Eastern Sierra - Wed Dec 23 2009

Eastern Sierra avalanche advisory

Posted December 23, 2009 by Sue Burak

Updated December 23, 2009 - 11:04 pm


Danger Rating: Moderate

The avalanche danger rating is estimated to be MODERATE on all wind loaded terrain. Small human triggered avalanches are possible in steep wind loaded terrain 35 degrees or greater on north and southeast to west facing slopes. Evaluate snow conditions carefully off ridgetops, in exposed high elevation bowls with recent wind loading and in convex terrain.

Hazard Assessment

The avalanche danger rating is estimated to be MODERATE - there are potential avalanche conditions in wind loaded terrain on all north facing and west facing terrain. Strong, sustained north winds will continue through tomorrow afternoon so be aware of hollow or drum like sounds and thick deposits of wind drifted snow. During MODERATE avalanche conditions, human triggered avalanches are possible. Evaluate snow conditions carefully off ridgetops and in exposed high elevation bowls with recent wind loading and in convex terrain. The snowpack is shallow, ranging from 1 foot around 7,000 ft to 4 ft in favored accumulation areas like the Mammoth Lakes Basin. I have not been out enough in the snow to provide a general description of the base of the snowpack, though I have seen thin depth hoar layers of about 1 inch sitting below 3 to 4 inches of coarse textured, breaks apart in your hand, Sierra depth hoar. Ice at the base of the snowpack has been reported on Mammoth Mountain. Based on these limited observations, the potential for large avalanches collapsing on weak snow on the ground is not very likely for now. A return to an extended period of dry, cold weather could change this situation - remember last January when there was no snow for three weeks! And finally, snowpacks are thin (1-4 ft) and about half of what they should be this time of year. Rocks and logs can be hidden just under the surface and ruin your day- watch for rocks!

 

During the storm, strong sustained west to southwesterly winds blew snow onto north facing slopes. As the storm moved east to Nevada and Utah, strong north winds gusting over 70 mph stripped the new storm snow from high elevation north facing bowls, up and over the ridges onto southwest to west facing slopes. Not all the snow was stripped from north terrain because wind eddies formed in the lee of the ridgetops and redeposited snow back onto north facing terrain. These wind eddies created localized areas of 2-5 inch thick wind slabs - we know and love this snow as breakable wind board. These thin slabs were easy to trigger but did not travel far down the slope. The storm snow appears to have bonded to the old snow surface in north facing trees.

The storm that brought up to 10-12 inches of new snowfall to the higher elevations moved to east and south yesterday. Strong, relentless north and northeast winds up to 100 mph will continue to plague high exposed terrain today. Temperatures will be cold with highs in the mid to upper 20's and lows in the teens to single digits for the next couple of days. As a high pressure ridge builds over the West Coast, the cold, dense air mass will create fog in the Mono Lake and Crowley Lake areas Wednesday through Christmas Eve. A weekend storm will pass to the north. Otherwise, below normal precipitation and normal temperatures are expected for the 8-14 day period.

There have been no reports of natural avalanches.