THIS AVALANCHE ADVISORY EXPIRED ON December 29, 2016 @ 6:58 am
Avalanche Advisory published on December 28, 2016 @ 6:58 am
Issued by Brandon Schwartz - Tahoe National Forest

Areas of MODERATE avalanche danger continue to exist at all elevations on slopes 32 degrees and steeper due to a combination of wind slabs and persistent slabs. A human triggered wind slab avalanche occurred yesterday with signs of persistent instability present in other areas. Human triggered avalanches are possible again today in a variety of areas. Exercise caution and conservative decision making during backcountry travel.

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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Over the past 36 hours, SW to W winds have transported recent storm snow on the ground to leeward areas and created new wind slabs in some areas. The amount of wind slab formation observed around the forecast area yesterday was highly variable. Conditions ranged across the spectrum from active blowing snow with human triggered avalanche activity to some signs of recent transport to minimal wind effects.

Recently formed wind slabs exist near treeline and above treeline on NW-N-NE-E-SE aspects and potentially some S aspects. Human triggered wind slabs remain a possibility again today. While these wind slabs will likely be more difficult to trigger today than yesterday, continued caution and conservative decision making is warranted. Identify areas of concern and make a robust plan for avalanche avoidance.

Avalanche Problem 2: Persistent Slab
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Persistent slabs remain an ongoing avalanche problem around the forecast area. In many but not all areas, faceted snow sitting above the Dec 15 rain crust and below recent storm snow continues as a problematic weak layer. This is found mainly in near treeline and below treeline areas on W-NW-N-NE-E aspects. This problem has also been observed in isolated areas above treeline. Locations were avalanches have occurred on this weak layer are frequently low on the slope and away from the traditional ridgetop wind slab start zones. The icy Dec 15 rain crust is generally easily identifiable at the bed surface.

Not separating the difference between a more typical wind slab avalanche problem from a persistent slab avalanche problem and focusing travel decisions heavily on the danger rating is misguided thinking. The approach to backcountry travel when managing a persistent slab problem should be more conservative, accounting for triggering low on the slope and the possibility of wide propagation into connected terrain. Adjust travel tactics accordingly.

Avalanche Problem 3: Loose Wet
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With continued warming, minor loose wet instability is possible. Most instability is expected in the form of roller balls. Isolated loose wet avalanche activity could occur in areas of direct sun exposure on slopes 37 degrees and steeper.

 

recent observations

An intentionally human triggered wind slab avalanche occurred yesterday on Red Lake Peak (Carson Pass area). The party was surprised by the larger than expected avalanche size. The avalanche was reported to have occurred around 9,200', in near treeline terrain, on a N aspect, 35 degree slope, with a crown height of 2 feet and a width of around 50 feet (more info, photos). A report was received yesterday from National Geographic Bowl (Backcountry terrain on Granite Chief, outside the boundary of Squaw Valley) indicating newly formed variable and stiff wind slabs 4 to 6 inches thick.

Southwest of Carson Pass in the Tragedy Spring to Squaw Ridge area some minor recent wind effects were reported. The main issue was numerous snowpit tests on N-NE-E aspects pointing to persistent slab problems associated with faceted snow sandwiched between the Dec 15 rain crust and the recent storm snow on the surface. On Ralston Peak snowpit data was collected near the site of some of the Dec 26 human triggered avalanche activity that occurred on N aspects. This snowpit data collected on a E aspect revealed a snowpack structure consistent with areas where the persistent slab avalanche problem in known to exist. Snowpit data received from near Tinkers Knob in the upper portion of the Cold Stream Drainage (Cabin Creek area) showed some indications of possible persistent slab problems.

Other observations received yesterday from Chickadee Ridge along the S side of Tahoe Meadows (Mount Rose area) showed minimal signs of instability.

Weather and CURRENT CONDITIONS
weather summary

High pressure is building over the forecast area and is expected to last into Friday. Ridgetop winds remain moderate in speed out of the SW this morning. Ridgetop winds are forecast to shift slightly to the W today and gradually decrease in speed through this afternoon. A warming trend will continue to occur today with maximum daytime air temperatures above 7,000' reaching the upper 30s to mid 40s. Similar air temperatures and light winds are forecast for tomorrow with continued sunny skies.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 26 to 38 deg. F.
Max. temperature in the last 24 hours: 34 to 41 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 21 mph
Maximum wind gust in the last 24 hours: 46 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: Along the Sierra Crest: 26 to 40 inches | In the Mt. Rose area: 58 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny skies. Clear skies. Sunny skies.
Temperatures: 40 to 45 deg. F. 19 to 25 deg. F. 40 to 45 deg. F.
Wind Direction: W Variable Variable
Wind Speed: Light winds Light winds Light winds
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Wednesday Wednesday Night Thursday
Weather: Sunny skies. Clear skies. Sunny skies.
Temperatures: 38 to 44 deg. F. 20 to 25 deg. F. 38 to 44 deg. F.
Wind Direction: W W Variable
Wind Speed: 20 to 30 mph with gusts to 60 mph, decreasing to 10 to 15 mph with gusts to 30 mph in the afternoon. 10 to 15 mph with gusts to 30 mph in the evening, becoming light. Light winds
Expected snowfall: 0 in. 0 in. 0 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