THIS AVALANCHE FORECAST EXPIRED ON March 16, 2018 @ 7:00 am
Avalanche Forecast published on March 15, 2018 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

MODERATE avalanche danger will exist during the day due to wind slabs formed in the last 24 hours. The avalanche danger will quickly increase as the next storm impacts the forecast area starting this afternoon. The avalanche danger should rise to HIGH tonight night as wind slab and storm slab avalanche problems become widespread. Human triggered wind slabs will become very likely and storm slabs will become likely during the night. Travel in or near avalanche terrain is not recommended.

4. High

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Above Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

4. High

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Near Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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The wind slabs that have formed in the last 24 hours will remain fragile today and human-triggered wind slab avalanches will remain possible.  As more snow and wind start to impact the forecast area this afternoon, wind-loading will add wind slabs on top of the existing ones and wind slabs will quickly grow much larger and more widespread. Wind slab avalanches will also become much easier to trigger as the new snow and wind load slopes faster than the wind slabs can bond to the existing snowpack. Natural wind slab avalanches will become likely tonight. Human triggered wind slab avalanches will become very likely tonight and into tomorrow.  Expect wind slabs on wind-loaded near and above treeline NW-N-NE-E-SE aspects. Some of these wind slabs avalanches could be large and destructive. 

Look for signs of recent wind slab avalanches, blowing snow, cornices, cornice formation, and wind pillows.  Avoid steep wind-loaded terrain and run out zones below corniced slopes.

Avalanche Problem 2: Storm Slab
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Storm slab avalanches will not become an issue until later this afternoon or during the night tonight. Changing conditions during the storm will create weak layers within the storm snow. Snowfall rates that exceed 2 in/hr during the storm will cause the new snow to accumulate too fast for those weak layers to adjust and storm slabs will become an avalanche problem. Expect human triggered storm slab avalanche to become likely during the night. 

Look for signs of cracking or cohesion of the new storm snow.  Hand pits could provide a good tool for looking at the bonding of the new storm snow.  Avoid slopes with signs of unstable snow.

Forecast discussion

Observations and data targetting the persistent weak layer responsible for the deep slab avalanche cycle have shown significant increases in that layer's strength and in the strength of the snow above that layer. Tests on that layer across the forecast area including several comparison tests in places where this layer failed and had clearly unstable test results now do not indicate signs of continued instability.

recent observations
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* Fragile wind slabs existed on wind-loaded slopes near the ridgelines yesterday. 2 natural wind slab avalanches were reported on the far east ridge of Tamarack Peak in the Mt. Rose area. On Castle Peak (Donner Summit area) and on Powderhouse Peak (Luther Pass area) ski kicks could trigger wind slabs from lower angle terrain above or close to the steeper wind-loaded slopes. On Castle Peak, they measured up to 2 ft in depth and up to 16 inches in depth on Powderhouse Peak.

* Skier-triggered shooting cracks occurred in these areas on lower angle wind-loaded slopes as well.

* Below yesterday's new snow a thin rain crust existed on Tamarack Peak and a layer of rain-soaked snow existed on Castle Peak and Powderhouse Peak. Observations indicated good bonding between these layers.  

* Snowpit tests and data on Powderhouse Peak targetting the old persistent weak layer buried about 3 ft below the surface showed significant gains in strength and did not reveal evidence of continuing instability associated with this layer. 

Weather and CURRENT CONDITIONS
weather summary

Most sensors and areas reported 6 to 12 inches of new snow in the last 24 hours with some places reporting 4 inches and some reporting more than 12 inches. Snowfall has decreased and should remain light this morning. After this short break in the weather, another stronger and colder winter storm should arrive over the forecast area starting this afternoon. Widespread snow should return this afternoon. Once the snowfall increases and becomes more widespread it should continue through Saturday with the most intense snowfall and highest accumulations occurring tonight and tomorrow. The forecast calls for ~1.5 to 3 ft. of new snow above 7000 ft. by the end of the day tomorrow with some areas receiving even higher amounts. Snowfall rates are predicted to exceed 2 in/hr at times tonight and tomorrow. The southwest winds should begin to increase again and remain strong throughout the storm. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 16 to 21 deg. F.
Max. temperature: 26 to 29 deg. F.
Average ridgetop wind direction: Southwest
Average ridgetop wind speed: 25 to 30 mph
Maximum ridgetop wind gust: 55 mph
New snowfall: 6 to 12 inches
Total snow depth: 57 to 90 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy with isolated snow showers in the morning. Snow becoming likely in the afternoon Snow Snow with slight chances of thunderstorms in the afternoon
Temperatures: 27 to 32 deg. F. 17 to 22 deg. F. 25 to 30 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Wind Speed: 15 to 25 mph with gusts to 35 mph increasing to 50 mph in the afternoon 20 to 30 mph with gusts to 60 mph 15 to 25 mph with gusts to 45 mph
Expected snowfall: up to 4 8 to 15 6 to 15
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy with isolated snow showers in the morning. Snow becoming likely in the afternoon Snow Snow with slight chances of thunderstorms in the afternoon
Temperatures: 22 to 28 deg. F. 14 to 20 deg. F. 21 to 27 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
Wind Speed: 20 to 30 mph with gusts to 55 mph increasing to 75 mph 30 to 45 mph with gusts to 95 mph 20 to 35 mph with gusts to 80 mph
Expected snowfall: up to 4 10 to 18 7 to 16
Disclaimer

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.

For a recorded version of the Avalanche Advisory call (530) 587-3558 x258