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 January 30, 2010:


January 30, 2010 at 7:47 am

Near and above treeline, pockets of MODERATE avalanche danger will form today in wind loaded areas on NW-N-NE-E aspects, 37 degrees and steeper. Below treeline, avalanche danger is LOW in wind protected areas.


Forecast Discussion:


A weak weather system is passing through the forecast area today. Snow shower activity is expected to occur throughout the day with new snow accumulation of 1 to 3 inches. Isolated areas may see accumulations of up to 5 inches today. Snow level started out around 5,000' this morning and will continue to lower during the morning hours. Ridgetop winds remain out of the southwest this morning and are increasing in speed. Winds are expected to remain moderate in speed through this evening. Expect air temperatures in the 20s in most locations today.

Observations:

Observations made yesterday on the west side of Mt. Lincoln (Donner Summit area) at the site of Thursday's skier triggered avalanche revealed that details regarding the aspect, slab characteristics, and weak layer differed significantly from the initial report. This is understandable given that the reporting party had just suffered a very close call. The avalanche occurred just above treeline and covered two adjacent gully features with the aspect wrapping W-NW-N. The slab was composed of the recent higher density storm snow over a weak layer of lower density snow (photos, crown profile, video, more info). This upside down snowpack structure was seen in several other skier triggered avalanches on Monday and Tuesday with snowpit data continuing to show this instability in some areas on Wednesday (more info).

At the Mt. Lincoln avalanche site, stability and fracture propagation tests indicated that fracture propagation remained possible on the low density weak layer. Several test pits performed yesterday at similar elevations and aspects nearby on Donner Peak and Mt. Judah showed a similar snowpack structure but no significant weakness. Observations made yesterday on Jake's Peak (West Shore Tahoe area) below treeline at 8,000' on a NE aspect showed no evidence of instability with the lower density weak layer becoming increasingly harder to detect. This matches well with other recent observations from around the forecast area made in the Mount Rose, Blackwood Canyon, and Desolation Wilderness areas (more info).

Avalanche concern #1:

The main avalanche concern for today focuses on newly forming wind slabs in near and above treeline terrain. Snowfall accumulation will be a bit hit and miss today due to the snow shower nature of today's system. The greatest areas of instability will occur in wind loaded areas below cornice features on NW-N-NE-E aspects. Instability will mainly be limited to the locations that receive the higher end of the forecast snowfall accumulation.

Avalanche concern #2:

It is unlikely but not impossible that in isolated areas snowpack failure could occur 1 to 2 feet below the old snow surface down on the lower density layer that has played a roll in recent avalanche activity. This layer has been uncharacteristically slow to stabilize for a storm snow type instability. This layer warrants continued observation, despite the fact that it will appear stable in the vast majority of areas.


The bottom line:

Near and above treeline, pockets of MODERATE avalanche danger will form today in wind loaded areas on NW-N-NE-E aspects, 37 degrees and steeper. Below treeline, avalanche danger is LOW in wind protected areas.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 21 to 25 deg. F.
Max. temperature in the last 24 hours: 32 to 39 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 24 mph
Maximum wind gust in the last 24 hours: 51 mph
New snowfall in the last 24 hours: trace to 2 inches
Total snow depth: 74 to 111 inches

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

For 7000-8000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy skies with numerous snow showers in the morning. Mostly cloudy skies with scattered snow showers in the afternoon. Mostly cloudy skies with isolated snow showers in the evening. Partly cloudy skies with a slight chance of snow after midnight. Partly cloudy skies
Temperatures: 26 to 32 deg. F. 13 to 23 deg. F. 25 to 32 deg. F.
Wind direction: WSW W SW
Wind speed: 10 to 15 mph with gusts to 25 mph in the morning. 10 to 15 mph. Light winds
Expected snowfall: 1 to 3 in. with isolated areas up to 5 in. 0 to trace in. O in.

For 8000-9000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy skies with numerous snow showers in the morning. Mostly cloudy skies with scattered snow showers in the afternoon. Mostly cloudy skies with isolated snow showers in the evening. Partly cloudy skies with a slight chance of snow after midnight. Partly cloudy skies
Temperatures: 18 to 23 deg. F. 10 to 18 deg. F. 22 to 27 deg. F.
Wind direction: WSW WSW SW
Wind speed: 10 to 20 mph with gusts to 35 mph. 10 to 15 mph with gusts to 25 mph in the evening. Up to 10 mph.
Expected snowfall: 1 to 3 in. with isolated areas up to 5 in. 0 to trace in. O in.

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