THIS AVALANCHE ADVISORY EXPIRED ON February 4, 2017 @ 6:50 am
Avalanche Advisory published on February 3, 2017 @ 6:50 am
Issued by Steve Reynaud - Tahoe National Forest

Avalanche danger will increase to considerable today due to wind slab, storm slab, and loose wet avalanche problems.  These avalanche problems will be likely in below treeline, near treeline, and above treeline terrain.  Dangerous avalanche conditions will exist throughout today.  Natural avalanches will be possible and human triggered avalanches will be likely.  Conservative decision making and cautious route finding are essential.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Below Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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Wind slabs will be likely on NW-N-NE-E-SE aspects near and above treeline as new snow and gale force SW winds impact our area today.  These wind slabs will continue to grow throughout the day and will be large enough in size to injure or bury a backcountry user.

Look for clues to where wind slabs are forming such as blowing snow, cornice formation, and wind pillows.  Avoid terrain with new wind slab development.

Avalanche Problem 2: Storm Slab
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As snowfall intensity increases this morning, storm slabs will become likely on all aspects in below treeline and near treeline terrain.  Snow levels are generally forecasted to drop throughout the day but an upside down storm snow structure could exist in many areas.  In isolated open below treeline terrain, there remains the possibility that surface hoar has been buried under the recent storm snow.  If this occurs, easily triggered avalanches would be likely on lower angle slopes.  

Look for clues such as snow surface cracking and avoid areas where unstable snow exists.   

Avalanche Problem 3: Loose Wet
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Areas of rain on snow today will be prone to loose wet avalanche activity. This is most likely to occur below 8000' on slopes steeper than 35 degrees and should diminish as snow levels are forecasted to drop to near 6000' later this morning.  Loose wet avalanche activity today could be either natural or human triggered. Terrain traps such as creek beds and road cuts could increase the depth of avalanche debris associated with otherwise small loose wet avalanches.

recent observations

Observations were made yesterday on Andesite Peak (Donner Summit area).  Small storm slabs and winds slabs were forming throughout the morning hours in this area and were able to be intentionally triggered.  3 to 5'' of new storm snow existed in this area.  Temperatures and snow levels were rising throughout the morning creating an upside down storm snow structure with heavier storm snow on top of lighter snow.  Loose wet avalanches were observed on the drive to the trailhead at elevations below 7200'.  Buried surface hoar was not found in many open below treeline locations in this area. 

Weather and CURRENT CONDITIONS
weather summary

A winter storm warning is in effect until 4am Saturday.  Overnight and into the morning hours, snow levels were in the 7500-8500' range.  They should drop this morning with the next band of moisture coming down to around 6000'.  The higher elevation Sierra could pick up 1 to 2 feet of snow by early Saturday morning.  On Saturday, most moisture will stay along the Sierra crest north of Hwy. 50 with on and off showers.  After a break on Sunday, another series of storms will move into the area late Sunday night through at least Wednesday.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 29 to 33 deg. F.
Max. temperature in the last 24 hours: 35 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 45 to 55 mph
Maximum wind gust in the last 24 hours: 93 mph
New snowfall in the last 24 hours: 3 to 4 inches
Total snow depth: 102 to 125 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Rain in the morning, snow through the day. Cloudy. Snow. Mostly cloudy. Chance of snow.
Temperatures: 31 to 36 deg. F. 23 to 28 deg. F. 34 to 39 deg. F.
Wind Direction: SW SW SW
Wind Speed: 20 to 30mph. Gust to 75mph. 20 to 30mph. Gusts to 75mph decreasing to 65mph after midnight. 20 to 30mph with gusts to 55mph.
Expected snowfall: 4 to 10 in. 4 to 8 in. Up to 1 in.
For 8000 ft. to 9000 ft.
Friday Friday Night Saturday
Weather: Cloudy. Snow. Cloudy. Snow in the evening then snow likely after midnight. Mostly cloudy. Chance of snow showers.
Temperatures: 28 to 33 deg. F. 22 to 27 deg. F. 29 to 35 deg. F.
Wind Direction: SW SW SW
Wind Speed: 35 to 45mph with gusts to 100mph. 30 to 50mph with gusts to 100mph. 25 to 40mph with gusts to 80mph.
Expected snowfall: 4 to 10 in. 6 to 8 in. Up to 1 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