Eastern Sierra - Sun Jan 31 2010
Eastern Sierra avalanche advisory |
The avalanche danger rating for north to east facing slopes steeper than 35 degrees is MODERATE for the Rock Creek and Bishop Creek areas. There is a possibility you could trigger a small slab avalanche in convex or steep windloaded terrain in the trees and above treeline. If you are unlucky, you could trigger a large avalanche in steep rocky area where a shallower snowpack makes it easier to collapse the underlying weak layer and trigger an avalanche across a wider slope. |
Hazard Assessment
Despite little snow accumulation over the last week and mild daytime temperatures, the snowpack south of Mammoth is sleeping like a dragon with one eye open. Thanks to the efforts of Sierra Mountain Guides’ Level 3 avalanche training course, we got a comprehensive look at the snowpack from 7,000 to 9,500 ft on north and east exposures in the McGee/Rock Creek area on Friday. Observations on Saturday in the Bishop Creek area were very similar to Friday’s results.
Above 8400 ft, all snowpit stability tests showed a slab of snow ranging from 50 cm to over a meter from the Martin Luther King week of storms, sitting on a thin layer of weak facets. These faceted grains rested on top of an old melt freeze or high density snow/rain mix crust sitting on facets and depth hoar. This bottom layer is the old snowpack from December and the first part of January.
The contrast between slab hardness and the hardness of the faceted layer, and consistent stability test results, indicating slab propagation on a variety of slope angles and aspects, should be a red flag to backcountry travelers. The trend of cooling temperatures over the next few days promotes faceting so weak snow will stick around for a while and remain weak.
The avalanche danger rating is only a guideline. How do you tell if the slope or gully you plan to ski won’t slide?
Today will be the final appearance of sunny skies for a week. Temperatures continue to fall with today’s highs at the 8,000 to 9,500 ft elevations in the upper 20's to low 30's. Nights will be colder with low temperatures around 15F. Higher terrain will be cold with highs in the low 20's and lows in the 10-15 F range. Winds at all mountain elevations are forecasted to be light from the west.
A short lived ridge of high pressure builds in from the west as the cyclonic flow with southwest winds transitions to weak flat ridging. Monday will have varying amounts of mid to high clouds. A storm centered off the Washington and Oregon coast rotates towards the region with several weak impulses moving towards the west coast beginning Monday. General circulation shows zonal flow across the Pacific with storms lined up from Asia to the West Coast.
Please note that the avalanche danger rating in this advisory expires in 24 hours. This advisory is our best interpretation of snow pack conditions and NWS forecasts issued today. Backcountry travelers should be aware that elevation and geographic distinctions are approximate and that a transition zone exists between upper and lower elevations. Avalanches do not happen by accident and most human involvement is a matter of choice not chance. Most avalanche accidents are caused by slab avalanches that are triggered by the victim of member of the victim's party. Even small slides can be dangerous. Always practice route finding skills and carry avalanche rescue gear. Remember that avalanche danger ratings are only general guidelines. Distinctions between geographic areas, elevations, slope aspects and slope angles should be made.
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