THIS AVALANCHE ADVISORY EXPIRED ON November 25, 2015 @ 7:00 am
Avalanche Advisory published on November 24, 2015 @ 7:00 am
Issued by Andy Anderson - Tahoe National Forest

The avalanche danger will increase to CONSIDERABLE on near and above treeline NW-N-NE-E-SE aspects 35 degrees and steeper by this afternoon and evening. Human triggered avalanches will become likely as new wind slabs will form on these slopes. MODERATE avalanche danger will form on the remaining aspects and elevations where avalanches could take the form of loose dry instabilities or isolated storm slabs by later this afternoon. 

3. Considerable

?

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

3. Considerable

?

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

2. Moderate

?

Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    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

As new snow and strong southwest winds impact the forecast area today, new wind slabs will develop on the leeward aspects in near and above treeline areas. These wind slabs will accumulate above a variety of snow surfaces ranging from wind scoured surfaces to melt freeze crusts to old hard wind slabs and will likely accumulate more quickly than they can bond to themselves or some of the old snow surfaces. Human triggereing of these wind slabs will become likely today.  By this afternoon wind slabs could reach 1-3 ft in thickness in heavily wind loaded areas. The majority of avalanches resulting from the failure of these wind slabs should fail at or above the old snow surfaces; however, they could break into older snow layers in areas where the wind slabs form on top of old hard slabs left over from previous wind events and storms. The most sensitive wind slabs will exist on wind loaded N-NE-E aspects and cross loaded NW and SE aspects in near and above treeline terrain. These wind slabs may extend into more sheltered areas as well given the strength of the winds.

Avalanche Problem 2: Loose Dry
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Some small loose dry snow sluffs may start to occur by this afternoon and evening as cold new snow falls onto frozen melt-freeze crusts and other old snow surfaces. If more snow falls than forecasted some small storm slabs may form as well. Near and below treeline NW-N-NE-E aspects sheltered from the winds hold the best potential for these kind of instabilities since they had more previous snow coverage. These instabilities could also form on other near and below treeline aspects where previous snow cover exists, but they will be more isolated on the SE-S-SW-W because the previous snow cover has melted away from most of these aspects. As more snow accumulates tonight and tomorrow these loose dry instabilities may give way to small storm slabs

recent observations

Most recent observations from around the forecast area have indicated a strengthening trend in the snowpack and a great deal of variability in snow coverage and snow surface conditions. Melt-freeze crusts exist on sun exposed aspects, scoured surfaces remain on exposed near and above treeline N-NE-E aspects, and some pockets of soft unconsolidated snow remain on sheltered and shaded northerly aspects. Yesterday on Carpenter Ridge, similar variable conditions existed. Most of the observations from this area yielded data consistent with an overall strengthening trend except in places where a hard wind slab existed on top of looser weaker snow layers. These hard slabs remained isolated in distribution and small in size, but all the data collected about them indicated that additional snow loading could cause them to fail. 

Weather and CURRENT CONDITIONS
weather summary

Southwest winds have increased across the forecast area as a cold winter storm approaches the region. Southwest winds should peak this afternoon with sustained ridgetop winds in the 60-70 mph range and gusts up to 100 mph. The forecast calls for snowfall beginning later this morning and continuing through tomorrow. The heaviest snowfall should occur this afternoon and evening. Some areas may see bands of more intense snow as convective showers move across the region this afternoon and evening. Above 8000 ft. 10 to 18 inches of new snow could accumulate by tomorrow afternoon. Between 7000 and 8000 ft. accumulation amounts look more like 6 to 14 inches. The winds and snowfall should begin to taper off tomorrow as the main part of the storm moves eastward. This storm will also bring much colder air to the region. Today's high temperatures should occur this morning before the storm arrives. The forecast calls for temperatures to fall into the the single digits and teens for tonight with daytime highs in the teens above 7000 ft. tomorrow. 

Weather observations from along the Sierra Crest between 8200 ft. and 8800 ft.
0600 temperature: 27 to 36 deg. F.
Max. temperature in the last 24 hours: 40 to 46 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 35 mph
Maximum wind gust in the last 24 hours: 70 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 8 to 16 inches
Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 7000 ft. to 8000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy with a chance of snow in the morning snow increasing in the afternoon Snow with a slight chance of thunderstorms in the evening. Snow likely
Temperatures: 27 to 34 deg. F. 10 to 16 deg. F. 13 to 20 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 35 to 40 mph with gusts to 65 mph increasing to 75 mph in the afternoon 30 to 35 mph with gusts to 60 mph decreasing to 20 to 30 mph with gusts to 40 mph after midnight 15 to 20 mph with gusts to 30 mph
Expected snowfall: 3 to 6 in. 2 to 5 in. 1 to 3 in.
For 8000 ft. to 9000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy with a chance of snow in the morning snow increasing in the afternoon Snow with a slight chance of thunderstorms in the evening. Snow likely
Temperatures: 24 to 31 deg. F. 3 to 10 deg. F. 9 to 16 deg. F.
Wind Direction: Southwest Southwest West
Wind Speed: 60 to 70 mph with gusts to 100 mph 55 to 60 mph with gusts to 85 mph decreasing to 35 to 40 mph with gusts to 60 mph after midnight 20 to 30 mph with gusts to 45 mph
Expected snowfall: 4 to 8 in. 4 to 6 in. 2 to 4 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.