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 April 8, 2011:


April 8, 2011 at 6:45 am

Near and above treeline, avalanche danger is LOW with pockets of MODERATE danger on all aspects on slopes 37 degrees and steeper. Below treeline, avalanche danger is LOW.


Forecast Discussion:


Another day of cloudy to mostly cloudy skies with off and on snow showers is expected across the region today. New snow amounts for the past 24 hours are running 3 to 5 inches across the forecast area. Storm totals from the past 36 hours are 7 to 16 inches, with the greatest accumulations along the Sierra Crest. Cold air is in place over the region this morning with air temperatures above 8,000' in the single digits to low teens. Around 10 degrees of daytime warming is expected to occur today. Ridgetop winds shifted from SW to NW during the day yesterday and then shifted to the NE overnight. Light NE winds early this morning are expected to increase to moderate in speed as the day progresses. More clouds and snow showers with below freezing air temperatures above 7,000' are forecast for tomorrow.

Observations:

Observations made yesterday along Frog Lake Ridge and on Elephant's Back (Carson Pass area) revealed small and shallow soft slabs in near and above treeline terrain on N-NE-E aspects. These slabs were generally 6 to 8 inches deep and responded to the weight of a skier with cracking up to 3 feet long. Areas of cracking were limited to within 25 feet of the ridgeline. During the afternoon hours, evidence of several very recent loose snow avalanches were observed above treeline on the ENE aspect of Elephant's Back triggered by natural cornice collapse. A few of these avalanches had distinct crowns 6 to 8 inches deep, but just below the crowns the avalanches behaved as loose snow avalanches rather than slab avalanches. In areas sheltered from wind, 8 to 12 inches of new snow with significant amounts of small graupel was well bonded to the hard crust that composes the old snow surface (pit profile, more info).

Observations made yesterday on Silver Peak (Pole Creek area) revealed new snow amounts of 6 to 10 inches in this area. No apparent weak layers or evidence of slab instability within the storm snow were observed while on tour in this area. Snowpit data collected in near treeline terrain at 8,000' on a N aspect 34 degree slope adjacent to an avalanche start zone did not produce any evidence of instability. The wind shift from SW to NW was starting to scour N aspects near treeline at mid day. Good bonding of new snow to the old snow crust allowed for only very minor sluffing within the top 1/3 of the storm snow in response to a ski cut on a below treeline NE aspect, 38 degree test slope at 7,400' (photo, pit profile, more info).

Primary Avalanche Concern: Wind Slabs

Areas of fairly small and shallow wind slabs created by shifting ridgetop winds yesterday and last night are expected to allow for pockets of instability on all aspects. Any snowpack failure that occurs today is expected to occur above the old/new snow interface. Slab depths generally less than 12 inches deep are expected today in wind loaded areas that can exist on any aspect due to the ridgetop wind shift from SW to NE. These fairly small slabs (in the relative sense of avalanches) are still large enough to bury a person or cause significant injury, especially when combined with secondary terrain hazards such as cliffs and trees. Natural avalanches are unlikely today. Human triggered avalanches are possible.


The bottom line:

Near and above treeline, avalanche danger is LOW with pockets of MODERATE danger on all aspects on slopes 37 degrees and steeper. Below treeline, avalanche danger is LOW.


Brandon Schwartz - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 6 to 11 deg. F.
Max. temperature in the last 24 hours: 19 to 27 deg. F.
Average wind direction during the last 24 hours: West shifting (clocking) to northeast
Average wind speed during the last 24 hours: 19 mph
Maximum wind gust in the last 24 hours: 58 mph
New snowfall in the last 24 hours: 3 to 5 inches
Total snow depth: 107 to 175 inches

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

For 7000-8000 ft:

  Friday: Friday Night: Saturday:
Weather: Cloudy skies with scattered snow showers. Mostly cloudy skies with scattered snow showers in the evening. Mostly cloudy skies with isolated snow showers in the afternoon.
Temperatures: 19 to 26 deg. F. 11 to 21 deg. F. 25 to 32 deg. F.
Wind direction: NE N NW shifting to W in the afternoon.
Wind speed: 10 to 15 mph with gusts to 25 mph. 10 to 15 mph with gusts to 25 mph. Up to 10 mph.
Expected snowfall: Up to 2 in. 0 to trace in. 0 to trace in.

For 8000-9000 ft:

  Friday: Friday Night: Saturday:
Weather: Cloudy skies with scattered snow showers. Mostly cloudy skies with scattered snow showers in the evening. Mostly cloudy skies with isolated snow showers in the afternoon.
Temperatures: 16 to 22 deg. F. 11 to 19 deg. F. 23 to 29 deg. F.
Wind direction: NE N NW
Wind speed: 20 to 30 mph with gusts to 45 mph. 15 to 25 mph with gusts to 45 mph. Gusts decreasing to 35 mph after midnight. 10 to 15 mph with gusts to 25 mph in the afternoon.
Expected snowfall: Up to 3 in. 0 to trace in. 0 to trace in.