THIS AVALANCHE FORECAST EXPIRED ON March 19, 2020 @ 7:00 am
Avalanche Forecast published on March 18, 2020 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest - Sierra Avalanche Center

While the new snow has started to consolidate, people recreating on the snow could still trigger avalanches in wind-loaded near and above treeline terrain where fragile cornices and slabs of wind drifted snow exist. A lingering deep slab avalanche problem also exists in some near and below treeline terrain where old weak snow lurks below the storm snow. MODERATE avalanche danger exists at all elevations. 

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.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    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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Slabs of wind drifted snow and fragile cornices still exist on many leeward facing slopes, and today's new snow and wind today will allow them to grow a little larger. In many areas, these wind slabs have become slightly more difficult to trigger. However, human-triggered wind slab avalanches will remain possible today. Some of these could still break several feet deep and could involve enough snow to bury a person or a car (D2-D3). Wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects in near and above treeline terrain will hold the largest of these wind slabs.

Signs that a wind slab may exist on the slope below included snow blowing onto that slope, smooth pillows of wind drifted snow, and cornices hanging over the slope. Traveling on more sheltered slopes where wind slabs don't exist will provide softer less wind affected snow as well as limiting your exposure to potentially unstable wind slabs. 

Avalanche Problem 2: Deep Slab
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In some areas, weak sugary snow existed just below the snow surface on near and below treeline NW-N-NE aspects prior to this storm. In other areas, the old snow surfaces were so hammered that this layer is discontinuous. Now that 3 to 5 feet of storm snow rests on top of this old weak snow, it is hard to tell where the old weak snow may be problematic. So far the only avalanche activity reported on this layer has occurred outside the forecast area to the south. Still, evidence and observations from within the forecast area suggest that large deep slab avalanches may be possible. They could occur in below treeline areas on slopes without a history of frequent avalanches.

Getting caught in a deep slab avalanche would have severe consequences and may not be survivable. Recent avalanches, whumpfing, or snowpack collapse can indicate potential deep slab instability, but these signs are not always present. Probing into the snowpack or digging a snowpit can also help determine if the weak layer exists. If you find the layer or have any uncertainty about whether or not it exists, choosing a different aspect or more conservative terrain can help avoid this problem. 

recent observations
Submit Observations

* Yesterday small wind-loaded test slopes reacted to snowmobile cuts in Indian Valley and ski kicks on Hidden Peak. The snowmobile cuts produced cracking a few feet deep within the wind slab. The ski kick produced a 6-8 inch deep wind slab failure. 

* Yesterday, snowmobilers in Indian Valley and skiers on Hidden Peak, Rubicon Peak, and Donner Summit all reported signs of settlement and consolidation in the new snow on slopes sheltered from the wind with little to no signs of storm slab instability.

* Snowpit tests targetting the old snow buried 4 to 5 feet below the new snow produced unstable results on Hidden Peak and Rubicon Peak yesterday indicating that if the old weak layer of facets does break, fractures could still propagate through this layer. 

* More avalanche activity that occurred on 3/16 was reported with a skier triggered avalanche in Cold Stream Canyon and some activity near the top of Hidden Peak with an unknown trigger.

* South of the forecast area on Sonora Pass, snowmobilers triggered two deep slab avalanches yesterday. These broke on the old weak snow below the new snow. These large avalanches measured at least 5 ft at the crowns. No one was caught or buried.

Weather and CURRENT CONDITIONS
weather summary

The forecast calls for more snow showers and SW winds today due to the lingering low-pressure system. Some of these snow showers could be heavy at times but they should only add between 1 and 3 inches of new snow accumulation. In some spots, the potential for more like 4 to 6 inches of new does exist. Showery weather should continue through tomorrow but expect less snow and wind during the day tomorrow as the system finally starts to depart the region. 

CONDITIONS ALONG THE SIERRA CREST BETWEEN 8200 FT. AND 9200 FT. IN THE LAST 24 HOURS
Weather Station Map
Weather Station Table
6am temperature: 11 to 19 deg. F.
Max. temperature: 16 to 27 deg. F.
Average ridgetop wind direction: SW
Average ridgetop wind speed: 15 to 25 mph
Maximum ridgetop wind gust: 56 mph
New snowfall: 0 to 1 inches
Total snow depth: 64 to 95 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy. Snow likely. Snow levels below 7000 feet. Chance of precipitation is 60%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 50%. Partly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 25%.
Temperatures: 27 to 32 deg. F. 13 to 18 deg. F. 30 to 35 deg. F.
Mid Slope Winds: Southwest around 15 mph in the morning becoming light. Gusts up to 30 mph. Light winds. Light winds.
Expected snowfall: 60% probability of 1 to 3 inches. 40% probability of 3 to 5 inches. | SWE = up to 0.20 inch. 60% probability of 1 to 2 inches. 40% probability of 2 to 4 inches. | SWE = up to 0.15 inch. up to 1 inch. | SWE = less than 0.10 inch.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Cloudy. Snow likely. Snow levels below 7000 feet. Chance of precipitation is 60%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 50%. Partly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 35%.
Temperatures: 22 to 28 deg. F. 10 to 15 deg. F. 25 to 31 deg. F.
Ridge Top Winds: Southwest 20 to 30 mph. Gusts up to 45 mph decreasing to 30 mph in the afternoon. West around 15 mph with gusts to 35 mph in the evening becoming light. Light winds.
Expected snowfall: 60% probability of 1 to 3 inches. 40% probability of 3 to 6 inches. | SWE = up to 0.25 inch. 50% probability of 1 to 2 inches. 50% probability of 2 to 4 inches. | SWE = up to 0.15 inch. up to 1 inch. | SWE = less than 0.10 inch.
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