THIS AVALANCHE FORECAST EXPIRED ON January 17, 2019 @ 7:00 am
Avalanche Forecast published on January 16, 2019 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

CONSIDERABLE avalanche danger will exist at all elevations today. This afternoon it will increase to HIGH avalanche danger as the next strong winter storm arrives over the region. Expect large widespread avalanche activity during this storm. Dangerous and complex avalanche conditions exist with a mix of wind slabs, storm slabs, and deep persistent slabs.

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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Wind slabs will exist on wind-loaded N-NE-E aspects and on cross-loaded NW and SE aspects. The largest wind slabs should exist in near and above treeline terrain but some could also lurk in exposed below treeline areas. Human-triggered wind slabs will be likely today and natural wind slabs are possible. Wind slab avalanches could be large and destructive. In areas where wind slabs may rest on buried surface hoar, remote triggering is possible and avalanches could propagate wider than normal or even across connected slopes. As snow accumulation and winds increase this afternoon/evening, wind slabs will quickly grow in size and become more widespread. Widespread natural wind slab avalanches will become likely this afternoon and during the night.

Blowing snow, cornices above a slope, drifted snow, and other wind-created textures can help identify where wind slabs may exist. Giving these areas a wide berth would be a prudent plan for today. Sheltered low angle slopes without overhead hazard could provide better and safer recreation opportunities.

Avalanche Problem 2: Storm Slab
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Storm slab avalanches large enough to bury a person are likely today. Some may involve even more snow. Changing conditions during the storm will have created weaknesses within the storm snow including some heavier snow on top of lighter snow. In addition to this upside-down layering, the new snow rests on top of a layer of weak feathery snow (surface hoar) in many areas. Storm slab problems could exist on any slopes steep enough to avalanche. Storm slabs could fracture wider than expected or be remotely triggered. They will become more widespread and larger with the arrival of the next storm. Natural storm slabs will become likely this afternoon and tonight.

Cracking and collapsing can indicate that storm snow weaknesses or buried surface hoar exist. Today's complex and dangerous avalanche conditions warrant conservative decisions and route choices. Sticking to lower angle terrain without overhead hazard could be one way to create appropriate safety margins. 

Avalanche Problem 3: Deep Slab
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The new snow has added a significant new load to the snowpack making deep persistent slab avalanches possible. The persistent weak layer now exists more than 3 to 5 ft below the snow surface. Smaller triggers in the right places like a person on a trigger point or larger triggers like cornice collapses or other avalanches could cause a deep slab to release. If a deep slab avalanche does occur, it would likely be large, destructive, and unsurvivable.  The additional snow load expected in the next storm may make deep slab avalanches more likely.    

The consequences of a large destructive deep slab avalanche are high. Unfortunately, these are hard to predict and the first sign that a deep slab avalanche may occur could be a deep slab avalanche. Avoiding steep terrain where this weak layer may exist represents one of the only ways to effectively deal with deep slab problems.   

recent observations

* Yesterday observations from Mt. Rose, Jakes Peak, Incline Lake Peak, and Ward Canyon all found widespread buried surface hoar. Ski cuts on steep slopes triggered loose dry sluffs that ran on the buried surface hoar. 

* In a more exposed area near the ridge of Mt. Rose, small wind slabs had started to form. Some of these test slopes failed easily with ski kicks and ski cuts. These wind slabs may have been resting on top of buried surface hoar as well. 

* Snowpit data from Jake's Peak found the old persistent weak layers 2 ft below the surface.

Weather and CURRENT CONDITIONS
weather summary

Snowfall amounts since yesterday have exceeded expectations especially in areas along the Sierra Crest north of Emerald Bay where sensors indicate up to 2 ft of new snow has fallen in the last 24 hours. South of Emerald Bay and east of Lake Tahoe sensors are showing 10 to 16 inches of new snow in that time frame. Snowfall rates decreased after midnight. Temperatures climbed steadily through the day yesterday before leveling off during the night. Southwest winds increased dramatically yesterday evening with gale force winds and gusts over 100 mph during the night. The forecast calls for a short lull in the weather today with light snow showers and continued strong winds before a strong winter storm arrives over the region this afternoon. Expect blizzard conditions through tomorrow with gale force winds in the mountains and intense snow with snowfall rates exceeding 2 inches per hour. The forecast calls for an additional 2.5 to 5 ft of new snow above 7000 ft. by tomorrow afternoon. Check in with the Reno NWS for the latest weather updates.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 25 to 30 deg. F.
Max. temperature in the last 24 hours: 26 to 30 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 10 to 15 mph until 8pm. After 8 pm 60 to 70 mph
Maximum wind gust in the last 24 hours: 111 mph
New snowfall in the last 24 hours: north of Emerald Bay: 16 to 24 inches | South of Emerald Bay: 10 to 16 inches
Total snow depth: 57 to 70 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. Widespread snow showers in the morning then scattered snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels below 7000 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels below 7000 feet. Chance of precipitation is 100%.
Temperatures: 30 to 35 deg. F. 25 to 30 deg. F. 27 to 32 deg. F.
Mid Slope Winds: Southwest 15 to 30 mph. Gusts up to 45 mph. South 20 to 35 mph with gusts to 65 mph. Southwest 20 to 30 mph with gusts to 60 mph.
Expected snowfall: 60% probability of 1 to 3 inches. 40% probability of 3 to 6 inches. | SWE = up to 0.30 inch. 80% probability of 12 to 18 inches. 20% probability of 18 to 24 inches. | SWE = 1.15-1.65 inches. 70% probability of 18 to 28 inches. 30% probability of 10 to 18 inches. | SWE = 1.10-2.10 inches.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Widespread snow showers in the morning then scattered snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels 7000 feet. Chance of precipitation is 100%. Cloudy. Snow. Snow levels below 7000 feet. Chance of precipitation is 100%.
Temperatures: 26 to 30 deg. F. 20 to 25 deg. F. 20 to 27 deg. F.
Ridge Top Winds: Southwest 40 to 65 mph. Gusts up to 100 mph. South 50 to 75 mph with gusts to 130 mph. Southwest 40 to 60 mph with gusts to 115 mph.
Expected snowfall: 60% probability of 2 to 4 inches. 40% probability of 4 to 8 inches. | SWE = up to 0.40 inch. 80% probability of 16 to 24 inches. 20% probability of 24 to 30 inches. | SWE = 1.10-2.10 inches. 70% probability of 24 to 32 inches. 30% probability of 15 to 24 inches. | SWE = 1.35-2.35 inches.
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