THIS AVALANCHE FORECAST EXPIRED ON December 13, 2020 @ 7:00 am
Avalanche Forecast published on December 12, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

The new snow has made the rocks and other obstacles harder to see and easier to hit. This storm's new snow and wind have also added potential avalanche concerns. Unstable slabs of wind drifted snow (wind slabs) may exist on open slopes near ridgelines on almost any aspect today. Human triggered avalanches will be possible. MODERATE avalanche danger exists in near and above treeline terrain. 

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.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    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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On its own, the 2 to 4 inches of new snow that fell last night may not seem like much, but the gale-force winds can transport that snow and form slabs of drifted snow deep enough to cause problems. With the winds forecasted to shift to the NE and E today, these wind slabs may move during the day. These newly formed wind slabs may not bond well to the existing snow surfaces. In some places, the possibility exists that the additional weight of a person on them could cause the slabs to break away from the slope resulting in an avalanche. Most of these would remain relatively small, but any size avalanche could have severe severe consequences in the current shallow snowpack. There are still numerous obstacles close to the snow surface and a collision with them would hurt.  

Clues like blowing snow, cornices above a slope, snowdrifts, and wind created surface textures can all help identify where wind slabs may exist. Use this information to adjust travel plans to minimize exposure to potential wind slabs. 

Forecast discussion

Last night's new snow accumulated above old weak snow on many open slopes sheltered from the winds in near and below treeline terrain. While this old snow is weak, an additional 2 to maybe 4 inches of new snow on top of it is unlikely to overload it. This new snow could represent the beginning of a slab layer on top of that old weak faceted snow. As more storms impact the area in the coming days enough snow could accumulate on top of the old weak sugary snow (facets) to create an avalanche problem in near and below treeline terrain. 

recent observations
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* Observations collected across the forecast area have shown a weak and shallow snowpack in many places. Of particular concern are the open northerly aspects in near and below treeline terrain. Weak sugary snow (facets) have formed on these slopes during the long dry spell since our last storm on Nov. 18. As new snow accumulates on top of this potential weak layer, it could overload the layer, and avalanche activity may result.

* Prior to this snowfall, a somewhat supportable snowpack 1-2 ft. deep exists on northerly aspects above 8600 ft. in the Mt. Rose area, above 7000 ft. along the northern Sierra Crest, and above 8000 ft. along the southern Sierra Crest. Coverage and snow surfaces varied widely. Many southerly aspects had melted back to bare ground. The near and below treeline northerly aspects hold the most snow. More sheltered areas hold a mix of firm melt-freeze snow, breakable crusts, and some areas of soft surface snow (near-surface facets) on sheltered northerly aspects. Wind-scoured and sculpted surfaces existed in exposed near and above treeline terrain.

Weather and CURRENT CONDITIONS
weather summary

Snowfall started last night and deposited 2 to 4 inches of heavy wet snow above 7000 ft. Snow levels hit about 6500 ft and below that the precipitation has been mostly rain. The SW winds increased during the night and have averaged between 30 and 50 mph with some gusts over 100 mph along the Sierra Crest. As this storm departs today the snow and rain should taper off this morning and the SW winds should decrease. By this afternoon the winds should shift to the NE/E and remain strong enough to move snow and scour slopes. A colder air mass moving into the area should also keep temperatures cooler today and tonight. By tomorrow another stronger and wetter system should move into the area. The forecast calls for the winds to shift back to the SW and for more snow especially tomorrow afternoon. Snow should continue through Sunday night. Right now the forecast calls for an additional 8 to 14 inches at the upper elevations by Monday morning. Check-in with the Reno NWS for more details.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 27 to 32 deg. F.
Max. temperature: 32 to 35 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 35 to 50 mph
Maximum ridgetop wind gust: 116 mph
New snowfall: 2 to 4 inches
Total snow depth: 14 to 20 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy then becoming partly cloudy. Chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 65%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 90%.
Temperatures: 31 to 36 deg. F. 23 to 29 deg. F. 33 to 38 deg. F.
Mid Slope Winds: West 10 to 20 mph with gusts to 40 mph in the morning becoming north around 10 mph with gusts to 30 mph. East around 10 mph becoming southwest 10 to 20 mph after midnight. Southwest 15 to 20 mph with gusts to 35 mph increasing to 20 to 30 mph with gusts to 50 mph in the afternoon.
Expected snowfall: up to 1 inch. | SWE = up to 0.10 inch. No accumulation. | SWE = none. 80% probability of 2 to 4 inches. 20% probability of 4 to 6 inches. | SWE = up to 0.60 inch.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Cloudy then becoming partly cloudy. Chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 75%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet increasing to 7000 feet in the afternoon. Chance of precipitation is 90%.
Temperatures: 28 to 33 deg. F. 22 to 27 deg. F. 29 to 34 deg. F.
Ridge Top Winds: West 40 to 70 mph with gusts to 100 mph becoming north 20 to 35 mph with gusts to 50 mph in the afternoon. Northeast 15 to 25 mph with gusts to 40 mph shifting to the southwest after midnight. Southwest 20 to 35 mph with gusts to 65 mph increasing to 30 to 45 mph with gusts to 85 mph in the afternoon.
Expected snowfall: up to 1 inch. | SWE = up to 0.10 inch. No accumulation. | SWE = none. 80% probability of 3 to 6 inches. 20% probability of 6 to 8 inches. | SWE = 0.55-0.80 inch.
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