THIS AVALANCHE ADVISORY EXPIRED ON January 3, 2018 @ 6:52 am
Avalanche Advisory published on January 2, 2018 @ 6:52 am
Issued by Andy Anderson - Tahoe National Forest

While avalanche activity is unlikely in most areas, the snowpack continues to weaken and human-triggered persistent slab avalanches remain possible in isolated areas where a slab layer exists above loose weak sugary snow (facets). As a result, the avalanche danger remains at MODERATE at all elevations. Identify and avoid areas where a slab layer exists on top of the persistent weak layer of faceted snow

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
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The prolonged dry period combined with a shallow snowpack have caused the existing snowpack to lose strength. A layer of loose weak sugary snow (facets) now exists on northerly aspects across the forecast area. In most areas this persistent weak layer only represents a future concern because no slab layer sits on top of it. However, slab layers do exist on some isolated NW-N-NE aspects in the form of hard wind slabs in near and above treeline terrain and some softer slabs in isolated below treeline terrain. Observations over the last week have indicated that in those isolated areas where the combination of persistent weak layer with a slab on top of it exists, the weak layer has weakened to the point that it can barely support the snow above and the added weight of a person can break that persistent weak layer.  

Look for cracking as a person weights the snowpack, whumpfing sounds, a hollow sounding snowpack, feeling a softer weaker layer of snow below the surface with a probe or pole or hand pit, unstable snowpit tests, and any signs that a slab may exist above weak faceted snow. Clues to instability may be difficult to find unless just the right conditions are present and an isolated human-triggered avalanche may represent the first clue to an unstable slope. Use safe travel protocols when traveling in and around avalanche terrain.  A shallow snowpack with numerous barely covered and still exposed obstacles would magnify the consequences of any size avalanche.

recent observations

* Observations in the Carson Pass, Desolation Wilderness, West Shore Tahoe,  Deep Creek, Pole Creek, and Donner Summit areas during the last week continue to reveal signs of persistent slab instability including snow surface cracking, whumpfing, and/or unstable snowpit test results. This instability exists on NW-N-NE aspects in some isolated areas where loose sugary snow (facets) exists below either a hard or soft slab.

* Loose weak sugary snow (near surface facets) without an overlying slab is fairly widespread across the forecast area on northerly aspects in wind protected, below treeline areas.

* Snow surface conditions range from wind scoured snow and firm ice in near and above treeline terrain to lingering areas of soft unconsolidated snow on sheltered northerly aspects below treeline.

* Northerly aspects above 8000 ft in the Mt. Rose area and along the Sierra Crest north of Emerald Bay hold the best snow coverage at 2 to 3+ feet. Overall less snow cover exists south of Emerald Bay, but decent coverage exists above 8,500 ft. on northerly aspects in the Carson Pass area.  Coverage becomes patchy on other aspects with large areas of bare ground on many southerly aspects at all elevations.

Weather and CURRENT CONDITIONS
weather summary

The forecast calls for temperatures to remain above average with daytime highs in the upper 40's and low 50's above 7000 ft. today and tomorrow. Winds should remain light today and increase some tomorrow ahead of a weak storm system. Some cloud cover should begin to move into the area today. These clouds should thicken tomorrow and bring a slight chance of some light precipitation by tomorrow afternoon as the small low-pressure system arrives over the region. The forecast calls for freezing levels to start around 8500 ft. tomorrow afternoon and stay above 7500 ft. due to warm air accompanying this system.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 37 to 42 deg. F.
Max. temperature in the last 24 hours: 46 to 49 deg. F.
Average wind direction during the last 24 hours: SW to E
Average wind speed during the last 24 hours: 5 to 15 mph
Maximum wind gust in the last 24 hours: 33 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 17 to 32 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy Mostly cloudy Mostly cloudy with a slight chance of rain showers in the afternoon. Snow level 8500 ft.
Temperatures: 47 to 52 deg. F. 31 to 37 deg. F. 49 to 54 deg. F.
Wind Direction: Variable Variable South
Wind Speed: Light Light Light in the morning increasing to 10 mph in the afternoon
Expected snowfall: 0 in. 0 in. 0 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy Mostly cloudy Mostly cloudy with a slight chance of showers in the afternoon. Snow level 8500 ft.
Temperatures: 44 to 50 deg. F. 31 to 37 deg. F. 45 to 51 deg. F.
Wind Direction: East to southeast South to southeast South
Wind Speed: 10 mph 10 mph increasing to 10 to 15 mph with gusts to 25 mph after midnight 10 to 20 mph with gusts to 30 mph in the morning increasing to 40 mph in the afternoon
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