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 high pressure ridge should remain over the area and continue to gain strength during the next few days. The forecast calls for another sunny and clear day today with daytime highs in the mid 30's to upper 30's above 8000 ft. Temperatures should continue to gradually climb through the week. The northeast winds decreased and shifted to the northwest last night. They should shift to the west today and continue to decrease. By tomorrow some cloud cover should move over the area as a small system passes north of the region.

Recent Observations:

Yesterday on Donner Peak (photos, videos, snowpit, more info) wind scoured surfaces and crusts represented the most common snow surface (photo). Sporadic small hard wind slabs did exist on some slopes. Most of these wind slabs extended down slope less than 10 ft and measured less than 10 ft wide. The largest wind slab observed covered an area about 25 ft wide and extended down slope for about 35 ft. Their depth ranged form 4-12 inches on the northerly aspects to 2-3 ft on some W-NW facing test slopes. Snowpit tests on these wind slabs consistently showed that fractures could propagate along a thin layer of weak snow below the wind slab and along the weak sugary snow below the old rain crust (snowpit and videos). Data and observations from this area seemed to indicate that the weak layers under the slab are losing strength. However, ski cuts and aggressively weighting test slopes did not produce any cracks or other signs of instability. On undercut slopes where the slabs were unsupported, some human triggered cracking did occur on NW and NE test slopes as a result of aggressively jumping on the slope without skis on (video). Other observations around the forecast area have shown similar hard wind slabs that remain difficult to trigger on all near and above treeline aspects. 

Soft unconsolidated snow still remains on the northerly aspects in areas sheltered from the winds. Snowpit data and observations in these non wind affected areas did not reveal any signs of instability.

Avalanche Problem #1: Wind Slabs

Pockets of hard stubborn wind slabs still exist on most aspects near ridgelines in near and above treeline terrain. These wind slabs remain relatively small and do not extend very far down slope due to a combination of a lack of snow available to construct larger slabs and significant wind scouring. Some data has started to indicate that the weak layers below the slabs are getting weaker in some areas. If these layers continue to weaken it may become less difficult to trigger these slabs. A great deal of variability exists concerning the location, thickness, and size of these slabs. Variability also surrounds the stability of these slabs. Some tests have indicated that they are unstable; however, most of the data indicates that they are difficult for a single person to trigger. Triggering one of these slabs remains unlikely in most areas. Finding an isolated pocket of terrain where these wind slabs exist on layers that have weakened enough for a single person to trigger one of these slabs will be possible today. Wind loaded or cross loaded complex or extreme near and above treeline terrain along the Sierra Crest represents the best terrain to find one of these isolated pockets of more fragile wind slabs.

Avalanche Problem #2: Loose Wet Snow

Sunshine, decreased winds, and slightly warmer temperatures may allow some small roller balls, small pinwheels, or small point release sluffs to form on the most sun-exposed SE-S-SW aspects especially at the lower elevations or near exposed rocks and cliffs. These wet instabilities should remain small and not involve enough snow to bury a person.

 

The bottom line:

On a regional scale the avalanche danger remains LOW for most elevations and aspects. Very isolated and sporadic pockets of MODERATE danger may exist in complex or extreme near and above treeline terrain where isolated unstable wind slabs rest on layers of weak snow. Continue to use normal caution when traveling in the backcountry.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	24-29 deg. F.
Max. temperature in the last 24 hours:	28-36 deg. F.
Average wind direction during the last 24 hours:	Northeast shifting to the northwest
Average wind speed during the last 24 hours:	10-25 mph mph
Maximum wind gust in the last 24 hours:	55 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	56-80 inches

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

For 7000-8000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Sunny then becoming partly cloudy	Partly cloudy	Mostly cloudy
Temperatures:	36-43 deg. F.	20-30 deg. F.	39-46 deg. F.
Wind direction:	Variable	Variable	Variable
Wind speed:	Light	Light	Light
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Wednesday:	Wednesday Night:	Thursday:
Weather:	Sunny then becoming partly cloudy	Partly cloudy	Mostly cloudy
Temperatures:	30-39 deg. F.	17-27 deg. F.	34-43 deg. F.
Wind direction:	Northwest shifting to the west	West	Northwest
Wind speed:	10-15 mph with gusts to 25 mph	10-15 mph with gusts to 25 mph in the evening	15-20 mph with gusts to 30 mph
Expected snowfall:	O in.	O in.	O in.