MissoulaAvalanche.org

Avalanche Information for West Central Montana

January 23, 2012 Avalanche Advisory

Current Danger Rating is CONSIDERABLE

On slopes above 6000 feet in elevation and steeper than 35 degrees there is CONSIDERABLE AVALANCHE DANGER in the West Central Montana backcountry. The instability in the upper snowpack from last week’s storm has gained strength; but old, persistent, weak layers in the snowpack still exist.

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for January 23, 2012. On slopes above 6000 feet in elevation and steeper than 35 degrees the avalanche danger is CONSIDERABLE. On other terrain in the advisory area steeper than 30 degrees there is MODERATE avalanche danger.

Weather and Snowpack Analysis

After the big dump from last week we continued to receive snow. The rain level went to almost 6000 feet on Saturday and then the temperatures cooled back down. Most of the area received from 8 to 11 inches of snow on Friday and Saturday. Snotel sites indicate the Hoodoo Pass area and the central Bitterroots have received the most snow (hence the most load) since Friday.  Gusty west and southwest winds have loaded and side-loaded slopes throughout the advisory area. The higher, steeper terrain is holding the instabilities right now and is our major concern.

The upper instability in the snowpack, due to temperature differences in last week’s storm, has strengthened. We are, however, still seeing weaknesses from the older, persistent layers of facets and buried surface hoar we have been describing since December. These persistent layers are a tricky avalanche problem. You don’t see the obvious signs of avalanche danger; there is no cracking or collapsing, or recent avalanche activity. Careful evaluation of snowpits along with identification of recently loaded slopes is necessary to avoid avalanche hazard. These are not the avalanche hazards we associate with new storm loads. The likelihood of triggering one of these slabs is moderate; the consequence of triggering one is high.

This video, Steve and I made yesterday, show how weak these older layers are.

Yesterday, Tim and Dave toured near Lolo Pass and found a much more stable snowpack at just above 6000 feet. But, they did observe snow transport on the higher Bitterroot peaks to the south of their location.

Observer Rich Raines got out yesterday also and found one of these persistent weak layers just over 7000 feet on a southeast slope neat Lost Trail Pass. His stability tests were failing much like the test in the video from the Rattlesnakes.

Weather Forecast and Avalanche Outlook

There is a winter weather advisory that ends this morning at 11am. The mountains along the Idaho border should receive most of the snow from this current system. After a short break it looks like widespread and steady precipitation beginning Tuesday night. Temperatures may again be on the warm side with rising snow levels. This means heavier snow with more potential for loading.

I would expect the snow to strengthen ever so slightly the next couple of days; then we will see what the new storm does on Wednesday. These older, weak layers are going to be with us for some time; I urge all winter backcountry users to look carefully for weaknesses in the snowpack and recent loading, due to snowfall and/or wind, before venturing onto or near steep slopes.

Tim will issue the next regular advisory Friday, January 27.

Ski and ride safe!

We have had good response from you on our public observations page. We sincerely appreciate the extra effort; it gives us all more information to work with.

If you get out want to send us quick snow observations please use our public observations form on the home page missoulaavalanche.org or write us at info@missoulaavalanche.org with any observations or questions.

This information is the sole responsibility of the Forest Service and does not apply to operating ski areas. The avalanche danger rating expires at midnight tonight but you can use the information we provide to help you make more informed decisions regarding travel in avalanche terrain for the next few days.

Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to Hoodoo Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information for the Lookout Pass/St. Regis Basin is available from the Idaho Panhandle Avalanche Center.