Special Warning: High Avalanche Danger
Dangerous conditions for most of all mountain ranges in NW Montana
by Stan Bones, USFS Glacier Country Avalanche Center
January 2, 2009
Special backcountry avalanche warning issued by the U.S. Forest Service Glacier Country Avalanche Center at 7:00 AM, Friday, January 2nd, 2009, for the Glacier Park and Flathead and Kootenai National Forest areas.
HAZARD ANALYSIS: Because of heavy new snowfall, we are currently rating the avalanche danger as HIGH over most all of the mountain ranges of Northwestern Montana.
Beginning late Thursday night a strong Pacific weather system began moving into NW Montana. Locations in the East and West Cabinet Ranges on the Kootenai and in the mountains around the Flathead and Mission valleys registered 0.4-0.8" of new snow water equivalent loading, in a 9-hr period Thursday night and into early Friday morning. This heavy new snowfall combines with 4-5" of new snow water equivalent received over the last seven days. The new snowfall over the last week has begun to completely bury the vegetative and terrain anchors that were widely existing earlier.
The new snowfall is building a more consolidated, dense, slab layer which is now sitting atop the weakly bonded and lower density snow received earlier in December. New snowfall Wednesday was also accompanied by moderate to strong winds in many locations. Conditions have combined to produce an unstable, inverted snowpack with higher density surface snow over-topping less dense, more weakly bonded buried snow.
Because of heavy new snowfall, often combined by ridge and mountaintop winds, unstable slab layers are likely on steep, open terrain in all of the mountainous areas of Northwestern Montana. Both natural and human triggered avalanches are likely. Very dangerous avalanche conditions exist. Travel in avalanche terrain is not recommended. Extensive skill, experience, and local knowledge are essential to safely travel in the backcountry currently.
The weather forecast is for snowfall to continue Friday, bringing a total of 6-12" of new snow depth to the valleys and mountains of NW Montana. Temperatures are expected to drop into the teens and single digits Friday night and Saturday as colder air moves in behind the Pacific front. Mountain winds are forecasted to be moderate through the weekend, blowing 5-15 MPH, mostly from the W-SW. These conditions should maintain the avalanche danger at an elevated level through the weekend.
This advisory message will next be updated on Tuesday, January 6th, 2009.
AVALANCHE DANGER RATING KEY
LOW DANGER Snow is generally stable with only isolated areas of instability. Natural avalanches are very unlikely, while human triggered avalanches are unlikely. Backcountry travel is generally safe. Normal caution is advised.
MODERATE DANGER Unstable slab layers are possible on steep terrain. Natural avalanches are unlikely, while human triggered avalanches are possible. Backcountry travelers should use caution in steeper terrain on certain aspects.
CONSIDERABLE DANGER Unstable slab layers are probable on steep terrain. Natural avalanches are possible, while human triggered avalanches are probable. Backcountry travelers should be very cautious in steeper terrain. Be aware of potentially dangerous areas of unstable snow.
HIGH DANGER Unstable slab layers are likely on a variety of aspects and slope angles. Both natural and human triggered avalanches are likely. Travel in avalanche terrain is not recommended. Safest travel is on windward ridges or on lower angle slopes without steeper terrain above.
EXTREME DANGER Extremely unstable slab layers exist on most aspects and slope angles. Widespread natural and human triggered avalanches are certain. Large destructive avalanches are possible. Travel in avalanche terrain should be avoided and travel confined to low angle terrain well away from avalanche path runouts.
For more information contact Stan Bones, Flathead National Forest, U.S. Forest Service Glacier Country Avalanche Center, 650 Wolfpack way, Kalispell, MT 59901, Phone: 406-758-5284 phone, Fax: 406-758-5363.
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