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.


This Avalanche Advisory was published on December 14, 2010:


December 14, 2010 at 7:58 am

A continual increase in avalanche danger is expected to occur now through midnight tonight. This morning, avalanche danger both above and below treeline will rise to MODERATE danger in open areas 35 degrees and steeper. This afternoon, avalanche danger is expected to further increase to CONSIDERABLE danger near and above treeline on NW-N-NE-E-SE aspects 35 degrees and steeper. Late this afternoon and evening, areas of locally higher avalanche danger may exist if the upper end of forecast snowfall amounts are met or exceeded.


Forecast Discussion:


A fast moving winter storm is impacting the forecast area this morning. Snow levels are starting out around 7,000' to 7,500' this morning and are expected to fall to 5,000' to 6,000' by this evening following cold front passage this afternoon. New snow amounts of 8 to 16 inches are expected to accumulate between now and midnight. There remains some uncertainty that a period of high intensity snowfall this afternoon and/or evening may exceed the current snowfall forecast. Ridgetop winds remain out of the southwest and have increased in speed overnight. Strong southwest winds are expected to continue until around midnight. Air temperatures this morning at 8,500' are in the upper 20s to mid 30s and are expected to cool around 5 degrees as the day progresses.

Observations:

Observations made yesterday on Tamarack Peak (Mount Rose area) and on Jake's Peak (West Shore Tahoe area) revealed supportable rain and melt freeze crusts on all aspects below 9,500'. Above 9,500' (on Tamarack Peak) supportable crusts existed on southerly aspects while a mixture of thin breakable crust and small pockets of unconsolidated wind affected snow existed on northerly aspects. Cloud cover exceeded forecast expectations, keeping snow surface warming significantly less than anticipated. Around 1/2 inch of melted surface snow existed on true south aspects below 9,000' that received direct sun exposure. Shaded true south aspects remained frozen. Recent observations from around the forecast area indicate that the existing snowpack is in good condition to handle new snow loading.

Avalanche Concern #1: Storm snow slabs

As the day progresses, new slabs of storm snow are expected to form on top of the existing crusts that make up the old snow surface in the vast majority of locations within the forecast area. Areas of instability are expected to form first in near and above treeline areas on wind loaded N-NE-E aspects. Areas of instability will then spread to cross loaded NW and SE aspects as well as open wind loaded areas below treeline on all aspects. Human triggered avalanches will become possible from the mid morning hours onward. Natural avalanche activity will become possible by this afternoon and continue into the overnight hours.

Avalanche Concern #2: Loose snow avalanches

The widespread supportable crusts that compose the old snow surface in the vast majority of locations within the forecast area will make an efficient bed surface for human triggered loose snow avalanche activity. This will mainly occur in steep wind protected areas below treeline and will not be restricted to specific aspects. By the afternoon hours, enough new snow may have accumulated to allow loose snow avalanches to become large enough to bury or injure a person, especially if terrain traps or other secondary exposures such as cliffs, trees, or exposed rocks exist within the runout zone.


The bottom line:

A continual increase in avalanche danger is expected to occur now through midnight tonight. This morning, avalanche danger both above and below treeline will rise to MODERATE danger in open areas 35 degrees and steeper. This afternoon, avalanche danger is expected to further increase to CONSIDERABLE danger near and above treeline on NW-N-NE-E-SE aspects 35 degrees and steeper. Late this afternoon and evening, areas of locally higher avalanche danger may exist if the upper end of forecast snowfall amounts are met or exceeded.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 28 to 35 deg. F.
Max. temperature in the last 24 hours: 45 to 49 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: Prior to 6pm 30 mph, since 6pm 42 mph
Maximum wind gust in the last 24 hours: 90 mph
New snowfall in the last 24 hours: 0 to 1 inches
Total snow depth: 28 to 48 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy skies with rain and snow. Cloudy skies with snow decreasing by midnight. Mostly cloudy skies with a slight chance of snow.
Temperatures: 28 to 35 degrees deg. F. 14 to 21 deg. F. 26 to 32 deg. F.
Wind direction: SW SW SW
Wind speed: 20 to 35 mph with gusts to 60 mph. 20 to 30 mph with gusts to 60 mph. Gusts decreasing to 45 mph after midnight. 10 to 15 mph.
Expected snowfall: 3 to 6 in. 4 to 8 in. 0 to trace in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Cloudy skies with snow. Cloudy skies with snow decreasing by midnight. Mostly cloudy skies with a slight chance of snow.
Temperatures: 27 to 32 deg. F. 12 to 19 deg. F. 20 to 28 deg. F.
Wind direction: SW SW shifting to W SW
Wind speed: 50 to 70 mph with gusts to 100 mph. 45 to 65 mph with gusts to 95 mph. Winds decreasing to 25 to 35 mph with gusts to 55 mph after midnight. 15 to 25 mph with gusts to 35 mph.
Expected snowfall: 4 to 8 in. 4 to 8 in. 0 to trace in.

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