THIS AVALANCHE ADVISORY EXPIRED ON February 19, 2016 @ 7:00 am
Avalanche Advisory published on February 18, 2016 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

UPDATED at 8:30 am.

CONSIDERABLE avalanche danger exists on NW-N-NE-E-SE aspects steeper than 35 degrees at all elevations with MODERATE danger elsewhere due wind slabs and storm slabs. Large human triggered avalanches that run long distances and involve large amounts of snow are likely today and natural avalanches may remain possible this morning. Careful snowpack evaluation, cautious route-finding and conservative decision-making are essential. Identify where wind slabs and storm slabs exist and avoid them and their run out zones.

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

UPDATE @8:30 am -- We have received reports of large natural avalanches that are ran long distances and hit the Alpine Meadows Road around 7:00-7:30 am. Other natural avalanches also occurred last night and early this morning. No details about what kind of avalanches these were or their failure layers are available. As mentioned below some similar natural avalanche activity may remain possible through this morning.   

Strong winds and significant amounts of new snow at all elevations since 10 pm last night mean that wind slabs have formed on open slopes at all elevations. The largest and most fragile wind slabs will exist on exposed wind loaded near and above treeline N-NE-E aspects and cross loaded NW and SE aspects, but wind slabs could also exist in more sheltered locations and in below treeline terrain due to the strength of the winds during this storm. These wind slabs will continue to grow in size and extent today as the strong winds continue to move a combination of snow that fell last night and the new snow expected today. Even though the best window for natural wind slab avalanches likely occurred last night and early this morning, some natural wind slab avalanches may remain possible especially before noon today. Human triggered wind slab avalanches large enough to bury and injure people will be likely today on wind loaded slopes 35 degrees and steeper. These wind slabs will most likely fail on weaknesses within the new snow, but some could break on the old snow surfaces.

Clues like cornices above a slope, blowing snow, wind drifted snow, ripples in the snow surface, and other wind created textures can help identify where wind slabs may exist. Using observations and clues like these to find where the wind slabs lurk and then avoiding those wind slabs represents a prudent choice for backcountry travel today.

Avalanche Problem 2: Storm Slab
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  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Even though temperatures have gotten progressively colder since the snowfall started, some subtle temperature changes and rapid snow accumulation could have created some weaknesses within the storm snow like graupel layers or small density changes. In some cases failures on these weaknesses will manifest as small loose dry point release avalanches. However, these storm snow weakness will also allow human triggered storm slabs to become possible on open slopes steeper than 35 degrees in near and below treeline terrain where the new snow has more slab characteristics.

recent observations

Yesterday observations from Powderhouse Peak on Luther Pass showed a strong, consolidated snowpack that should handle new snow loading well. The 1 to 2 inches of new snow that had already fallen in that area had already started to bond to the old snow surfaces that consisted of melt freeze crusts and moist snow on the sheltered northerly aspects and scoured crusts on the exposed aspects. Observations from other places around the forecast area this week have told a similar story with a well bonded snowpack capped by a a mix of melt freeze crusts, scoured surfaces, and old expoesed rain crusts.

UPDATE @8:30 am -- We have received reports of large natural avalanches that are ran long distances and hit the Alpine Meadows Road around 7:00-7:30 am. Other natural avalanches also occurred last night and early this morning. No details about what kind of avalanches these were or their failure layers are available.

Weather and CURRENT CONDITIONS
weather summary

During the day yesterday, precipitaion remained light and fell as a mix of rain and snow with snow levels hovering around 7000 ft. More precipitation started to fall last night and quickly changed to snow as a cold front arrived over the area causing temperatures and snow levels to fall dramatically between 10 pm and 1 am. Since then 8 to 14 inches of new snow has accumutlated below 8000 ft. and 12 to 18 inches have accumulated above 8000 ft. So far remote sensors north of Hwy 50 have reported the higher new snow amounts. Snow should start to taper off by mid morning and the forecast only calls for another 3 to 5 inches above 7000 ft. today. The southwest winds accompaning this storm have averaged in the 40 to 50 mph range with some gusts in the 90's. These strong winds should persist through today before decreasing tonight. After a lull in the weather tonight and tomorrow morning, winds and cloud cover should increase again tomorrow as a smaller storm approaches the area for Friday night. For more details check in with the Reno NWS.

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 20 to 24 deg. F.
Max. temperature in the last 24 hours: 38 to 42 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 40 to 50 mph
Maximum wind gust in the last 24 hours: 99 mph
New snowfall in the last 24 hours: 12 to 18 inches
Total snow depth: 68 to 92 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with snow in the morning. Snow tapering off to a chance of snow showers in the afternoon Mostly cloudy with a slight chance of snow Mostly cloudy with a slight chance of snow in the morning and a better chance of snow in the afternoon
Temperatures: 28 to 35 deg. F. 19 to 27 deg. F. 32 to 39 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 30 to 40 mph with gusts to 60 mph 35 to 40 mph with gusts to 60 mph decreasing to 20 to 25 mph with gusts to 40 mph after midnight 30 to 40 mph with gusts to 60 mph
Expected snowfall: 3 to 5 in. trace to 0 in. up to 1 in.
For 8000 ft. to 9000 ft.
Thursday Thursday Night Friday
Weather: Cloudy with snow in the morning. Snow tapering off to a chance of snow showers in the afternoon Mostly cloudy with a slight chance of snow Mostly cloudy with a slight chance of snow in the morning and a better chance of snow in the afternoon
Temperatures: 25 to 32 deg. F. 16 to 23 deg. F. 30 to 37 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 50 to 60 mph with gusts to 90 mph 50 to 55 mph with gusts to 85 mph decreasing to 30 to 35 mph with gusts to 50 mph after midnight 45 to 50 mph with gusts to 80 mph increasing to 55 to 60 mph with gusts to 95 mph in the afternoon
Expected snowfall: 3 to 5 in. trace to 0 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.