THIS AVALANCHE FORECAST EXPIRED ON January 20, 2020 @ 7:00 am
Avalanche Forecast published on January 19, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

The potential size and dire consequences of avalanches failing on a weak layer of snow at least 2 ft below the surface (or much deeper in areas with wind drifted snow) means our travel advice is still to avoid steep slopes. Plenty of great snow exists on lower angle terrain for recreation. At the upper elevations, some unstable wind drifted snow may also still linger. CONSIDERABLE avalanche danger continues below treeline and near treeline with MODERATE avalanche danger above treeline.

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Persistent Slab
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Large avalanches that fail on weak snow (facets) around the Jan 4 rain crust have occurred on each of the last 4 days. These human-triggered persistent slab avalanches will remain possible today, because of the ongoing instability associated with this persistent weak layer. Since recent snow has buried the weak layer at least 2 ft deep in sheltered areas and much deeper where wind drifted snow exists above it (up to 8 ft), any avalanches that result from the failure of this layer would be large (D2-D3). They could have severe consequences including trauma from collisions with trees or other terrain features. These kinds of avalanches can propagate farther than expected and even break around terrain features. They also can be triggered from a distance or from an area where the weak layer is closer to the surface and break out into an area where the weak layer is much deeper. 

NW-N-NE-E facing slopes in near and below treeline terrain represent the most likely places to find this layer. However, observers have found it in a few isolated areas in more exposed upper elevation terrain. This persistent weak layer will not gain strength very quickly and could remain unstable for some time.

If you encounter signs of recent avalanches, evidence of snow surface cracking, snowpack collapse or whumfing, or unstable snowpit test results, avoid travel on or below steep slopes in the area. Persistent slab avalanches often fail after several people have traveled on the slope so previous tracks are not an indication of stability. If you have any uncertainty about whether or not this layer exists on a slope, it is best to just avoid it and pick something else.

Choosing lower angle terrain (maximum slope angles less than 30 degrees) without steeper terrain above or to the side represents a way to avoid this problem completely. The quality of the snow is so good right now that these lower angle slopes are providing excellent skiing, snowmobiling, and snowboarding experiences.

See the photos below for some of the avalanches triggered on this layer from Friday and Saturday.

 

   

Avalanche Problem 2: Wind Slab
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Some unstable slabs of wind drifted snow may still linger on steep wind-loaded slopes especially in complex or extreme terrain. Human triggered wind slab avalanches involving enough snow to bury a person remain possible.

Identify where wind slabs might exist by using clues like recent avalanche activity, cracking in drifted snow, cornices above a slope, wind drifted snow, and wind textured surfaces. Use this information to build safety margins that prevent these wind slabs from turning the day into an avalanche incident. 

recent observations
Submit Observations

* Snowbikers triggered a persistent slab avalanche involving enough snow to damage a car (D3) yesterday. This avalanche occurred on a steep E facing aspect and failed on the Jan 4 crust/facet layer. The crown on this avalanche ranged between 2 and 8 ft. deep. It was triggered from an area where the weak layer was closer to the surface (more details here). Several other avalanches have been reported on this layer including a full burial near Independence Lake on Friday (first-hand account here). Neither of these resulted in serious injury to the parties that triggered these large avalanches. 

* The Jan 4 crust/facet weak layer continues to exhibit signs of instability in places across the forecast area. Skier triggered snowpack collapse with whumpfing occurred yesterday in the Castle Peak, Deep Creek, Ophir Creek, and Red Lake Peak areas. Snowpit data also yielded unstable test results in even more places including the Blue Lakes, Deep Creek, Johnson Canyon, Yuba Pass, and Echo Summit areas.

* The Jan 4 crust/facet weak layer is most prevalent and widespread in the 7,000' to 8,500' elevation range but has also been observed at higher elevations in more isolated distribution.

* Ongoing wind slab instability was inconsistent around the forecast area yesterday with continued unstable wind slabs in some areas and limited evidence of unstable wind slabs in other areas.

Weather and CURRENT CONDITIONS
weather summary

The forecast calls for quiet weather today and tomorrow. Expect slightly warmer than normal temperatures, mostly cloudy skies, and light SE winds for both today and Monday. Some areas along the ridgetops could see some stronger gusts of wind starting this afternoon. Winds should begin to increase on Tuesday as the next storm system moves into the area.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 29 to 38 deg. F.
Max. temperature: 37 to 42 deg. F.
Average ridgetop wind direction: SSW
Average ridgetop wind speed: 15 mph
Maximum ridgetop wind gust: 31 mph
New snowfall: 0 inches
Total snow depth: 47 to 56 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy. Cloudy Cloudy
Temperatures: 41 to 47 deg. F. 27 to 32 deg. F. 37 to 42 deg. F.
Mid Slope Winds: Light winds Light winds Light winds
Expected snowfall: None None None
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Mostly cloudy Cloudy Cloudy
Temperatures: 41 to 47 deg. F. 24 to 29 deg. F. 32 to 38 deg. F.
Ridge Top Winds: Light winds becoming SE around 15 mph with gusts to 30 mph in the afternoon. South 15 to 25 mph with gusts to 50 mph. South 15 to 30 mph with gusts to 45 mph.
Expected snowfall: None None None
Disclaimer

This avalanche forecast is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This forecast 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 forecast applies only to backcountry areas outside established ski area boundaries. This forecast describes general avalanche conditions and local variations always occur. This forecast expires 24 hours after the posted time unless otherwise noted. The information in this forecast is provided by the USDA Forest Service who is solely responsible for its content.

For a recorded version of the avalanche forecast call (530) 587-3558 x258