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 8, 2011:


March 8, 2011 at 7:55 am

Near and above treeline, avalanche danger is MODERATE on all aspects on slopes 35 degrees and steeper. Below treeline, avalanche danger is MODERATE on E-SE-S-SW-W aspects 35 degrees and steeper. On NW-N-NE aspects below treeline, avalanche danger is LOW with pockets of MODERATE danger on slopes 37 degrees and steeper.


Forecast Discussion:


High pressure will build off the coast today allowing for decreasing cloud cover, warming air temperatures, and decreased wind speeds. Remote sensors at all elevations have shown a warming trend since around midnight last night. Maximum daytime air temperatures are expected to warm well above freezing below 8,000' with near to above freezing air temperatures for many locations below 9,500'. Ridgetop winds out of the west decreased to light in speed overnight and have become calm early this morning. A period of increasing winds with moderate gusts is expected this morning before winds decrease again this afternoon. Warmer air temperatures and decreased winds are forecast for tomorrow.

Observations:

Widespread but small natural and human triggered avalanches were reported yesterday from the Donner Summit, West Shore Lake Tahoe, and Emerald Bay areas. On Donner Peak and Mt. Judah (Donner Summit area) small natural avalanches were noted to occur around 8:30am in wind loaded near treeline terrain on steep N-NE aspects. Several skier or snowboarder triggered avalanches were noted to occur throughout the day on NW-N-NE-E aspects. Crown depth for all of the avalanches in this area measured 4 to 10 inches deep with wind slabs up to 1.5 feet deep noted in the most heavily wind loaded areas (more info, photos, videos). On Jake's Peak (West Shore Tahoe area) evidence of instability below treeline was limited to minor skier triggered sluffing while near treeline shooting cracks to 20' and small slab avalanches with crown depths 8 to 10 inches were observed in wind loaded areas (photos, pit profile, more info). Similar to the Donner Summit area the unstable wind slabs sat on top of lower density recent storm snow. On South Maggie's Peak (Emerald Bay area) several skier triggered avalanches occurred on N-NE-E-SE-S micro aspects within the "Dogleg Chute" that descends east and south off of the peak. Here again crown depth measured 4 to 8 inches with many areas of wind loading observed (photos, more info). Observations made on Becker Peak (Echo Summit area) revealed evidence of widespread avalanche activity from Sunday. Snowpit data collected near treeline at 8,300' on a N aspect revealed ongoing instability within the upper portion of the snowpack (pit profile, more info).

Avalanche Concern #1: Wind Slabs and Storm snow instability

Ongoing instability is expected today with a mix of wind slabs and storm snow instability. All of this will remain limited to the upper portion of the snowpack. For areas between 6,500' to 8,300' the rain crust that formed within the recent storm snow and was buried on Saturday is showing weak layer properties in snowpit data. As the recent storm snow above this layer settles and gains strength shifting the area of relative weakness down to this crust, human triggered slab avalanches 1 to 2 feet deep will become increasingly possible. Wind slabs created by ongoing wind loading yesterday and to some degree at times today will remain of concern in wind loaded areas near and above treeline. Below treeline areas with open exposure to wind loading by west winds that act much more like near treeline terrain will also hold this concern. In many areas yesterday, these wind slabs were noted to sit on top of a weak layer of lower density recent storm snow that fell Saturday morning prior to rising snow levels.

Avalanche Concern #2: Warming instability

As the sun comes out today and air temperatures warm to near and above freezing, warming instability will occur in sun exposed areas. Much of the instability will occur as roller balls and shallow loose snow point release avalanches, but human triggered slab avalanches are possible. S-SE-E aspects that were cross loaded or directly wind loaded by westerly winds over the past two days will be the most suspect. Sun angles are getting higher this time of year which will allow for solar radiation to reach the snow surface in many more locations that just one month ago. Per the air temperature forecast for today, slopes below 8,000' hold the greatest potential for significant warming instability with comparatively lower but still significant potential for warming instability on slopes 8,000' to 9,500'.


The bottom line:

Near and above treeline, avalanche danger is MODERATE on all aspects on slopes 35 degrees and steeper. Below treeline, avalanche danger is MODERATE on E-SE-S-SW-W aspects 35 degrees and steeper. On NW-N-NE aspects below treeline, avalanche danger is LOW with pockets of MODERATE danger on slopes 37 degrees and steeper.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 24 to 28 deg. F.
Max. temperature in the last 24 hours: 24 to 28 deg. F.
Average wind direction during the last 24 hours: West
Average wind speed during the last 24 hours: Prior to 6 pm 35 mph, since 6 pm 20 mph
Maximum wind gust in the last 24 hours: 56 mph
New snowfall in the last 24 hours: Trace to 2 inches
Total snow depth: 98 to 132 inches

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

For 7000-8000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Mostly cloudy skies in the morning, becoming partly cloudy. Partly cloudy skies. Mostly cloudy skies in the morning, becoming partly cloudy.
Temperatures: 35 to 45 deg. F. 20 to 30 deg. F. 42 to 52 deg. F.
Wind direction: W W W
Wind speed: 10 to 15 mph with gusts to 30 mph in the afternoon. 10 to 15 mph with gusts to 30 mph in the evening. Up to 10 mph.
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Tuesday: Tuesday Night: Wednesday:
Weather: Mostly cloudy skies in the morning, becoming partly cloudy. Partly cloudy skies. Mostly cloudy skies in the morning, becoming partly cloudy.
Temperatures: 28 to 38 deg. F. 20 to 30 deg. F. 33 to 43 deg. F.
Wind direction: W W W
Wind speed: 20 to 30 mph with gusts to 50 mph. Gusts decreasing to 35 mph in the afternoon. 10 to 15 mph with gusts to 35 mph. Gusts decreasing to 25 mph after midnight. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: O in. O in. O in.