the wind slabs but could still reach a few feet in depth. They could easily bury a person or push him/her into an area that has serious consequences like a stand of trees, over a cliff, into a gully or treewell, or into some other terrain trap. These kind of slides could occur on any steep, open slopes.

The bottom line:

Near and above treeline, the avalanche danger will remain HIGH on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Below treeline the avalanche danger is  CONSIDERABLE on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. On the S-SW-W aspects the avalanche danger will remain CONSIDERABLE above treeline and MODERATE below treeline on slopes steeper than 35 degrees. Large natural and human-triggered avalanche will remain likely today.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	13-17 deg. F.
Max. temperature in the last 24 hours:	20-25 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	55-65 mph
Maximum wind gust in the last 24 hours:	143 mph
New snowfall in the last 24 hours:	17-24 inches
Total snow depth:	142-214 inches

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

For 7000-8000 ft:
	Friday:	Friday Night:	Saturday:
Weather:	Snow and blowing snow	Snow and blowing snow	Snow
Temperatures:	20-25 deg. F.	15-20 deg. F.	24-30 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	20-30 mph with gusts to 60 mph	15-25 mph with gusts to 50 mph	20-30 mph with gusts to 50 mph
Expected snowfall:	4-8 in.	3-6 in.	6-12 in.
For 8000-9000 ft:
	Friday:	Friday Night:	Saturday:
Weather:	Snow and blowing snow	Snow and blowing snow	Snow
Temperatures:	12-20 deg. F.	10-15 deg. F.	17-27 deg. F.
Wind direction:	Southwest	West	Southwest
Wind speed:	30-45 mph with gusts to 70 mph	30-45 mph with gusts to 75 mph	45-65 mph with gusts to 95 mph
Expected snowfall:	4-8 in.	3-6 in.	6-12 in.