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Monday, December 20, 2010

Sierra Avalanche Danger is High on slopes greater than 35 degrees

 #CaWx Where: CONSIDERABLE avalanche danger on slopes greater than 35 degrees on all aspects below tree line and on easterly (SE/E/NE) aspects above tree line. MODERATE elsewhere.
  The Mt. Shasta Avalanche Center - US Forest Service

Update Monday, December 20, 7 am:  Temps cooled down last night and it dumped.  In the last 24 hours, 9-13 inches of snow has fallen in the surrounding mountains.  Winds have been steady from the WSW, averaging 22 and gusting to 42, loading mostly NE and E aspects above tree line.  Concerns continue with wind slabs above tree line and storm slabs below tree line.  should be a great powder day, but slopes 35 degrees and steeper below tree line will likely be touchy and northerly and easterly aspects above tree line will be loaded and touchy, especially for machines. 

The Eastern Sierra Avalanche Center 
Danger Rating: Extreme
http://esavalanche.org/advisory
Extreme avalanche danger exists for the Mammoth area. High Avalanche danger exists for all other areas in the forecast region. A great deal of new snow, up to 10', has fallen with strong winds over the last several days, and will continue to fall today. Widespread avalanche activity reported by Mammoth and June Mtn ski patrols, with most paths capable of avalanching doing so this am as result of control work. Avoid avalanche terrain, including areas UNDER steep slopes as well. Be careful underneath roofs with snow build up as well! Click here for full advisory.


Up to 10 feet of new snow has fallen over the past few days with very strong SW winds, including 2-3 feet of lower density snow fallen last night. Mammoth hasn't seen a storm like this since 1986. Temperatures have been very warm up to yesteday evening, with snow levels at times above 8000'. The resulting snow has been very dense. Another foot+ expected today, with diminished, yet still moderate winds gusting out of the SW into the 50s. Temperatures are expected to be MUCH cooler with a high near 20 above 9000'.
Backcountry observations have been extremely limited at this time due to the dangers and difficulties of skinning travel in this type of snow.
Mammoth Mtn ski patrol and June Mtn ski patrol have both reported very sensitive avalanche conditions over the past 3 days with large slides releasing and propagating large distances, mostly in upper storm snow layers. Wide range of results on Mammoth Mtn ranging from foot crowns, to many 3-4' crowns, to an 8 ft crown in the Upper Cliffs this morning. Numerous natural slides have been reported, and there are sure to have been many many more that no one has been able to see. Quick observations made near Panorama Dome yesterday revealed 2 meters of new snow over the past few days, and stability tests indicating very sensitive surface instabilities. Click here for more details.What has been seen in the backcountry up to yesteday however indicates that the lower layers of this storm have been settling relatively quickly with the previously warm air-mass. However with the quick addition of this great amount of snow weight, it may be possible for deeper instabilities that were less of a worry before to become reactive.

Bottom Line: Extreme avalanche danger exists for the Mammoth area. High Avalanche danger exists for all other areas in the forecast region. A great deal of new snow, up to 10' has fallen around Mammoth with strong winds over the last several days, and will continue to fall today. Widespread avalanche activity reported by Mammoth and June Mtn ski patrols, with most paths capable of avalanching doing so this am as result of control work. Avoid avalanche terrain, including areas UNDER steep slopes as well. Be careful underneath roofs with snow build up as well!

The Tahoe National Forest Sierra Avalanche Center 
http://www.sierraavalanchecenter.org/advisory
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. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.

Today's Avalanche Advisory:


December 20th, 2010 at 06:46 AM
Near and above treeline, avalanche danger is CONSIDERABLE on NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in open areas on all aspects, 35 degrees and steeper.

Forecast Disscussion:
The storm system that has affected the forecast area since Friday is starting to taper off. Cold front passage occurred yesterday afternoon/evening decreasing air temperatures and snow level across the region. New snowfall amounts in the last 24 hours at 8,500' are 7 to 16 inches, bringing storm totals up to 29 to 49 inches with 8+ inches of rain water equivalent. Air temperatures this morning at 8,500' are in the upper teens with around 5 degrees of daytime warming expected. Ridgetop winds remain out of the southwest this morning and have decreased from gale force to moderate in speed. Winds are expected to further decrease in speed this afternoon. Snowfall will be much less organized today as a short wave passes through the area. This will produce snow showers with new snow accumulations highly varied by location. An additional 3 to 6 inches of new snow is expected to accumulate over most of the forecast area today with up to 12 inches forecast for the far southern portion of the forecast area (Alpine County).
Observations:
Yesterday on Silver Peak (Pole Creek area), evidence of a large recent natural avalanche was observed. The avalanche is believed to have occurred around 10:30am Sunday morning. The start zone was a convex wind loaded E aspect bowl above treeline at 8,350'. The crown was approximately 600' wide, 3' deep and the avalanche ran 1000 vertical feet. This avalanche was full clean out of the avalanche path and ran the full distance of the path (photos, more info). Below treeline at 7,500', ski cutting a convex test slope produced a small slab avalanche involving the top 6 to 8 inches of rain wetted new snow (photo, more info). Test pits were dug on this slope and a second location nearby in an area that was noted previously to have surface hoar up to 4 cm at the base of the storm snow. Both pits revealed no evidence of ongoing instability with this already collapsed surface hoar layer. Snow level had risen to around 7,700' by mid day in this area.
Avalanche Concern #1: Wind Slabs
The combination of new snow and wind over the past 24 hours is expected to have further enhanced existing slab formation in wind loaded areas. Natural slab avalanches remain possible today while human triggered slab avalanches remain likely near and above treeline on steep NW-N-NE-E-SE aspects. Most of the avalanche activity that occurs today is expected to involve new snow from the past 24 hours. However, large and destructive avalanches with snowpack failure deep within the recent storm snow remain possible.
Avalanche Concern #2: Loose snow avalanches

Cooling air temperatures since yesterday afternoon have placed lower density new snow on top of higher density new snow. In wind protected areas below treeline, this will limit slab formation and allow most snowpack failure to occur as loose snow avalanches (sluffs) involving the upper portion of the recent storm snow. Secondary terrain hazards such as cliffs, terrain traps, trees, and exposed rocks will drastically increase the consequences of becoming caught in a loose snow avalanche.

The bottom line:

Near and above treeline, avalanche danger is CONSIDERABLE on NW-N-NE-E-SE aspects, 35 degrees and steeper. Below treeline, avalanche danger is MODERATE in open areas on all aspects, 35 degrees and steeper.

Find your Avalanche Center n the United States - http://www.avalanche.org/

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