THIS AVALANCHE FORECAST EXPIRED ON December 18, 2018 @ 7:00 am
Avalanche Forecast published on December 17, 2018 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

Human-triggered avalanches will be possible today due to wind slab and storm slab avalanche problems. MODERATE avalanche danger exists at all elevations. Maintain awareness of the conditions and terrain while traveling in the backcountry and use this information to create safety margins that avoid the avalanche problems. 

2. Moderate

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

2. Moderate

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

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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New snow and strong southwest winds during the night will have created new wind slabs on wind-loaded N-NE-E aspects and on some cross-loaded NW and SE aspects in near and above teeline terrain. These new wind slabs will continue to grow today as more snow and wind impact the forecast area. In some places, these slabs may rest on top of the weak surface snow that existed before the storm. Human-triggered wind slabs will be possible today and some of them could involve enough snow to bury or injure a person. Some of these wind slabs could be remotely triggered or propagate across a larger area than expected. 

Maintain awareness of the conditions and terrain by looking for signs of wind-loading like cornices above a slope, blowing snow, or wind created textures on the snow surface. Avoiding these wind-loaded areas where fragile wind slabs may exist and planning to recreate in lower angle more sheltered terrain will help avoid this potential avalanche problem and find fun soft snow. 

Avalanche Problem 2: Storm Slab
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Human-triggered storm slab avalanches may be possible today especially where the new snow sits on top of the weak surface snow that existed on most northerly aspects. If storm slab avalanches do occur, they should remain relatively small due to the amount of new snow accumulation. Small avalanches can still have large consequences especially when terrain traps like cliffs, gullies, trees, lakes, etc are involved. They could also help push a person into any of the numerous early season obstacles. 

Look for clues like recent avalanche activity, shooting cracks, or feeling a softer layer of old snow under the new snow to help figure out where storm slabs may exist. Use this information to avoid those potentially problematic areas. When in doubt, assume the slope may be unstable and move to lower angle terrain where avalanches are unlikely and fun recreation conditions still exist.

recent observations
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* Yesterday observations from Powderhouse Peak (Luther Pass), Tamarack Peak (Mt. Rose backcountry), and Castle Peak (Donner Summit) found weak unconsolidated snow on the surface on northerly aspects. This data is consistent with other observations from around the region this week showing widespread near surface facets on northerly aspects.

* A mix of scoured surfaces, wind packed snow, and crusts existed on the more exposed slopes before this storm.

* On less exposed southerly aspects, variable crusts existed on many of the slopes prior to the storm.

* Early season conditions still exist with lots of obstacles on or near the snow surface in many places.

Weather and CURRENT CONDITIONS
weather summary

Light precipitation started falling over the Sierra Crest yesterday evening but the snow did not start to accumulate until later in the night. As of 6 am the mountains had only received about 3 to 5 inches of new snow. The forecast calls for continued snow showers this morning with up to 4 more inches of new snow. Snow levels should drop to around 5500 ft as colder air associated with this front reaches the area. The southwest winds increased overnight and should remain strong enough to move snow through today. By this afternoon, the storm should taper off and winds should decrease. Expect a partly cloudy and cooler day tomorrow. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 25 to 32 deg. F.
Max. temperature: 33 to 40 deg. F.
Average ridgetop wind direction: Southwest
Average ridgetop wind speed: 50 to 55 mph
Maximum ridgetop wind gust: 112 mph
New snowfall: 3 to 5 inches
Total snow depth: 26 to 32 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy then becoming partly cloudy. Scattered snow showers in the morning. Snow levels below 7000 feet. Chance of precipitation is 75%. Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 5%.
Temperatures: 34 to 39 deg. F. 21 to 29 deg. F. 40 to 45 deg. F.
Mid Slope Winds: Southwest 10 to 15 mph with gusts to 45 mph in the morning becoming light. Light winds. Southwest 10 to 15 mph with gusts to 30 mph.
Expected snowfall: up to 4 inches. | SWE = up to 0.20 inch. No accumulation. | SWE = none. No accumulation. | SWE = none.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy then becoming partly cloudy. Snow showers in the morning. Snow levels below 7000 feet. Chance of precipitation is 85%. Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Partly cloudy. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 5%.
Temperatures: 30 to 35 deg. F. 21 to 26 deg. F. 37 to 42 deg. F.
Ridge Top Winds: Southwest 25 to 35 mph with gusts to 85 mph decreasing to 15 to 25 mph with gusts to 50 mph in the afternoon. Southwest 15 to 20 mph with gusts to 35 mph. Southwest 15 to 30 mph with gusts to 55 mph increasing to 25 to 35 mph with gusts to 75 mph in the afternoon.
Expected snowfall: 70% probability of up to 2 inches. 30% probability of 2 to 4 inches. | SWE = up to 0.25 inch. No accumulation. | SWE = none. No accumulation. | SWE = 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