THIS AVALANCHE ADVISORY EXPIRED ON December 15, 2015 @ 6:56 am
Avalanche Advisory published on December 14, 2015 @ 6:56 am
Issued by Brandon Schwartz - Tahoe National Forest

MODERATE avalanche danger exists on slopes 32 degrees and steeper above treeline on all aspects, near treeline on NW-N-NE aspects, and below treeline NW-N-NE aspects. For other areas LOW avalanche danger exists. Today's avalanche problems are a combination of persistent slabs and wind slabs. Natural avalanches are unlikely today. Human triggered avalanches are possible and to a significant degree expected today near treeline and below treeline on NW-N-NE aspects where areas of buried surface hoar have not yet collapsed.

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: Persistent Slab
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Persistent slab avalanche problems remain an issue today following yesterday's rapid new snow loading and subsequent avalanche activity. Areas of buried surface hoar linger near treeline and below treeline on NW-N-NE aspects, most likely above 7,000' to 7,500'. This is an uncommon weak layer for this region and it certainly does not abide by any mythical "24 hour rule". The potential for human triggered avalanches involving all of the new storm snow with the potential for remote triggering and wide propagation, wrapping around corners, etc requires a more conservative approach to both uphill and downhill travel. Question traditional route placement and give large steep open areas below treeline a wide birth until the presence or absence of the weak layer is verified. Ongoing easy shears in hand pits at the old/new snow interface are a potential indication of buried surface hoar that remains intact as a weak layer. Surface hoar crystals may be visible on the lower side of the snowpack failure interface.

Developing near crust facets around the Dec 10 rain crust also pose a potential persistent slab avalanche problem. These facets have been noted as most well developed in the Carson Pass and Ebbetts Pass/Bear Valley areas on NW-N-NE aspects above about 8,000'. In other areas this weak layer has not been seen.

Avalanche Problem 2: Wind Slab
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Last night's wind shift from SW to NE will redistribute snow in above treeline areas. New wind slabs have formed on SE-S-SW-W-NW aspects. Wind scouring will occur on N-NE-E aspects, but pockets of wind slab will remain on N-NE-E aspects in complex terrain. Look for areas of blowing snow, recent cornice formation, and wind pillows below ridgelines. These are all indications of potential wind slabs on the slopes below.

 

recent observations

Observations made yesterday in the Deep Creek area revealed natural and skier triggered avalanche activity occurring on buried surface hoar in open areas near treeline and below treeline on N-NE aspects. In any areas where buried surface hoar had not yet collapsed, it was readily identified at the failure interface in hand pits and was reactive in snowpit tests. On Becker Ridge (Echo Summit area) better stability was observed with an absence of buried surface hoar. Very difficult to trigger but still potentially problematic rounded and moist facet remnants at the base of the snowpack were noted to produce sudden collapse failure characteristics in snowpit tests on northerly aspects in this area. On Castle Peak (Donner Summit area) a party reported sensitive wind slabs near treeline on N aspects with unstable snowpit tests leading to test slope failure.

Weather and CURRENT CONDITIONS
weather summary

The storm system that deposited 10 to 17 inches of new snow across the forecast area is exiting the region. High intensity snowfall tapered to snow showers in most areas by 10 pm last night. Ridgetop winds shifted from SW to NE behind the departing low pressure trough. Ridgetop winds moderate to strong in speed are forecast for today and tomorrow. Colder northerly flow will keep maximum daytime air temperatures above 7,000' in the low teens to low 20s today. Single digit overnight lows will be followed by highs in the 20s tomorrow. Lingering cloud cover today will allow for isolated snow showers with no significant additional accumulation expected. Cloud cover is expected to thin overnight and give way to sunny skies tomorrow.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 11 to 16 deg. F.
Max. temperature in the last 24 hours: 26 to 30 deg. F.
Average wind direction during the last 24 hours: SW shifting to NE
Average wind speed during the last 24 hours: SW 40 mph | NE 21 mph
Maximum wind gust in the last 24 hours: SW 117 mph | NE 43 mph
New snowfall in the last 24 hours: 10 to 17 inches
Total snow depth: 34 to 39 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy skies with isloated snow showers. Mostly cloudy skies becoming partly cloudy. Isolated snow showers in the evening. Sunny skies.
Temperatures: 17 to 24 deg. F. 2 to 9 deg. F. 22 to 27 deg. F.
Wind Direction: N to NE N to NE NE
Wind Speed: 15 to 25 mph with gusts to 35 mph. 15 to 25 mph with gusts to 40 mph. 15 to 20 mph with gusts to 35 mph.
Expected snowfall: 0 to trace in. 0 to trace in. 0 in.
For 8000 ft. to 9000 ft.
Monday Monday Night Tuesday
Weather: Mostly cloudy skies with isloated snow showers. Mostly cloudy skies becoming partly cloudy. Isolated snow showers in the evening. Sunny skies.
Temperatures: 12 to 18 deg. F. 4 to 10 deg. F. 22 to 27 deg. F.
Wind Direction: N to NE N to NE NE
Wind Speed: 25 to 35 mph with gusts to 50 mph. 25 to 35 mph with gusts to 55 mph. 20 to 30 mph with gusts to 45 mph.
Expected snowfall: 0 to trace in. 0 to trace 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.