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 25, 2010:


December 25, 2010 at 7:58 am

The avalanche danger remains LOW in most areas above and below treeline. Pockets of MODERATE avalanche danger exist near and above treeline on wind-loaded NW-N-NE-E aspects, 35 degrees and steeper. As more snow and wind impact the forecast area tonight, expect the avalanche danger to increase.


Forecast Discussion:


A low pressure west of the forecast area should arrive later this afternoon and evening bringing 5-10 inches of snow to the mountains above 7000 ft tonight. The southerly winds and cloud cover should continue to increase today as this fast-moving system approaches. By tomorrow this system should continue to the east, and the showers and winds should taper off. Temperatures should decrease into the teens tonight and remain cold during the day tomorrow.

Observations:

As the winds increased yesterday, they transported enough snow to form shallow but sensitive wind slabs on the most wind-loaded slopes near and above treeline. On Meiss Ridge (Carson Pass area) and on Incline Lake Peak (Mt. Rose area), these new slabs failed easily on slopes steeper than 39 degrees in response to a skiers weight by the afternoon. On Incline Lake Peak, these slabs only reached 2-5 inches in depth and got quickly shallower moving down from the ridgelines (photos, pit profile). On Meiss Ridge where the most wind-loading occurred, slabs up to 12 inches had formed. One N-facing, 40 degree slope had slid naturally resulting from a large natural cornice collapse. A skier remotely triggered another similar slide (photos and pit profile) adjacent to this natural avalanche. On Mt Judah (near Donner Summit) some minor cracking occurred on wind loaded slopes. Cornices formed by the recent winds remain very sensitive to human triggering where ever they have formed. Layer bonding tests on less wind-loaded areas and in below treeline areas indicated that bonds between the recent snow and the older snow as well as between the deeper layers in the snowpack continue to strengthen. Soft, unconsolidated snow remained on the surface on sheltered, shaded slopes yesterday. On the sun-exposed southerly aspects wet, sticky snow and a thin sun crust existed on Incline Lake Peak and Mt Judah. Below 7300 ft in the Mt. Judah area several roller-balls occurred due to daytime warming yesterday.

Primary avalanche concern: Wind Slabs

Human triggered avalanches will remain possible today on the most heavily wind-loaded, NW-N-NE-E aspects near and above treeline on slopes steeper than 35 degrees. Additional wind-loading today will allow these slabs to continue to grow. Most avalanches due to  failure of these wind slabs should remain shallow within the most recent snow; however, serious consequences could result from these small slides due to their locations. Wind-loaded complex or extreme terrain (unsupported slopes, couloirs, cliffy areas, etc) and steep, open, wind-loaded slopes near ridgelines will be the most likely areas for these pockets of instability. Cornices that exist above wind-loaded slopes will also remain sensitive to human triggering today. As more snow and wind impact the forecast area tonight expect these slabs to become more widespread and the avalanche danger to increase.


The bottom line:

The avalanche danger remains LOW in most areas above and below treeline. Pockets of MODERATE avalanche danger exist near and above treeline on wind-loaded NW-N-NE-E aspects, 35 degrees and steeper. As more snow and wind impact the forecast area tonight, expect the avalanche danger to increase.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 31-36 deg. F.
Max. temperature in the last 24 hours: 31-36 deg. F.
Average wind direction during the last 24 hours: South southwest
Average wind speed during the last 24 hours: 35-40 mph
Maximum wind gust in the last 24 hours: 70 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 62-86 inches

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

For 7000-8000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy with a chance of isolated snow showers later in the day. Snow Cloudy with scattered snow showers in the morning. Showers should taper off in the afternoon.
Temperatures: 33-40 deg. F. 15-22 deg. F. 24-31 deg. F.
Wind direction: South Southwest West
Wind speed: 10-20 mph with gusts to 40 mph 15-20 mph with gusts to 35 mph increasing to 20-30 mph with gusts to 45 mph 15-25 mph with gusts to 45 mph decreasing to 30 mph in the afternoon
Expected snowfall: trace in. 3-8 in. trace in.

For 8000-9000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Cloudy with a chance of isolated snow showers later in the day. Snow Cloudy with scattered snow showers in the morning. Showers should taper off in the afternoon.
Temperatures: 30-37 deg. F. 9-16 deg. F. 23-29 deg. F.
Wind direction: South Southwest West
Wind speed: 30-45 mph with gusts to 60 mph increasing to 70 mph 30-50 mph with gusts to 75 mph 30-45 mph with gusts to 70 mph decreasing to 25-35 mph with gusts to 55 mph
Expected snowfall: trace in. 4-10 in. trace in.

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