THIS AVALANCHE FORECAST EXPIRED ON March 17, 2018 @ 6:57 am
Avalanche Forecast published on March 16, 2018 @ 6:57 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

HIGH avalanche danger exists for all elevations today due to widespread wind slab and storm slab avalanche problems and a possibility of isolated deep slab problems. Any avalanches that occur today could be very large and destructive and have very serious consequences. Travel in or near avalanche terrain or in lower angle terrain below avalanche terrain is 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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Strong winds and significant new snow accumulation over the last 24 to 72 hours have created large unstable wind slabs on near and above treeline slopes exposed to wind-loading. These wind slabs will continue to grow in size and become more unstable as more snow and wind impact the forecast area today. Large natural and human-triggered wind slab avalanches are very likely to certain today. Expect wind slabs on wind-loaded N-NE-E and cross-loaded NW and SE aspects. Some wind slabs may even form in more traditionally sheltered spots or extend into below treeline areas. Wind slab avalanches could entrain enough snow to bury cars (D3) or even trains (D4) or destroy a swath of forest.  

Look for signs of recent wind slab avalanches, blowing snow, cornices, cornice formation, and wind pillows.  Avoid steep wind-loaded terrain and run out zones below corniced or wind-loaded slopes.

Avalanche Problem 2: Storm Slab
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Rapid snow accumulation will overload weaknesses that have formed in the storm snow. Human-triggered storm slab avalanches are very likely today and natural storm slab avalanches are likely. Due to the amount of new snow storm slab avalanches could certainly involve enough snow to bury or injure a person (D2) and some of these storm slab avalanches could be large enough to bury a car (D3).

Recent avalanche activity and shooting cracks, as well as hand pits to feel for weak layers and slab layers in the new snow, can help identify where storm slabs may exist. This avalanche problem could exist in areas traditionally thought of as safe like tree-covered slopes. Avoid slopes with signs of unstable snow.

Avalanche Problem 3: Deep Slab
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Most observations and data targetting the persistent weak layer responsible for the previous deep slab avalanche cycle have shown significant increases in that layer's strength and in the strength of the snow above that layer. However, a few pieces of data (some of which were collected just yesterday) still indicate that with enough new loading or a large enough trigger (like the other avalanches mentioned above) this layer could possibly reactivate. A significant amount of uncertainty exists as to whether or not this storm could provide enough loading to reactivate this layer or if the old persistent weak layer has gained enough strength to support the new load. In many areas, this layer can support the new load, but it is possible that in some places it cannot. If it does break the resulting deep slab avalanches would be huge, unsurvivable, and could run very long distances (size D4 to D5).

Due to this combination of uncertainty, variability in the data, and the catastrophic consequences of a deep slab, avoiding slopes where even a remote possibility of a deep slab may exist is the best choice for today.  

recent observations
Submit Observations

* New fragile wind slabs had formed on wind loaded slopes on Jakes Peak (near Emerald Bay), Elephant's Back, and Elephant's Hump (Carson Pass area) by yesterday afternoon. Numerous natural wind slab avalanche had occurred on Elephant's Back sometime since March 14. Ski cuts easily triggered wind slabs up to 1 ft. deep on Jakes Peak yesterday afternoon. Skier and snowboarder triggered shooting cracks also occurred on wind-loaded lower angle slopes in both of these areas. 

* A skier-triggered storm slab avalanche 12 to 16 inches deep was reported on Powderhouse Peak yesterday around midday. 

* While most data and evidence has indicated that the old persistent weak layer buried deep below the surface has gained strength, snowpit tests and data in the Elephant's Hump area and in a backcountry area near Squaw Creek yesterday indicated that this layer could reactivate with enough new loading. 

* The new snow sits on top of a rain crust and refreezing wet snow in many areas. 

Weather and CURRENT CONDITIONS
weather summary

12 to 24 inches (with 1.6 to 2.5 inches of snow water equivalent) of new snow has fallen across the forecast area in the last 24 hours. Add this to the previous new snow and new snow totals this week are pushing 3 to 4 ft in many areas. Intense snowfall with accumulation rates at or higher than 2 in / hr at times should continue through today. Strong southwest winds will also continue through today. By this evening the storm should start to move out of the area with snowfall and winds decreasing during the night. While the main storm should have moved out of the area by tomorrow some snow showers could still continue across the region during the day 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 24 deg. F.
Max. temperature: 22 to 25 deg. F.
Average ridgetop wind direction: Southwest
Average ridgetop wind speed: 50 mph
Maximum ridgetop wind gust: 115 mph
New snowfall: 12 to 24 inches
Total snow depth: 70 to 102 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Snow with a slight chance of thunderstorms in the afternoon Snow with a slight chance of thunderstorms in the evening. Mostly cloudy with snow showers likely
Temperatures: 23 to 28 deg. F. 12 to 17 deg. F. 26 to 31 deg. F.
Mid Slope Winds: Southwest Southwest Southwest
Wind Speed: 15 to 25 mph with gusts to 50 mph 10 to 15 mph with gusts to 30 mph becoming light overnight 10 to 15 mph with gusts to 35 in the morning becoming light in the afternoon
Expected snowfall: 5 to 12 2 to 6 1 to 3
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Snow with a slight chance of thunderstorms in the afternoon Snow with a slight chance of thunderstorms in the evening. Mostly cloudy with snow showers likely
Temperatures: 19 to 24 deg. F. 10 to 16 deg. F. 21 to 27 deg. F.
Ridge Top Winds: Southwest Southwest Southwest
Wind Speed: 25 to 40 mph with gusts to 85 mph decreasing to 20 to 30 mph with gusts to 75 mph in the afternoon 15 to 25 mph with gusts to 55 mph decreasing to 45 mph after midnight 20 to 30 mph with gusts to 50 mph in the morning becoming light in the afternoon
Expected snowfall: 8 to 15 2 to 7 1 to 4
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