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.

During the night another 3-4 inches of snow fell across the the forecast area as the first of several more storm systems began to approach the area. Temperatures increased a few degrees this morning due to this system pushing some warmer air ahead of it. Snowfall should continue this morning and become more intense this afternoon as this storm arrives. By the end of the day the mountains above 7000 ft could see another foot of new snow with 4-8 more inches tonight. The southerly winds should also increase as this system arrives and shift more to the southwest tonight. As this system moves out of the region tomorrow, a second strong storm should replace it. The forecast calls for periods of intense snowfall (2in/hr or more) and strong southwest winds (gusts to 100 mph over the Crest). When this second storm departs on  Thursday another 2-3 ft of snow could have fallen above 7000 ft.

Observations:

A skier-triggered avalanche occurred on the NE side of the Fireplug in the Mt. Rose backcountry yesterday. The slide carried the skier who triggered it several hundred feet and partially buried him. Fortunately, he only incurred minor injuries. This slide measured about a foot deep. The slab consisted of thin layer of new snow sitting on top of a more dense wind crust and a lighter layer of softer snow below the wind crust. The boundary between that lighter layer of snow at the bottom of the slab and a heavier layer of snow below it represented the weakness that the slide fractured along. This slide occurred on a wind-affected, near treeline, NE facing slope and started as the skier went over a convex roll near the top of the slope (pit profile, photos, and video). Farther south on Mt. Tallac and on Trimmer Peak, similar snowpack structure existed. In both of these areas intentionally triggered avalanches also occurred. They measured 1-2 in depth, but their widths and run-outs were much smaller than the one that occurred in the Mt. Rose area. On Trimmer Peak a skier triggered the avalanche, and on Mt. Tallac a large piece of cornice dropped onto the slope triggered the slide. Some natural avalanches also occurred during the night on Monday or early Tuesday morning that were similar in size to the avalanche mentioned above. Observers on Tallac saw one of these natural slides on the slope next to the one that they dropped the cornice on. In Mt. Rose area, a natural avalanche had occurred on a wind-loaded slope on the far east ridge of Tamarack Peak.

Avalanche Concern #1: Wind Slabs

Human-triggered avalanches will remain likely on wind-loaded slopes today. The old wind slabs have not fully bonded to the snow below them as evidenced by yesterday's avalanches. More new snow and wind forecasted for today will put more weight on top of those old wind slabs by creating new wind slabs above them. As the snow accumulates, triggering these wind slabs will become easier and easier today. Natural avalanche activity will become possible during the day due to the added weight of the new snow. The largest wind slabs will exist on the wind-loaded NW-N-NE-E-SE aspects near and above treeline. Smaller but still dangerous wind slabs could also exist in wind-affected below treeline areas. Avalanches involving these wind slabs could reach several feet in depth and will grow larger and more likely as snow and wind continue over the next few days.

Avalanche Concern #2: Storm Snow Weaknesses

The new snow that falls today will land on top of a mix of snow surfaces ranging from sun-affected snow to soft light snow in non-wind affected areas. Bonding between these layers and the new snow should not occur very quickly. Changing temperatures and winds during the storm will also create other weaknesses withing the new snow. As the new snow accumulates today, human-triggered soft slab avalanches and loose snow sluffs will become likely. These will remain smaller than the wind slabs but may reach up to 1.5 ft in depth. They could easily push someone into an area that has serious consequences or could trap enough snow to bury a person 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 that receive significant new snow today.

The bottom line:

Near and above treeline, CONSIDERABLE avalanche danger will form on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees today. Below treeline pockets of CONSIDERABLE avalanche danger will form on these aspects on slopes steeper than 35 degrees. On the S-SW-W aspects the avalanche danger is MODERATE on slopes steeper than 35 degrees.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	21-26 deg. F.
Max. temperature in the last 24 hours:	22-29 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:	85 mph
New snowfall in the last 24 hours:	3-4 inches
Total snow depth:	122-188 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Snow	Snow	Chance of snow in the morning becoming more widespread in the afternoon
Temperatures:	23-29 deg. F.	18-23 deg. F.	20-26 deg. F.
Wind direction:	South	Southwest	Southwest
Wind speed:	10-20 mph with gusts to 30 mph increasing to 50 mph in the afternoon	10-20 mph with gusts to 40 mph	15-30 mph with gusts to 45 mph increasing to 70 mph in the afternoon
Expected snowfall:	5-10 in.	4-8 in.	8-14 in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Snow	Snow	Chance of snow in the morning becoming more widespread in the afternoon
Temperatures:	20-26 deg. F.	10-17 deg. F.	16-22 deg. F.
Wind direction:	South	Southwest	Southwest
Wind speed:	15-30 mph with gusts to 50 mph increasing to 25-40 mph with gusts to 70 mph	20-35 mph with gusts to 60 mph	35-50 mph with gusts to 70 mph increasing to 40-65 mph with gusts to 100 mph in the afternoon
Expected snowfall:	6-10 in.	5-8 in.	8-14 in.