Avalanche Forecast published on December 9, 2018 @ 6:57 am
This Avalanche Forecast expires in 14 hours, 5 minutes
This Forecast is valid for 24 hours
Issued by Andy Anderson - Tahoe National Forest

Increased winds during the night and continued windy conditions today may mean that growing wind slabs could exist on top of a buried weak layer. The avalanche danger is MODERATE in near and above treeline terrain. Human triggered avalanches may be possible today.

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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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
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As the winds increased last night, they may have grown strong enough to move soft snow remaining on windward aspects across ridges to leeward aspects. This wind-loading could have started to form some new wind slabs on NW-N-NE-E-SE aspects in near and above treeline terrain. Since the wind speeds have hovered near the threshold for transporting snow, significant uncertainty exists concerning the size and extent of these new wind slabs. If wind slabs do exist, they could rest on top of weak layers in the form of buried surface hoar making them easier for people to trigger. Wind transport could continue today allowing these wind slabs to become larger and more widespread. Avalanches resulting from these growing wind slabs could involve enough snow to bury or injure a person (size D2) in some areas. 

Signs that wind slabs may exist on a slope include cornices above a slope, wind drifts/pillows, ripples and other wind-created features on the snow surface, and current blowing snow. While many of these wind slabs may remain small, the fact that they may rest on a layer of buried surface hoar would make them more difficult to manage. Surface hoar can allow remote triggering of avalanches and can allow avalanches to propagate across wider areas and run farther downslope. If doubts or uncertainty exist about whether or not an unstable wind slab has formed on a slope, assume the slope is unstable and avoid it. Avoiding wind-loaded terrain where problematic wind slabs may exist represents a great way to manage today's potential avalanche problem and to enjoy the cold soft snow that still exists in many more-sheltered, non-wind-exposed areas.

recent observations

* Observers have noted the surface hoar that was buried on 12/4 across the forecast area. Yesterday, it was found on Elephant's Hump (Carson Pass), on Castle Peak (Donner Summit), on Mt. Judah (Donner Summit), and in Johnson Canyon (Donner Summit). Widespread surface hoar also existed on the top of the snowpack in these areas.

* No evidence of wind slab formation was found by observers yesterday

* Decent snow coverage exists above 7,000' to 7,500' along the Sierra Crest with the best coverage in the northern and far southern Sierra Crest portions of the forecast area.  Snow coverage declines to the east. Early season conditions exist.

Weather and CURRENT CONDITIONS
weather summary

Southwest winds increased to 20 to 30 mph along ridgetops with gusts into the 40's during the night. A small storm system approaching the region should allow the increased winds to continue today and bring some cloud cover to the area. Expect the southwest winds to strengthen more tonight with gusts into the 60's as the system arrives. This system will also bring a chance of light snow starting late tonight and continuing into tomorrow. The forecast calls for up to an inch during the day tomorrow.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 35 deg. F.
Max. temperature in the last 24 hours: 39 to 45 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 10 to 20 mph
Maximum wind gust in the last 24 hours: 41 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 27 to 35 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Today Tonight Monday
Weather: 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 5%. Mostly cloudy. Chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 45%.
Temperatures: 37 to 42 deg. F. 20 to 26 deg. F. 32 to 37 deg. F.
Mid Slope Winds: Light winds becoming southwest 10 to 15 mph with gusts to 25 mph in the afternoon. Southwest 10 to 15 mph with gusts to 30 mph. Southwest 10 to 15 mph with gusts to 25 mph in the morning becoming light.
Expected snowfall: No accumulation. | SWE = none. in. No accumulation. | SWE = none. in. 80% probability up to 1 inch. 20% probability of 1 to 2 inches. | SWE = less than 0.10 inch. in.
For 8000 ft. to 9000 ft.
Today Tonight Monday
Weather: Partly cloudy then becoming mostly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Mostly cloudy then becoming partly cloudy. Snow levels below 7000 feet. Chance of precipitation is 5%. Mostly cloudy. Chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 40%.
Temperatures: 35 to 40 deg. F. 18 to 23 deg. F. 29 to 34 deg. F.
Ridge Top Winds: Southwest 15 to 25 mph. Gusts up to 40 mph increasing to 50 mph in the afternoon. Southwest 15 to 30 mph with gusts to 65 mph. Southwest 15 to 30 mph with gusts to 60 mph.
Expected snowfall: No accumulation. | SWE = none. in. No accumulation. | SWE = none. in. 80% probability up to 1 inch. 20% probability of 1 to 3 inches. | SWE = less than 0.10 inch. 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

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