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 March 7, 2011:


March 7, 2011 at 8:03 am

Near and above treeline pockets of CONSIDERABLE avalanche danger exist on N-NE-E-SE aspects on slopes 35 degrees and steeper. MODERATE danger exists on other near and above treeline aspects. Below treeline pockets of MODERATE avalanche danger will exist on any slopes steeper than 35 degrees.


Forecast Discussion:


A fast and cold storm impacted the forecast area last night bring 24 hour snow totals to 8-14 inches of snow in the mountains. This system has already started to exit the region, and precipitation should taper off to showers today. The cold air left behind by this storm should keep temperatures in the upper teens and 20's until tomorrow morning. By then a high pressure ridge should start to build over the forecast area allowing skies to start clearing and temperatures to start climbing. The winds should remain out of the west today before shifting more toward the north as the storm exits. They should start to decrease tomorrow.

Observations:

Near Donner Summit on Andesite Ridge light rain fell up to 8200 ft. during the day yesterday. This rain fell on 8-14 inches of new snow and caused several natural avalanches on N-NE-E facing, 35-40 degree slopes. Most of these slides occurred during the morning and on wind-loaded slopes. One of them occurred on a non-wind-loaded slope, and one remotely triggered avalanche occurred on a wind-loaded test slope in the afternoon. These avalanches entrained all of the new snow and fractured up to 800 ft wide (photos, more info). Observations on Red Lake Peak near Carson Pass told a different story. In that area the rain only reached up to 7600 ft around mid-day then turned back to snow all the way down to 7200 ft as temperatures cooled during the afternoon. Snowpit data and observations when the snowpack was the warmest showed weakness at the base of the new snow (pit profile, video). However, as the temperatures cooled in the afternoon, these instabilities started to strengthen. By mid afternoon in this area, ski cuts on steep, wind-loaded test slopes produced no results. In this area observers did not report any large natural avalanche activity only small roller balls and wet loose snow sluffs on east aspects.

As the snowpack cooled last night the instabilities caused by the rain and warmth should have started to freeze and bond to the snow layer around them. Remote sensors indicate that the temperatures got progressively colder as new snow fell last night. These decreasing temperatures should have allowed lighter snow to accumulate on top of heavier snow creating a right side up snowpack in most areas.

Avalanche Concern #1: Wind Slabs

More snow and wind last night will have allowed the wind slabs to grow larger during the night. As the winds continue today, these slabs will continue to grow. These winds slabs have formed faster than the bonds that will eventually hold them in place. Some natural avalanche activity could occur as these slabs and the cornices above them overload those nascent bonds on the most wind-loaded aspects especially during the morning hours. Human-triggered avalanches will also remain likely on wind-loaded slopes today. Avalanches involving these wind slabs could reach several feet in depth. The wind over the last 24 hours has come from a more westerly direction than usual, so it has created wind slabs in different places than usual. The N-NE-E-SE wind-loaded aspects near and above treeline will hold the most fragile wind slabs. Some cross-loaded NW and S aspects may also contain these wind slabs.

Avalanche Concern #2: Storm Snow and Loose Snow Sluffs

As yesterday's wet snow freezes, it should bond well to itself and the snow above it creating a more stable snowpack. However, this freezing will take time to occur, and wet snow will still exist below the new snow today. Fortunately, the heavy wet snow should be able to support the lighter new snow above it in most areas. Some small weaknesses may have formed in the new snow as it fell last night due to changes in the storm conditions. Bonds within the new snow have also not had enough time to fully form. This combination of storm snow weaknesses and unconsolidated new snow will allow some instabilities to persist in the new snow today. Most avalanche activity resulting from these instabilities should manifest as loose snow sluffs that pose little threat to backcountry travelers. However, some human-triggered soft slab avalanche activity will also be possible on steep slopes especially if they have any wind-affect at all. Steep open slopes with new snow on them are susceptible to this kind of instability today. 


The bottom line:

Near and above treeline pockets of CONSIDERABLE avalanche danger exist on N-NE-E-SE aspects on slopes 35 degrees and steeper. MODERATE danger exists on other near and above treeline aspects. Below treeline pockets of MODERATE avalanche danger will exist on any slopes steeper than 35 degrees.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 20-24 deg. F.
Max. temperature in the last 24 hours: 29-31 deg. F.
Average wind direction during the last 24 hours: West
Average wind speed during the last 24 hours: 30-35 mph
Maximum wind gust in the last 24 hours: 65 mph
New snowfall in the last 24 hours: 8-12 inches
Total snow depth: 101-150 inches

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

For 7000-8000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy with scattered snow showers. Mostly cloudy with a chance of snow showers Mostly cloudy with a slight chance of snow showers in the morning
Temperatures: 21-31 deg. F. 17-23 deg. F. 30-38 deg. F.
Wind direction: West West West
Wind speed: 15-25 mph with gusts to 45 mph 10-20 mph with gusts to 40 mph 10 -15 mph
Expected snowfall: up to 1 in. 0- trace in. O in.

For 8000-9000 ft:

  Monday: Monday Night: Tuesday:
Weather: Mostly cloudy with scattered snow showers. Mostly cloudy with a chance of snow showers Mostly cloudy with a slight chance of snow showers in the morning
Temperatures: 16-26 deg. F. 15-25 deg. F. 24-34 deg. F.
Wind direction: West West shifting to the northwest Northwest
Wind speed: 25-40 mph with gusts to 65 mph 25-40 mph with gusts to 65 mph 25-35 mph with gusts to 65 mph decreasing to 15-25 mph with gusts to 30 mph in the afternoon
Expected snowfall: 1-2 in. 0-trace in. O in.