Tuesday, December 20, 2011

 This advisory expires in 24 hours

BOTTOM LINE:

Natural avalanches are unlikely, human triggered possible. Small avalanches in specific areas or large avalanches in isolated areas. Pay special attention to high elevation wind loaded pockets on steep North to West aspects and cross loaded gullies. Avoid skiing above or in terrain traps that can magnify the consequences of small and/or shallow avalanches. Choose runs with good runnouts.

TREND:

Improving stability, unless there are more wind events. Watch for NOAA predicted 60mph gusts this afternoon/evening. This could build more wind slabs that will require a new evaluation of the hazard.

DISCUSSION:

The first wind slabs from early last week bonded well to the old snow surface. Since then numerous high wind events have formed more wind slabs that have bonded fairly well to the old surfaces. Most observations have indicated that this bond is good, however there are some locations indicating an easy failure below the newest wind slabs, 6 – 12″ thick. This spatial variability should alert you to thoroughly evaluating the new wind slabs bond on your run of choice especially in wind loaded steep North to West aspects.

The rain crust layer from December 3rd is still a good layer to watch in your snowpits, however it has bonded well. This is a layer to watch for the future as it facets out and creates a possible weaker layer.

The snow surface has been scraped by strong SE winds. Ridgelines like Presidents and the standard Marmot up-track have been scoured down to rock. In some places the wind stripped the available snow down to the rain crust, in others it left shallow sastrugi. This crust can be firm ice or carveable stiff velvet. Gullies in these areas are loaded with pockets and pillowed wind deposits.

The skiing conditions are variable from wind slab to soft cakey powder in wind protected bowls.

–Jed Workman