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 13, 2012:

March 13, 2012 at 7:00 am

MODERATE avalanche danger will become more widespread on NW-N-NE aspects on slopes 35 degrees and steeper in both above and below treeline terrain due to ongoing deep slab instability, new snow, and wind loading. The possibility of large destructive human triggered avalanches remains in these areas. Pockets of MODERATE danger should also form on wind-loaded and cross-loaded E and SE aspects. Some natural avalanche activity may become possible tonight due to more intense snowfall and continued wind loading.

Forecast Discussion:

The winds and cloud cover increased during the night with winds averaging between 60 and 80 mph with gusts over 105 mph. The winds should continue to increase today with the forecast calling for average speeds of 80-90 mph with gusts of 135 mph along the ridge tops. These winds should decrease by 10-15 mph tonight and another 10 mph tomorrow. Snowfall started early this morning with 1-2 inches accumulating above 7000 ft by 6 am. Snow should continue through tomorrow with the heaviest snowfall occurring tonight. The forecast calls for up to 6 inches of snow today with another 6 to 12 inches tonight and up to 6 more inches tomorrow above 7500 ft. This morning snow level remains around 5500-6000 ft. Snow level should rise to 7000 ft today and stay there through tonight. Snow level should continue to climb reaching 7500 ft tomorrow. Another colder stronger storm should arrive later this week bringing more wind and snow. Click here for a weather briefing from the Reno NWS.

Recent Observations:

Yesterday on Meiss Ridge and on Incline Lake Peak snowpit data showed that the persistent weak layer of crusts and weak sugary snow (facets) still exists a few feet down in the snowpack. Tests on this layer still indicate that even though it may be difficult to start a fracture in these facets, once one starts it can travel along this weak layer. In these areas as in other places around the region, the snow above this weak layer has consolidated into a strong slab layer. Breakable crusts existed on the surface of most northerly aspects in these places yesterday. Some of the most sheltered, upper elevation, north facing slopes on Incline Lake Peak still held pockets of softer snow. On the more sun-exposed southerly aspects frozen melt-freeze conditions prevailed.

Today's Primary Avalanche Concern: Persistent Deep Slabs

New wind slabs and new snow will add weight to the snowpack today. Even though the six inches of snow forecasted to fall today should not add enough load to cause the persistent weak layer to fail naturally, it should make it easier for a person to start a fracture in this layer. Human-triggered deep slab avalanches will become possible in more areas on the NW-N-NE aspects in above and below treeline terrain where this weak layer exists. Areas near exposed rock outcrops and cliffs, steep unsupported slopes, and slopes with trigger points represent locations where triggering a deep slab avalanche would be easiest. Due to the way that these kind of layers behave, areas of stability and areas of instability could exist within a few feet of each other on the same slope. As this storm continues, avalanches resulting from the failure of this persistent weak layer should become more widespread and easier to trigger. Some of these avalanches may start to occur naturally during the night. Caution and conservative decision making remain prudent due to the variability of this layer and the consequences of its failure.

Today's Secondary Avalanche Concern: Wind Slabs and Storm Slabs

The new snow and wind will combine to form new wind slabs on the leeward slopes today. These slabs should start out small this morning but could grow large enough to pose problems during the day on the most heavily wind loaded slopes. Adding a person on top of these new slabs could break the bonds holding them to the snowpack resulting in an avalanche. Even though most of these avalanches should remain small, they could step down into the lower weak layers turning a small wind slab avalanche into a much deeper and more dangerous deep slab avalanche. Some natural avalanche activity involving these wind slabs as well as new storm slabs that form as more snowfall accumulates could become possible tonight due to the continued strong winds and increased snowfall causing these slabs to become larger and more widespread.

Today's Third Avalanche Concern: Rain on snow

Rain adds weight to the snowpack while at the same time weakening the snowpack by melting the bonds holding it together. This combination will typically cause avalanche activity. Due to snow levels predicted to stay at or below 7000 ft today, this kind of instability should remain unlikely. However, snow levels are notoriously difficult to forecast. If snow level happens to rise higher than 7000 ft today or tonight, rain on snow could produce some wet avalanche activity. As snow levels climb during the next 24 hours this kind of instability may become more of a problem.


The bottom line:

MODERATE avalanche danger will become more widespread on NW-N-NE aspects on slopes 35 degrees and steeper in both above and below treeline terrain due to ongoing deep slab instability, new snow, and wind loading. The possibility of large destructive human triggered avalanches remains in these areas. Pockets of MODERATE danger should also form on wind-loaded and cross-loaded E and SE aspects. Some natural avalanche activity may become possible tonight due to more intense snowfall and continued wind loading.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature: 20-26 deg. F.
Max. temperature in the last 24 hours: 27-38 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 45-60 mph
Maximum wind gust in the last 24 hours: 107 mph
New snowfall in the last 24 hours: 0-2 inches
Total snow depth: 27-48 inches

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

For 7000-8000 ft:
  Tuesday: Tuesday Night: Wednesday:
Weather: Snow Snow Snow and rain with snow level between 7000 and 7500 ft.
Temperatures: 28-34 deg. F. 25-30 deg. F. 30-36 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 40-55 mph with gusts to 70 mph 30-45 mph with gusts to 60 mph 25-40 mph with gusts to 55 mph
Expected snowfall: up to 6 in. 6-12 in. up to 6 in.

For 8000-9000 ft:
  Tuesday: Tuesday Night: Wednesday:
Weather: Snow Snow Snow
Temperatures: 24-29 deg. F. 23-28 deg. F. 25-30 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 80-90 mph with gusts to 135 mph 60-85 mph with gusts to 100 mph 50-60 mph with gusts to 90 mph
Expected snowfall: up to 6 in. 6-12 in. up to 6 in.