THIS AVALANCHE ADVISORY EXPIRED ON January 21, 2018 @ 6:58 am
Avalanche Advisory published on January 20, 2018 @ 6:58 am
Issued by Brandon Schwartz - Tahoe National Forest

MODERATE avalanche danger exists near treeline and below treeline due to isolated areas of a persistent slab avalanche problem. Any isolated human triggered persistent slab avalanches could be large with significant consequences. Above treeline, avalanche danger is LOW.

1. Low

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Above Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.

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

Evidence of a low probability, high consequence persistent slab avalanche problem continues to emerge from snowpit data collected in three locations as recently as yesterday. Isolated areas of a difficult to trigger (stubborn) but problematic snowpack structure exist in isolated locations around the forecast area. The layer of concern for this avalanche problem is loose grain (sugary) faceted snow generally found 1 to 2.5 feet deep in the snowpack. There is significant spatial variability around the forecast area regarding the presence or absence of this avalanche problem. No clear explanation exists for the variations in stability between similar terrain at different locations in regards to this avalanche problem.

Difficult to trigger (low probability) large avalanches size D2 to D3 in treed terrain (high consequence) are a concern near treeline and below treeline on NW-N-NE aspects. Areas with fair to good protection from NE wind scouring are the most suspect. Plan accordingly for and take measures to manage the potential for avalanche triggering to occur in the mid to lower portions of the slope for this particular avalanche problem.

Avoid avalanche terrain in any areas where snowpack collapse with a possible associated whumpfing sound or unstable snowpit test results are occurring. Keep in mind that snowpits dug high on the slope may miss evidence of a problem that exists in the mid to lower portions of the slope.

Avalanche Problem 2: Wind Slab
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    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
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    Very Large
    Large
    Small

Some signs of wind slab instability were reported yesterday along ridgelines near cornice features and wind pillows. Improving stability is expected today, but a lingering unstable wind slab is not impossible near treeline or above treeline in a recently wind loaded area on a NW-N-NE-E-SE aspect.

Look for and identify areas of concern adjacent to or below cornices and wind pillows. Human triggered cracking near the edges of the wind slab are often the easiest signs of instability to identify.

recent observations

* Continued evidence of the persistent slab avalanche problem was reported yesterday from N aspects near treeline in the Upper Blue Lake and Twin Lake areas (South of Carson Pass) as well as from a NE aspect in the headwaters of the Cold Stream Drainage (Donner Summit area).

* New snow amounts from the Jan 18-20 snowfall events ranged from 6 to 14 inches around the forecast area.

* No avalanche activity was reported yesterday.

Weather and CURRENT CONDITIONS
weather summary

Snow showers set up over the Job/Freel, Luther Pass, and Carson Pass areas last night, quickly depositing 5 to 8 inches of new snow. Short lived high pressure will create decreasing cloud cover today and light N winds. Air temperatures are in the teens for most locations this morning, with a few 20s at the lowest elevations and a few single digits at the highest elevations. Winds are forecast to shift back to the SW tomorrow afternoon ahead of the next approaching weather system.

 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 15 to 18 deg. F.
Max. temperature in the last 24 hours: 24 to 29 deg. F.
Average wind direction during the last 24 hours: NW
Average wind speed during the last 24 hours: 8 mph
Maximum wind gust in the last 24 hours: 34 mph
New snowfall in the last 24 hours: 0 to 5 inches
Total snow depth: 24 to 48 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy skies, becoming sunny. Isolated snow showers in the morning. Partly cloudy skies. Mostly cloudy skies with a slight chance of snow showers.
Temperatures: 26 to 31 deg. F. 15 to 20 deg. F. 30 to 35 deg. F.
Wind Direction: N N SW
Wind Speed: Light winds Light winds Light winds increasing to 10 to 15 mph with gusts to 30 mph in the afternoon.
Expected snowfall: 0 to trace in. 0 in. 0 to trace in.
For 8000 ft. to 9000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy skies, becoming sunny. Isolated snow showers in the morning. Partly cloudy skies. Mostly cloudy skies with a slight chance of snow showers.
Temperatures: 21 to 26 deg. F. 12 to 17 deg. F. 26 to 31 deg. F.
Wind Direction: N N SW
Wind Speed: 10 to 15 mph in the morning, becoming light. 10 to 15 mph in the evening, becoming light. Gusts up to 25 mph. 15 to 25 mph. Gusts up to 30 mph increasing to 40 mph in the afternoon.
Expected snowfall: 0 to trace in. 0 in. 0 to trace. 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