THIS AVALANCHE ADVISORY EXPIRED ON December 15, 2016 @ 6:57 am
Avalanche Advisory published on December 14, 2016 @ 6:57 am
Issued by Brandon Schwartz - Tahoe National Forest

Areas of MODERATE avalanche danger exist above 8,000' both above and below treeline on slopes 35 degrees and steeper due to a combination of newly formed wind slabs above 8,000' and difficult to trigger persistent slabs above 9,000'. Below 8,000' to 9,000' firm icy surfaces exists in many areas and could pose dangerous travel conditions with long sliding falls possible.

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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A new round of wind slabs is expected to have formed last night above 8,000' in near and above treeline areas on NW-N-NE-E-SE aspects due to the combination of new snow and wind. In the majority of areas that received just 1 to 3 inches of new snow, any new wind slabs will be small and not present a significant hazard to backcountry travelers. In the isolated areas above 9,000' that potentially received up to 7 inches of new snow last night, wind slabs could be large enough to present a significant hazard to backcountry travelers today. Look for new cornice formation, new wind pillows, and areas of human triggered snow surface cracking that point towards recent wind loading. In locations where signs of recent wind loading point to the presence of this wind slab avalanche problem, avoid slopes 35 degrees and steeper.

Avalanche Problem 2: Persistent Slab
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The persistent slab avalanche problem is an ongoing issue above 9,000' on NW-N-NE aspects both above and below treeline. Likelihood of triggering is temporarily decreased today with the break between large new snow loading events on the snowpack. That said, triggering a large, destructive, difficult to survive avalanche is not impossible today, just less likely than over the past several days. The most likely scenario for a large avalanche to occur today is for a human triggered wind slab avalanche to occur on the slope first and then step down with additional snowpack failure on the deeper persistent weak layer of old faceted snow.

Likelihood of triggering is expected to increase again Thursday-Friday as significant amounts of new snow above 9,000' drastically increase the amount of stress on the faceted weak layer. Significant amounts of new snow are also expected to transition this persistent slab avalanche problem to a deep persistent slab avalanche problem in the coming days.

recent observations

Observations made yesterday on Tamarack Peak (Mount Rose area) and on Elephant's Back (Carson Pass area) pointed towards ongoing issues with faceted snow above 9,000' on NW-N-NE aspects. This is the weak layer associated with the ongoing persistent slab avalanche problem. Snowpit tests indicated increasing difficulty in triggering during the lull between new snow loading events stressing the snowpack. These tests also indicated that once snowpack failure is initiated, large destructive avalanche activity remains likely.

Other recent observations have shown firm icy conditions on all aspects from recent rain on snow events below 8,000' to 9,000'. Last night's rain is expected to have done little to soften this icy snow surface. The potential for long sliding falls exists in some areas.

Weather and CURRENT CONDITIONS
weather summary

A lull in precipitation will occur today as a weak system from last night moves out of the area and a much stronger storm system moves into the region tomorrow morning. Last night snow levels were around 8,000' to 8,500' with most locations picking up 0.1 to 0.3 inches of rain water equivalent. Isolated areas received 0.5 to 0.7 inches of rain water equivalent. Above snow level this translated to 1 to 3 in of new snow in most areas with some isolated locations of potentailly 5 to 7 inches of new snow above 9,000'. Ridgetop winds out of the SW increased to gale force in speed overnight. Winds are forecast to further increase in speed today with gusts to 120 mph possible. The next storm system for tomorrow is another Atmospheric River. Snow levels are currently forecast at 8,000' or higher until Friday morning. New snow amounts measured in feet are expected over the upper elevations with significant rain at the mid and lower elevations. Gale force SW winds are expected to continue into Friday.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 32 to 36 deg. F.
Max. temperature in the last 24 hours: 32 to 37 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 45 mph
Maximum wind gust in the last 24 hours: 82 mph
New snowfall in the last 24 hours: Most locations 1 to 3 in | Isolated areas potentially 5 to 7 inches
Total snow depth: Along the Sierra Crest: 16 to 24 inches | In the Mt. Rose area: 42 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy skies, becoming partly cloudy. Slight chance of rain in the morning, Partly cloudy skies with a slight chance of rain after midnight. Mostly cloudy skies, becoming cloudy. A chance of rain in the morning. Rain in the afternoon.
Temperatures: 45 to 51 deg. F. 33 to 38 deg. F. 42 to 48 deg. F.
Wind Direction: SW SW SW
Wind Speed: 15 to 25 mph with gusts to 60 mph, increasing to 25 to 35 mph with gusts to 80 mph in the afternoon. 25 to 35 mph with gusts to 80 mph. 25 to 35 mph with gusts up to 80 mph.
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy skies, becoming partly cloudy. Slight chance of rain and snow in the morning, Partly cloudy skies with a slight chance of rain and snow after midnight. Mostly cloudy skies, becoming cloudy. A chance of rain and snow in the morning. Rain and snow in the afternoon.
Temperatures: 37 to 45 deg. F. 31 to 36 deg. F. 37 to 45 deg. F.
Wind Direction: SW SW SW
Wind Speed: 30 to 45 mph with gusts to 110 mph, increasing to 35 to 55 mph with gusts to 120 mph in the afternoon. 40 to 60 mph with gusts to 130 mph. 35 to 55 mph with gusts up to 125 mph.
Expected snowfall: 0 in. 0 in. 2 to 4 in.
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