THIS AVALANCHE FORECAST EXPIRED ON January 17, 2020 @ 7:00 am
Avalanche Forecast published on January 16, 2020 @ 7:00 am
Issued by Brandon Schwartz - Tahoe National Forest

Dangerous avalanche conditions are expected in near treeline and below treeline areas today caused by rapid new snow loading of a problematic weak layer in the upper snowpack.  New snow and wind will also form unstable slabs of wind drifted snow in near treeline and above treeline areas. A rapid increase to HIGH avalanche danger is forecast for all elevations today with travel in avalanche terrain 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.

4. High

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Below Treeline
Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
    Very dangerous avalanche conditions. Travel in avalanche terrain not recommended.
Avalanche Problem 1: Wind Slab
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Gale force winds combined with high intensity snowfall today will create unstable slabs of wind drifted snow. These unstable wind slabs will mainly exist in near treeline and above treeline areas on NW-N-NE-E-SE aspects. Unstable wind slabs may form in isolated areas on other aspects or below treeline due to gale force winds. Avalanche size is anticipated at D2 with up to D3 possible if failing on the Jan 4 crust/facet sugary snow weak layer.

Travel in or below avalanche terrain is not recommended today. If traveling non-avalanche terrain in the backcountry today, make a detailed plan for how to navigate in limited visibility conditions in order to successfully avoid exposure to avalanche terrain when you cannot see it above or around you. Natural avalanches are expected today.

Avalanche Problem 2: Storm Slab
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In my mind, this is the scarier of the two avalanche problems today. High intensity snowfall will cause rapid loading and stressing of the weak sugary snow around Jan 4 crust/facet layer today. Avalanches that fail on this weak layer could be naturally triggered, human triggered from afar (remotely triggered), are expected to have wide propagation, have the ability to wrap around corners, and fracture above and behind a person who is a direct trigger on the slope.

The first known avalanche on this weak layer occurred yesterday, well before today's rapid loading event has even begun.  This Jan 4 crust/facet weak layer exists near treeline and below treeline on NW-N-NE-E aspects. Avalanche size is expected at D2. High intensity snowfall is expected to spread storm slab instability to other aspects as well simply due to weakness within the new snow itself. 

"Staying in the trees" will not avoid this avalanche problem today. This can be a way to avoid a wind slab problem, but not a way to avoid this storm slab problem. Travel in or below avalanche terrain is not recommended today. If traveling non-avalanche terrain today, avoid areas connected to avalanche terrain above or to the side. Anticipate poor visibility limiting your ability to see the terrain.

  Photo: First reported avalanche on Jan 4 weak layer.

recent observations
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* The first report of an avalanche failing on the Jan 4 crust/facet layer was reported to have occurred yesterday on Donner Ridge (Donner Summit area) in wind loaded near treeline terrain on a N to NE aspect around 7,500'. Crown depth was reported at approximately 2 feet. Trigger unknown.

* SAC has been tracking the weak sugary snow around the Jan 4 rain crust for a while now. Snowpit tests using an artificial new snow load have indicated that snowpack failure and propagation along this layer is likely under new snow loading conditions.

* In most locations the Jan 4 crust/facet layer is 4 to 6 inches below the old snow surface, but buried deeper under wind drifted snow in isolated areas. It exists in near treeline and below treeline terrain on NW-N-NE-E aspects. It is most prevalent in the 7,000' to 8,500' elevation range but has been observed with concerning weakness in more isolated distribution at higher elevations.

Weather and CURRENT CONDITIONS
weather summary

An approaching storm system will impact the forecast area today. The onset of snowfall is expected around 9 to 10 am this morning. Snowfall rates are forecast to quickly increase to 2+ inches per hour and last into this evening. New snow amounts of 1 to 2 feet are expected over an 18 hour period. Ridgetop winds are strong out of the SW early this morning and are forecast to become gale force from sunrise to noon, gusting to 110 to 130 mph. Ridgetop winds are forecast to remain out of the SW, gradually decreasing to moderate speed tonight through tomorrow. This storm is expected to end tonight with partial clearing of cloud cover forecast for 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: 21 to 26 deg. F.
Max. temperature: 23 to 28 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 40 mph
Maximum ridgetop wind gust: 87 mph
New snowfall: 0 inches
Total snow depth: 41 to 46 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Cloudy. Slight chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Cloudy. Snow in the evening, then snow likely after midnight. Snow levels below 7000 feet. Chance of precipitation is 95%. Mostly cloudy. Slight chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 20%.
Temperatures: 28 to 33. deg. F. 12 to 17. deg. F. 28 to 34. deg. F.
Mid Slope Winds: South 25 to 35 mph with gusts to 80 mph. Southwest 15 to 25 mph. Gusts up to 45 mph decreasing to 35 mph after midnight. Southwest around 15 mph with gusts to 30 mph in the morning becoming light.
Expected snowfall: 80% probability of 6 to 12 inches. 20% probability of 10 to 18 inches. | SWE = 0.55-0.80 inch. 70% probability of 3 to 7 inches. 30% probability of 6 to 10 inches. | SWE = 0.15-0.40 inch. 60% probability up to 1 inch. 40% probability no accumulation. | SWE = trace amounts.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Cloudy. Slight chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Cloudy. Snow in the evening, then snow likely after midnight. Snow levels below 7000 feet. Chance of precipitation is 95%. Mostly cloudy. Slight chance of snow in the morning. Snow levels below 7000 feet. Chance of precipitation is 20%.
Temperatures: 24 to 30. deg. F. 9 to 14. deg. F. 24 to 30. deg. F.
Ridge Top Winds: South 50 to 70 mph with gusts up to 120 mph decreasing to 35 to 55 mph with gust to 75 mph in the afternoon. Southwest 20 to 35 mph with gusts to 60 mph. Southwest 15 to 30 mph. Gusts up to 50 mph decreasing to 35 mph in the afternoon.
Expected snowfall: 80% probability of 8 to 14 inches. 20% probability of 14 to 18 inches. | SWE = 0.50-1.00 inch. 70% probability of 3 to 7 inches. 30% probability of 6 to 10 inches. | SWE = 0.20-0.45 inch. 60% probability up to 1 inch. 40% probability no accumulation. | SWE = trace amounts.
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