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 wave of wind and precipitation should impact the forecast area today allowing the gale force southwest winds to continue along the Sierra Crest. Snowfall rates should increase today and average 1.5 to 3 inches per hour for most of the day bringing another another 15-30 inches of snow to the mountains. Snow levels should continue to oscillate between 6000 ft and 7000 ft this morning. By this afternoon snow levels should decrease quickly due to a cold front reaching the region. The forecast calls for temperatures to remain cold through tomorrow. Snowfall and winds should slow down tonight, and the storm should start to taper off during the day tomorrow. Click here for an online briefing from the Reno NWS.

Observations:

Even though most remote sensors report 5-6 inches of water accumulation since Dec. 17th, only 22-33 inches of snow has fallen, meaning that most of this precip fell as very wet, very heavy snow or rain. The storm did start colder, and those colder temperatures and mild winds during the day on the Dec. 17th allowed a lighter, softer snow layer to accumulate. By that night snow levels started to climb and have fluctuated between 6000 ft and 7500 ft ever since resulting in very dense wet snow. This wet, heavy snow (average of 25-30% density above 7500ft) that fell yesterday and most of last night formed a very dense, heavy, cohesive slab on top of the lighter snow that fell during the day on Dec 17th. This heavy layer on top of a lighter layer has created an upside down snowpack.

Observations on the far east ridge of Tamarack Peak (photo and pit profile) yesterday showed that the the lighter layer of storm snow is having difficulty supporting the strong, heavy slab on top of it. Layer bonding tests on this interface failed with light to moderate force, and ski cuts on test slopes resulted in shooting cracks that broke on this interface and traveled up to 30 ft through the slab. The winds increased and wind slabs quickly increased in depth and extent in this area. The surface hoar layer buried on Dec. 17th appeared to have collapsed during the storm in this area without resulting in as much avalanche activity as anticipated. Visibility did not allow observers to see if other steeper areas in the Mt. Rose area had slid or not. Similar observations concerning this surface hoar layer came from Talking Mountain near Echo Summit on Dec 17th. Other observations on the 17th showed that the new snowfall buried the surface hoar layer intact and that avalanche activity was possible due to its failure. 

Avalanche Concern #1: Buried Surface Hoar

In areas where the buried surface hoar has not already collapsed, it cannot support the new heavy snow, rain, and wind slabs that have fallen on top of it (potato chips trying to hold up concrete-photo of what buried surface hoar can look like in the snowpack). Large, deep, destructive natural and human triggered avalanches that run long distances will remain likely today in areas where this layer has not already collapsed. Buried surface hoar layers are unusual for the Sierra and will make traditionally safe areas suspect. This kind of weak layer allows people to trigger avalanches on lower angle terrain, in open trees, and even allows people to remotely trigger avalanches on nearby slopes from seemingly safe terrain like meadows. This kind of weak layer requires extra caution and re-evaluation of everything from approaches to safe zones to descents. Uncertainty surrounding where this layer remains intact and where it has collapsed leads to more uncertainty about stability from slope to slope.

Avalanche Concern #2: Wind Slabs

The strong winds and continued snowfall will add depth, size, and weight to the wind slabs that exist on wind-loaded aspects. These wind slabs rest on lower density snow and buried surface hoar layers that cannot support them. Large, deep, destructive natural and human triggered avalanches involving these wind slabs will remain likely today. The largest wind slab avalanches will occur on wind-loaded N-NE-E and cross loaded NW and SE aspects near and above treeline but could occur in less common areas (open slopes below treeline or even in the trees) due to strong winds transporting snow longer distances than normal. Even if these slopes have already slid once, the continued snow and wind will quickly reload them allowing them to avalanche multiple times.

Avalanche Concern #3: Rain on New Snow and an Upside Down Snowpack:

Rain falling on new snow and more dense snow falling on lighter snow will continue to occur this morning as freezing levels fluctuate. Rain adds weight, destroys bonds, and adds lubrication to the already fragile snowpack. The rain on new snow will cause avalanches on any slopes that receive rain. Even on slopes that do not receive rain, heavy layers of snow exist on top of light layers of snow that cannot support them. This upside down snowpack will also lead to avalanche activity today.

Overall several serious instabilities exist in the current snowpack. Combined with the uncertainty surrounding the buried surface hoar layer and the serious consequences associated with triggering a large slide today, these conditions will keep the avalanche danger at HIGH.

The bottom line:

The avalanche danger is HIGH on NW-N-NE-E-SE aspects and CONSIDERABLE on S-SW-W aspects steeper than 30 degrees at all elevations due to a combination of heavy snow and rain, high winds, and uncertainty surrounding the weak layer of buried surface hoar. Widespread, large natural and human-triggered avalanches are likely to continue today. Travel in or near avalanche terrain including low elevation road cuts and other steep hillsides is not recommended.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	27-30 deg. F.
Max. temperature in the last 24 hours:	29-32 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	75 mph
Maximum wind gust in the last 24 hours:	119 mph
New snowfall in the last 24 hours:	12-14 inches
Total snow depth:	61-92 inches

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

For 7000-8000 ft:
	Sunday:	Sunday Night:	Monday:
Weather:	Snow mixed with rain below 7000 ft this morning. Snow level quickly falling this afternoon.	Snow showers	Snow showers
Temperatures:	30-34 deg. F.	18-24 deg. F.	24-29 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	20-35 mph with gusts to 60 mph	20-35 mph with gusts to 60 mph	15-25 mph with gusts to 40 mph
Expected snowfall:	15-25 in.	6-12 in.	2-4 in.
For 8000-9000 ft:
	Sunday:	Sunday Night:	Monday:
Weather:	Snow	Snow showers	Snow showers
Temperatures:	25-30 decreasing this afternoon to 20-25 deg. F.	12-18 deg. F.	19-24 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	40-55 mph with gusts to 90 mph ridge gusts to 125 mph	40-55 mph with gusts to 80 mph ridge gusts to 100 mph	25-35 mph with gusts to 55 mph
Expected snowfall:	18-30 in.	6-12 in.	3-6 in.