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 13, 2012:


April 13, 2012 at 6:59 am

On slopes steeper than 35 degrees near and above treeline, CONSIDERABLE avalanche danger will exist on N-NE-E aspects with pockets of CONSIDERABLE danger on the NW and SE aspects due to new snow and wind loading. Pockets of CONSIDERABLE danger may also exist on below treeline NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Human triggered avalanches are likely today, and natural avalanche are possible.


Forecast Discussion:


In the last 24 hours, another 11-16 inches of new snow has fallen along the Sierra Crest. On the east side of Lake Tahoe less new snow accumulated with sensors only showing 6-7 inches. Most of this snow fell during the night as the second stronger winter storm of this week arrived over the region. This brings storm totals to 15-22 inches since Wednesday. Snowfall should continue today with the most intense precipitation occurring this morning as the main part of this system moves into the area. The forecast calls for snowfall rates of 1-2 inches per hour during this period of heavier snowfall. Later this morning and this afternoon, the snow showers should decrease in intensity, and they should start to taper off tonight. By the end of the day, the forecast calls for 6-10 inches of additional accumulation along the crest and 3-6 inches in the Carson Range. Snow showers should end by tomorrow. The strong southwest winds should start to decrease some this afternoon and begin shifting towards the northwest and north this evening and tomorrow. Temperatures should remain cold through tonight before they begin slowly warming during the day tomorrow.

Recent Observations:

On Flagpole Peak and on Tamarack Peak yesterday, wind slabs up to 2ft deep had formed in wind loaded areas. In both of these areas, the wind slabs rested on top of lower density snow. Skiers triggered shooting cracks (up to 20 ft long on Tamarack) in these wind slabs. Ski cuts on undercut and unsupported test slopes produced slab failures on the slopes where these wind slabs existed (photos from Tamarack, photo from Flagpole). Snowpit tests on wind loaded slopes also showed that the bonds between the lower density snow and the heavier wind slabs above remain weak and that fractures can easily travel along that weak interface (video from Flagpole). The strong wind loading had also formed large fragile cornices along the near and above treeline ridgelines on the east ridge of Tamarack Peak. On Flagpole Peak swirling winds had deposited wind slabs in below treeline areas as well. Some small loose sluffing occurred in response to ski cuts on steep less wind affected test slopes on Tamarack Peak. Several snow squaws passed over the region yesterday producing intense bursts of graupel (ball bearing shaped snow).

Today's Primary Avalanche Concern: Wind Slabs

The combination of wind slabs (the slab layer) on top of a layer of lighter snow (the weak layer) on top of frozen crusts (a bed surface) will make human triggered avalanches likely and natural avalanches possible today. These wind slabs have grown significantly over the last 24 hours and could easily measure several feet in depth and several hundred feet in width in many areas. These slabs will continue to grow faster than the snowpack can adjust to them as the snow and wind continue today. The failure of these wind slabs would result in large dangerous avalanches. The largest and most sensitive wind slabs will exist on wind loaded N-NE-E aspects and cross loaded NW and SE aspects in near and above treeline terrain. Smaller but still serious wind slabs may also exist on leeward aspects in open areas below treeline.

Today's Secondary Avalanche Concern: Storm Slabs and Loose Dry Sluffs

Changing winds, snowfall rates, and temperatures have all created weaknesses within the new snow. These weaknesses will remain active today. On steep slopes where the new snow has more cohesion and can act more like a slab human-triggerable storm slab avalanches will be likely. In other areas where the new snow remains unconsolidated, widespread loose snow sluffs will continue to occur on any steep slopes.

Other things to think about: The Persistent Weak Layer

Due to the depth (5 to 8 ft down) of the persistent weak layer on the NW-N-NE aspects and the strength of the snow above it, triggering an avalanche on this layer has become unlikely. At this time data indicates breaking this layer would require an extremely large trigger or significant weakening of the snowpack. Spring melting could eventually allow free water to percolate down through the snowpack and cause enough weakening for a deep wet slab avalanche cycle to occur. Conditions that would create enough melting for this scenario remain weeks to months away.


The bottom line:

On slopes steeper than 35 degrees near and above treeline, CONSIDERABLE avalanche danger will exist on N-NE-E aspects with pockets of CONSIDERABLE danger on the NW and SE aspects due to new snow and wind loading. Pockets of CONSIDERABLE danger may also exist on below treeline NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Human triggered avalanches are likely today, and natural avalanche are possible.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 19-25 deg. F.
Max. temperature in the last 24 hours: 22-29 deg. F.
Average wind direction during the last 24 hours: Southwest to south
Average wind speed during the last 24 hours: 25-35 mph
Maximum wind gust in the last 24 hours: 75 mph
New snowfall in the last 24 hours: Along the Sierra Crest: 11-16 inches | East of Lake Tahoe: 6-7 inches
Total snow depth: 46-104 inches

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

For 7000-8000 ft:

  Friday: Friday Night: Saturday:
Weather: Snow Snow showers decreasing after midnight Mostly cloudy becoming partly cloudy with a chance of snow showers
Temperatures: 21-26 deg. F. 13-18 deg. F. 28-35 deg. F.
Wind direction: Southwest shifting to the west in the afternoon Northwest North
Wind speed: 10-20 mph with gusts to 30 mph decreasing to 10-15 mph in the afternoon 10-20 mph with gusts to 30 mph 15-25 mph with gusts to 35 mph
Expected snowfall: Along the Sierra Crest: 6-10 in. | In the Carson Range: 3-6 in. 1-3 in. up to 1 in.

For 8000-9000 ft:

  Friday: Friday Night: Saturday:
Weather: Snow Snow showers decreasing after midnight Mostly cloudy becoming partly cloudy with a chance of snow showers
Temperatures: 15-21 deg. F. 10-17 deg. F. 23-30 deg. F.
Wind direction: Southwest shifting to the west in the afternoon Northwest North
Wind speed: 25-35 mph with gusts to 50 mph decreasing to 15-20 mph with gusts to 30 mph in the afternoon 10-20 mph with gusts to 30 mph increasing to 30-40 mph with gusts to 50 mph after midnight 25-35 mph with gusts to 55 mph
Expected snowfall: Along the Sierra Crest: 6-10 in. | In the Carson Range: 3-6 in. 1-3 in. up to 1 in.

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