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.

Snow totals since early Wednesday morning range from 15-24 inches with the areas north of Emerald Bay receiving the most snow. The southwest winds have averaged between 30 and 50 mph with gusts close to 100 mph since Tuesday. After tapering off some yesterday afternoon, snowfall started to pick back up this last night and should continue through this morning due to a second wave of this winter storm arriving over the forecast area. Another 3-5 inches fell during the night, and the forecast calls for another 5-10 inches above 7000 ft by midday today. The southwest winds should remain strong through today with average speeds above 40 mph and gusts over 100 along the ridgetops. As this storm starts to exit the region this evening the winds should begin to shift towards the east and decrease some and snowfall should end. A high pressure ridge should move into the area tomorrow bringing more east winds, slightly warmer temperatures, and mostly sunny skies.

Recent Observations:

Observations on Andesite Ridge (photos, video, snowpit, more info) yesterday showed two main weaknesses and several minor ones in the current snowpack. The minor weaknesses existed within the storm snow and formed due to changing conditions during the storm. Even though these minor weaknesses broke easily, stability tests indicated that the resulting fractures did not travel very far through the snowpack. The light snow that fell on Monday and the snow that has fallen since early Wednesday morning has created a density inversion that comprises one of the main weaknesses. This density inversion remained widespread across most aspects and elevations yesterday. The Jan. crust/facet combination represented the other main weakness. Hand pits and pole probes showed that the crust/facet combination remains on most NW-N-NE aspects. Ski cuts on tests slopes caused shooting cracks that broke on the interface between Monday's snow and Wednesday's snow (photo). One ski cut on a NE facing, 38 degree, wind loaded slope triggered a small avalanche that stepped down to this weak interface (photo). Tests showed that a person on top of the snowpack could easily break either of these main weaknesses, and that once a fracture started, it could travel through these weaknesses (video). The strong winds had affected most open slopes both above and below treeline in this area creating winds slabs on top of the snowpack. The thickest wind slabs existed on N-NE-E aspects in near and above treeline terrain.

Today's Primary Avalanche Concerns: Wind Slabs and Storm Slabs

The wind slabs that have formed during this storm will continue to grow in size and extent today. These slabs will remain fragile. Adding a person on top of them will likely result in an avalanche today. Natural avalanche activity will also remain possible today on wind loaded slopes. Avalanches involving the newly formed winds slabs could be large, deep, and travel long distances. They could also step down to the lower weak layers mentioned above. N-NE-E aspects near and above treeline will hold the largest and most fragile wind slabs; however, dangerous wind slabs will also exist on cross-loaded NW and SE aspects as well as open wind affected areas below treeline due to the high wind speeds at all elevations. The slightly heavier snow that has fallen since Wednesday morning could act as a soft slab in less wind affected areas. Since the previously mentioned weaknesses still exist in these areas, human triggered soft slab avalanches will also remain likely today on steep open slopes below treeline especially on NW-N-NE aspects.

Today's Secondary Avalanche Concern: Persistent Deep Slabs

The persistent weak layer of crusts and facets that exists below the recent snow remains weak. The new snow, soft slabs, and wind slabs accumulating on top of it will likely be enough to overload this layer or at least push it to the tipping point today. Any avalanches that occur today could easily step down into this layer making the resulting avalanches deeper and even more dangerous. Observations made around the forecast area over the past week have shown that this weak layer remains on NW-N-NE aspects in near and below treeline terrain.

The bottom line:

Near and above treeline CONSIDERABLE avalanche danger remains on NW-N-NE-E-SE aspects, on slopes 35 degrees and steeper. Areas of  CONSIDERABLE avalanche danger also exist near and below treeline on NW-N-NE aspects steeper than 35 degrees with pockets of CONSIDERABLE danger on the below treeline E and SE aspects. Dangerous human-triggered avalanches remain likely and natural avalanches remain possible today due to high winds, new snowfall, and a weak snowpack.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	11-16 deg. F.
Max. temperature in the last 24 hours:	18-22 deg. F.
Average wind direction during the last 24 hours:	Southwest
Average wind speed during the last 24 hours:	30-40 mph
Maximum wind gust in the last 24 hours:	92 mph
New snowfall in the last 24 hours:	North of Emerald Bay 7-11 inches | South of Emerald Bay 3-7 inches
Total snow depth:	32-61 inches

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

For 7000-8000 ft:
	Thursday:	Thursday Night:	Friday:
Weather:	Cloudy with snow showers	Mostly cloudy becoming partly cloudy with isolated snow showers	Mostly sunny
Temperatures:	24-30 deg. F.	3-10 deg. F.	28-35 deg. F.
Wind direction:	Southwest	West shifting to the east after midnight	East
Wind speed:	35-45 mph with gusts to 60 mph	20-30 mph with gusts to 45 mph	20-30 mph with gusts to 45 mph
Expected snowfall:	5-10 in.	O in.	O in.
For 8000-9000 ft:
	Thursday:	Thursday Night:	Friday:
Weather:	Cloudy with snow showers	Mostly cloudy becoming partly cloudy with isolated snow showers	Mostly sunny
Temperatures:	18-24 deg. F.	2-9 deg. F.	26-33 deg. F.
Wind direction:	Southwest	West shifting to the east after midnight	East
Wind speed:	65-75 mph with gusts to 110 mph	40-50 mph with gusts to 70 mph decreasing to gusts to 55 mph after midnight	35-45 mph with gusts to 65 mph
Expected snowfall:	6-10 in.	O in.	O in.