THIS AVALANCHE FORECAST EXPIRED ON December 14, 2020 @ 7:00 am
Avalanche Forecast published on December 13, 2020 @ 7:00 am
Issued by Brandon Schwartz - Tahoe National Forest

Avalanche danger will rapidly increase from midday onwards. Isolated and still unstable slabs of wind drifted snow from yesterday (wind slabs) will be a problem this morning and added to this afternoon with increasing size and distribution. A persistent slab avalanche problem is expected to develop during today's storm. Avalanche danger will increase from MODERATE danger this morning to CONSIDERABLE this afternoon. HIGH avalanche danger is possible tonight if the upper end of the forecast snowfall amount is reached.

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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Unstable wind slabs were reported yesterday from the Carson Pass zone with a few small skier triggered avalanches. Slabs of wind drifted snow were deposited on top of weak faceted (sugary) snow in near treeline and above treeline terrain. These wind slabs are expected to continue to be reactive this morning. Following the onset of snowfall today, gale force SW ridgetop winds will drift more snow and deposit larger and more widespread wind slabs near treeline and above treeline on NW-N-NE-E aspects across the forecast area.

Any slopes near ridgetops where blowing and drifting snow is depositing are suspect. Any slopes with newly formed cornices or drifted pillows of snow along the upper portion of the slope are likely to hold unstable wind slabs. Snow surface cracking is a sign of unstable snow in the immediate area. Use low angle or wind protected terrain to avoid the areas where wind drifted snow is depositing as a wind slab. Take care not to venture onto slopes with a persistent slabs problem while trying to avoid the wind slab problem.

 Crown of a small human triggered wind slab avalanche yesterday near Frog Lake Cliffs, Carson Pass, CA.

Avalanche Problem 2: Persistent Slab
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The existing snowpack is in poor condition to handle new snow loading. The long dry spell from Nov 18 to Dec 11 caused the existing snowpack to weaken and turn sugary from the faceting process. This weak snow is widespread below treeline and near treeline on W-NW-N-NE-E aspects. It has been noted in isolated areas above treeline as well. The forecast snowfall amounts today and tonight are expected to bring this weak snow either very close or to the point of failure. Persistent slab avalanches will break in the snowpack below the old/new snow interface and could have wide propagation, wrapping around corners and other terrain features that often confine wind slabs and storm slabs. Persistent slab avalanches can be triggered in low angle terrain from a distance to the side, above, or from below (remote triggering).

Staying on wind protected slopes in the trees is not a tactic for avoiding persistent slabs. It is a tactic for avoiding wind slabs. You have to route plan to manage both today. The situation is expected to suddenly become complex and difficult to manage as today's storm progresses. The easy answer is to pull back into low angle, conservative terrain that is less than 30 degrees in slope angle and without steeper slopes adjacent or above.

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* Small wind slab avalanches were reported yesterday from the Carson Pass area. In the Ward Canyon, Donner Summit, and Mt. Rose zones wind slabs were notably absent yesterday and winds were below forecast speeds.

* A freezing rain event occurred up to at least 8,400' at the onset of precipitation the evening of Dec 11. The exact geographic distribution of the rain crust that formed from this event is unknown. This supportable rain crust was observed just below the new snow yesterday in the Ward Canyon and Donner Summit zones. Some evidence of the rain crust was also reported from the Luther Pass/Jobe/Freel zone.

* Weak faceted (sugary) snow in the mid and upper snowpack is widespread below treeline, at treeline, and in some areas above treeline on W-NW-N-NE-E aspects. This weak snow is in poor condition to handle new snow loading. Check out the videos in yesterday's forecast.

* Friday night's rain and snow buried weak faceted snow that was the old snow surface. This weak faceted snow is now named the Dec 11 weak layer. This layer is expected to act as a persistent weak layer with additional new snow load.

Weather and CURRENT CONDITIONS
weather summary

A storm system is expected to arrive late morning to early afternoon today bringing snowfall and gale force SW ridgetop winds. Total snowfall amounts of 8 to 16 inches are expected to accumulate above 6,500' by early tomorrow morning. This is a fast moving storm system with snowfall rates of 1 inch per hour or greater expected. Clearing conditions are forecast for tomorrow with light north winds. The next storm system is on track to arrive Wednesday night into Thursday.

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 34 deg. F.
Max. temperature: 31 to 36 deg. F.
Average ridgetop wind direction: E shifting to SW
Average ridgetop wind speed: E 25 mph | SW 30 mph
Maximum ridgetop wind gust: E 56 mph | SW 44 mph
New snowfall: Trace inches
Total snow depth: 15 to 19 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Sunday Sunday Night Monday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Mostly cloudy. Snow in the evening, then scattered snow showers after midnight. Snow levels below 7000 feet. Chance of precipitation is 100%. Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 5%.
Temperatures: 32 to 37. deg. F. 16 to 22. deg. F. 25 to 30. deg. F.
Mid Slope Winds: Southwest 15 to 30 mph with gusts to 45 mph. Southwest 15 to 25 mph with gusts to 35 mph. North to northeast 5 to 15 mph.
Expected snowfall: 80% probability of 3 to 8 inches. 20% probability of 8 to 12 inches. | SWE = 0.50-0.75 inch. 70% probability of 3 to 6 inches. 30% probability of 6 to 8 inches. | SWE = 0.25-0.50 inch. No accumulation. | SWE = none.
For 8000 ft. to 9000 ft.
Sunday Sunday Night Monday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Mostly cloudy. Snow in the evening, then scattered snow showers after midnight. Snow levels below 7000 feet. Chance of precipitation is 100%. Partly cloudy then becoming sunny. Snow levels below 7000 feet. Chance of precipitation is 5%.
Temperatures: 29 to 34. deg. F. 13 to 18. deg. F. 20 to 26. deg. F.
Ridge Top Winds: Southwest 25 to 40 mph with gusts to 60 mph increasing to 40 to 60 mph with gusts up to 95 mph in the afternoon. Southwest 30 to 50 mph with gusts to 90 mph becoming west 20 to 35 mph with gusts to 70 mph after midnight. North to northeast 10 to 20 mph.
Expected snowfall: 80% probability of 5 to 10 inches. 20% probability of 10 to 14 inches. | SWE = 0.55-1.05 inches. 70% probability of 4 to 8 inches. 30% probability of 8 to 10 inches. | SWE = up to 0.60 inch. 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