Avalanche Warning December 30, 2010

This is Steve Karkanen with an avalanche warning for Thursday December 30, 2010. The 2011 New Years Resolution for everyone is to avoid being caught in an avalanche!

Current Avalanche Danger

An AVALANCHE WARNING is in effect for the mountains of West Central Montana above 5000 feet. This includes the Bitterroot Mountains from Lost Trail to Lookout Passes, the Rattlesnake and Southern Swan and Mission Mountains north of Missoula.

Throughout the entire advisory area at elevations above 5000 feet, the avalanche danger is HIGH on all wind loaded terrain steeper than 30 degrees. Very dangerous avalanche conditions exist on this terrain, natural avalanches are likely, human-triggered avalanches are very likely. Travel in avalanche terrain is not recommended. Winds have been from the W-SW during the heavy snow event so N to E slopes will be the most suspect.

Other slopes may be cross-loaded and sheltered steep slopes steeper than 30 degrees have a CONSIDERABLE avalanche danger. Natural avalanches are possible, human-triggered avalanches are likely. Careful snowpack evaluation, cautious route-finding and conservative decision making is essential for safe travel in avalanche terrain.

Avalanche Incident

An avalanche incident occurred in the Rattlesnake Wilderness yesterday. Preliminary information is still being gathered but two individuals were caught and one slightly injured in an avalanche triggered on an east aspect about one half air mile from Snowbowl Ski Area. More information will be posted once we have complete data.

Weather and Snowpack Analysis

Nearly 2.5 inches of snow water equivalent or close to 20 inches of snow has fallen in the northern part of our advisory area at Stuart Peak. Montana Snowbowl is reporting 15 inches of new snow and the ski patrol reported significant avalanche activity during avalanche control this morning. Winds are currently 15-20 mph from the west at the top of Snowbowl.

The primary concern is the new load failing at the new-old snow interface where we were finding a variety of crusts and near surface facets on our tours last Sunday. A secondary concern is the now deeply buried surface hoar that was still showing signs of weakness in spotty locations. 2 inches of SWE is a lot of weight to be dropped in a 24 hour period and snow needs time to adjust to this shock. I would not be surprised to see a soft slab avalanche step down to this deeper weakness. This would be an unsurvivable and destructive avalanche.

The below video was shot last Sunday and gives you a good idea why we are concerned with the added weight:

During our stability testing in several pits the buried surface hoar that formed in early December is still an obvious concern and was giving us very mixed results yesterday. The video we shot of a compression test then an extended column test tells the story best. The main concern is the tendency for the now stiff slab to propagate across the slope with energy. The spotty, variable nature of this persistent weakness is still a problem so we strongly recommend looking for it before committing to anything steep.

Weather Forecast and Avalanche Outlook

 BACKCOUNTRY FORECAST FOR WESTERN MONTANA AND NORTH CENTRAL IDAHO
 NATIONAL WEATHER SERVICE MISSOULA MT
 310 PM MST WED DEC 29 2010
HEAVY SNOW WILL CONTINUE TO ACCUMULATE OVER THE HIGH TERRAIN OF
WESTERN MONTANA AND NORTH CENTRAL IDAHO. STORM TOTAL SNOW
ACCUMULATIONS WILL BE MEASURED IN FEET BY THURSDAY MORNING.
LITTLE TO NO ADDITIONAL SNOW ACCUMULATION IS ANTICIPATED THURSDAY
 THROUGH SATURDAY. BRUTALLY COLD TEMPERATURES ARE ANTICIPATED
THURSDAY THROUGH SATURDAY AS ARCTIC AIR MOVES WEST OF THE
CONTINENTAL DIVIDE. AFTERNOON TEMPERATURES IN THE BACKCOUNTRY
WILL STRUGGLE TO REACH ZERO DEGREES ON FRIDAY AND WILL NOT BE
MUCH WARMER ON THURSDAY OR SATURDAY.

The avalanche danger is HIGH in many areas. This is a good time to ski within the boundaries unless you have excellent route-finding and snow assessment skills.

Dudley will update the New Years Eve advisory. Enjoy the new snow but above all, be safe, so you can enjoy the new year!