THIS AVALANCHE FORECAST EXPIRED ON January 18, 2020 @ 6:57 am
Avalanche Forecast published on January 17, 2020 @ 6:57 am
Issued by Brandon Schwartz - Tahoe National Forest

There is a high degree of uncertainty today in regards to snowpack instability near and below treeline. A significant loading event onto a weak layer in the upper snowpack has occurred near treeline and below treeline. Significant drifting of new snow has occurred near treeline and above treeline. CONSIDERABLE avalanche danger is forecast for all elevations.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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This is the simpler of today's two avalanche problems to figure out. The combination of wind and new snow during the last 24 hours has created some slabs of wind drifted snow in lee areas. These wind slabs exist mainly near treeline and above treeline on NW-N-NE-E-SE aspects. Avalanche size is expected mostly at D2 with up to D3 possible in the most heavily wind drifted areas.

The wind direction has not changed so use the clues of cornice formations, wind pillows, snow surface cracking, and any signs of recent avalanches to determine where unstable wind slabs are most likely to be encountered. Travel around these specific areas with caution, anticipating instability.

Avalanche Problem 2: Storm Slab
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This is the more complicated of the two avalanche problems today and there is great uncertainty here. A rapid new snow loading event occurred on all aspects in wind protected areas near treeline and below treeline. At a basic level, weakness within the new snow may allow for continued instability. More complex is how the sugary weak snow of the buried Jan 4 crust/facet layer in adjusting to the new snow load. Information at this time is very limited and we simply do not know if this will stabilize like a typical storm slab or if there will be persistent instability. For now, anticipate instability and account for uncertainty by increasing margins of safety until the situation becomes more clear. Avalanche size is expected at D2.

The Jan 4 crust/facet weak layer exists near treeline and below treeline on NW-N-NE-E aspects. In most below treeline locations, it is found 4 to 6 inches below the old/new snow interface. In some near treeline locations, it is found deeper under older wind drifted snow.

"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. If you encounter signs of recent avalanches near to below treeline, evidence of snow surface cracking, snowpack collapse (possible whumpf sound), or unstable snowpit test results, avoid travel on or below steep slopes in the area.

recent observations
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* With the delayed start to yesterday's storm, evidence of instability observed during the midday hours was limited.

* Snowpit tests from the Donner Summit, Ward Canyon, Brockway Summit, and Luther Pass areas produced a mix of results. Snowpit test results showing failure around the Jan 4 crust/facet weak layer and indicating likely propagation were reported late day from Ward Canyon. Tests from other areas and those performed closer to midday provided limited to minimal indications of instability.

* With poor visibility and the peak in snowpack instability having occurred around or after sunset yesterday, it is unknown how much avalanche activity has occurred.

Weather and CURRENT CONDITIONS
weather summary

New snow amounts from the past 24 hours range from 11 to 22 inches. The greatest reported accumulations were along the Sierra Crest in the northern half of the forecast area. Clearing weather conditions are forecast for today. Ridgetop winds remain moderate to strong out of the SW this morning but are expected to decrease as the day progresses. Moderate speed ridgetop gusts will continue through tomorrow. Any isolated light snow showers early this morning should come to an end shortly after sunrise. Decreasing cloud cover is forecast with a mix of sun and periods of mid to high level cloud cover today and tomorrow. Air temperatures tomorrow are forecast to climb above freezing for many areas.

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 10 to 15 deg. F.
Max. temperature: 22 to 27 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 52 mph
Maximum ridgetop wind gust: 116 mph
New snowfall: 11 to 22 inches
Total snow depth: 57 to 68 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy. Isolated snow showers in the morning. Snow levels below 7000 feet. Chance of precipitation is 10%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 27 to 33. deg. F. 16 to 21. deg. F. 36 to 41. deg. F.
Mid Slope Winds: Southwest around 15 mph with gusts to 25 mph in the morning becoming light. Light winds becoming southwest around 15 mph after midnight. Gusts up to 30 mph. Southwest around 15 mph with gusts to 30 mph.
Expected snowfall: 100% probability up to 1 inch. | SWE = none. No accumulation. | SWE = none. No accumulation. | SWE = none.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Mostly cloudy. Isolated snow showers in the morning. Snow levels below 7000 feet. Chance of precipitation is 10%. Mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 0%.
Temperatures: 24 to 30. deg. F. 14 to 19. deg. F. 35 to 41. deg. F.
Ridge Top Winds: Southwest 15 to 30 mph. Gusts up to 60 mph decreasing to 40 mph in the afternoon. Southwest 15 to 30 mph. Gusts up to 45 mph increasing to 55 mph after midnight. Southwest 15 to 30 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 100% probability up to 1 inch. | SWE = none. 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