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 December 18, 2010:


December 18, 2010 at 8:03 am

The avalanche danger will remain at EXTREME on NW-N-NE-E-SE aspects and HIGH on S-SW-W aspects steeper than 30 degrees at all elevations due to a combination of heavy snow and rain, high winds, and a very widespread weak layer of buried surface hoar. Widespread large natural and human-triggered avalanches are certain to continue today. Travel in or near avalanche terrain including low elevation road cuts and other steep hillsides is not recommended.


Forecast Discussion:


After about 10 inches of light, cold snow fell in the mountains during the day yesterday, another 6-12 inches of dense heavy snow fell on top of that last night. Some areas near and below 7000 ft even received some rain. The current round of wind, snow, and some rain near and below 7000 ft  should continue today as the low pressure in the Gulf of Alaska continues to push moisture into the region.  A second wave of the storm should arrive this evening and has even more moisture and wind associated with it. The forecast calls for southwest winds averaging 40-55 mph with ridge top gusts up to 125 mph during the next 24 hours. An additional 30-55 inches of snow could accumulate between 7000 ft and 8000 ft with 38-64 inches possible above 8000 ft. Snow levels should decrease to below 6000 ft this afternoon and tonight but rise back to around 7000 ft during the day tomorrow.

Observations:

Light to moderate winds and cold temperatures allowed yesterday's snowfall to bury the layer of surface hoar. Widespread buried surface hoar existed in the Deep Creek area, the Echo Summit area, and in the Mt. Rose backcountry. Layer bonding tests and snowpit data from Mt. Rose and Echo summit earlier in the day showed that this layer remained very weak, but that the new snow had not consolidated into enough of a slab to produce slab avalanches. By mid-day the new snow had enough cohesion for skier triggered soft slabs to start occurring due to failure of the buried surface hoar layer in the Deep Creek area. These fractures occurred on slopes as low as 30 degrees. Later in the day it became possible to remotely trigger soft slab avalanches that ran through open trees from even shallower slopes, and some natural avalanches started to occur.

Avalanche Concern #1: Buried Surface Hoar

The buried surface hoar cannot support the new heavy snow, rain, and wind slabs that have fallen on top of it (potato chips trying to hold up concrete-photo of what buried surface hoar can look like in the snowpack). Large, deep, destructive natural and human triggered avalanches that run long distances will continue to occur today as this layer fails. This kind of weak layer allows people to trigger avalanches on lower angle terrain, in open trees, and even allows people to remotely trigger avalanches on nearby slopes from seemingly safe terrain like meadows. Fractures can easily propagate long distances along surface hoar layers sometimes going around corners in the terrain or connecting obstacles like trees or rocks. Buried surface hoar layers are unusual for the Sierra and will make traditionally safe areas suspect. This kind of weak layer requires extra caution and re-evaluation of everything from approaches to safe zones to descents. Any slopes steeper than 30 degrees are suspect today.

Avalanche Concern #2: Wind Slabs

New, heavy wind slabs have formed on wind-loaded aspects. These wind slabs rest on lower density snow and buried surface hoar layers that cannot support them. Large, deep, destructive natural and human triggered avalanches involving these wind slabs will occur today. The largest wind slab avalanches will occur on wind-loaded N-NE-E aspects near and above treeline. These slopes may slide multiple times as the snow and wind continually reload these slopes over the next 24 hours.

Avalanche Concern #3: Rain on New Snow and an Upside Down Snowpack:

As freezing levels fluctuate during the next 24 hours, rain falling on new snow and more dense snow falling on lighter snow will create even more problems. Rain adds weight, destroys bonds, and adds lubrication to the already fragile snowpack. The rain on new snow will cause avalanches on any slopes that receive rain. Even on slopes that do not receive rain the changing temperatures will deposit heavy layers of snow on top of light layers of snow that cannot support them. This upside down snowpack will also lead to avalanche activity today.

Overall several serious instabilities exist in the current snowpack. Any one of these instabilities would be enough to create HIGH avalanche danger with serious consequences. They will all work together to compound the danger today pushing it to EXTREME. Making matters worse, avalanches that do occur today could step down into the snow from Dec. 14th.


The bottom line:

The avalanche danger will remain at EXTREME on NW-N-NE-E-SE aspects and HIGH on S-SW-W aspects steeper than 30 degrees at all elevations due to a combination of heavy snow and rain, high winds, and a very widespread weak layer of buried surface hoar. Widespread large natural and human-triggered avalanches are certain to continue today. Travel in or near avalanche terrain including low elevation road cuts and other steep hillsides is not recommended.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


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

0600 temperature: 27-30 deg. F.
Max. temperature in the last 24 hours: 29-32 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 20 mph before 2pm yesterday after that 50 mph
Maximum wind gust in the last 24 hours: 89 mph
New snowfall in the last 24 hours: 19-21 inches
Total snow depth: 53-82 inches

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

For 7000-8000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Snow mixed with rain at times. Snow level hovering near 7000 ft. Snow Snow mixed with rain at times. Snow level hovering near 7000 ft.
Temperatures: 32-36 deg. F. 26-30 deg. F. 30-34 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 25-40 mph with gusts to 60-75 mph 20-35 mph with gusts to 55-70 mph 20-35 mph with gusts to 55-70 mph decreasing to 25-30 mph in the afternoon
Expected snowfall: 7-14 in. 10-17 in. 12-24 in.

For 8000-9000 ft:

  Saturday: Saturday Night: Sunday:
Weather: Snow Snow Snow
Temperatures: 28-32 decreasing this afternoon to 25-30 deg. F. around 25 deg. F. 24-31 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 40-55 mph with gusts to 100 mph ridge gusts to 125 mph 40-55 mph with gusts to 90 mph ridge gusts to 115 mph 40-55 mph with gusts to 90 mph ridge gusts to 125 mph
Expected snowfall: 10-17 in. 10-17 in. 18-30 in.

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