Western Wyoming Avalanche Bulletin
December 31-January 3, 2007
by Bridger-Teton National Forest Avalanche Center
December 31, 2006
Issued: Sun December 31, 2006 4:15 PM
SITUATION
High pressure over Western Wyoming began to break down as clouds from an approaching weak disturbance moved over the area. Temperatures at 9000 feet throughout the region climbed into the mid and upper twenties before the cloud cover obscured the sun. Cold valley temperatures rose into the low teens but inversion conditions continued. Light winds varied but were generally westerly at ten miles per hour at ridge tops. Surface hoar has persisted on most aspects at low elevations and in shaded areas at mid and upper elevations. Sun crusts are up to one inch thick on southerly aspects but with the low angle of the sun at this time of year the crusts are restricted to areas with direct sun exposure.
In the Tetons, small surface slabs continue to be triggered on wind loaded slopes near ridge crests: just below the summit ridge on northeast aspects of Mount Taylor and the north side of Disappointment Peak in Grand Teton National Park. In addition to the possibility of these small slides, deeper instabilities of weak faceted snow continue, and are more widespread in the shallower snowpacks of the Greys River and Togwotee Pass areas. They can also be found in the Tetons on scoured, windward aspects and areas of thin coverage over rock outcroppings. Substantial triggers, such as snowmobiles or large groups, are more likely to initiate such a slide, but smaller triggers may be enough if a shallow, weak zone is encountered. The trigger points may be the top, middle or base of such slopes. These types of slides are very unpredictable but the consequences can be severe.
FORECAST FOR Monday January 01, 2007
A weak disturbance will bring a chance of very light snow to the upper elevations tonight into Monday morning. Winds will be northerly at ten miles per hour and 9500 foot temperatures will rise into the upper teens. High pressure will begin to build again by Monday night.
The General Avalanche Hazard is expected to remain MODERATE. Pockets of surface slab 6 to 12 inches in depth exist, and could be triggered on steep, wind loaded aspects near ridge crests. Deeper instabilities associated with buried weak layers of faceted snow persist, especially in steep, rocky areas with a shallow snowpack.
TREND FOR TUESDAY AND WEDNESDAY
Tuesday and Wednesday will be dry under high pressure and inversion conditions then a chance of accumulating snow returns Wednesday night. The Avalanche hazard will slowly increase in shallow snowpack areas as faceting continues to develop.
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Extreme: Wide spread areas of unstable snow exist and avalanches are certain on some slopes. Backcountry travel should be avoided.
High: Mostly unstable snow exists on a variety of aspects and slope angles. Natural avalanches are likely. Travel in avalanche terrain is not recommended.
Considerable: Dangerous unstable slabs exist on steep terrain on certain aspects. Human triggered avalanches probable. Natural avalanches possible.
Moderate: Areas of unstable snow exist. Human triggered avalanches are possible. Larger triggers may be necessary as the snowpack becomes more stable. Use caution.
Low: Mostly stable snow exists. Avalanches are unlikely except in isolated pockets.
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Related Links:
Bridger-Teton National Forest Avalanche Center
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