Mar 9, 2012 @ 6:03 am

March 9, 2012 Avalanche Advisory

On slopes that are 35 degrees and steeper and on wind-loaded slopes there is MODERATE AVALANCHE DANGER in the West Central Montana backcountry. Human-triggered avalanches are possible. Expect the avalanche danger on the steep terrain to ramp up to CONSIDERABLE during the heat of the afternoon.

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for March 9, 2012.

 Weather and Snowpack Analysis

On Monday Tim reported on a persistent weak layer from surface hoar that formed in the early part of February. This layer is the primary concern right now. Over the last week I have found this layer in snow pits near Lost Trail Pass, in the Rattlesnakes, and in the crown of an avalanche near Seeley Lake. It was the weak layer in this snowmobile-triggered avalanche that happened last Sunday. You can read Steve’s incident report from this link or look in the documents link on our home page.

Yesterday in the Rattlesnakes this layer was failing with high stability scores that indicate strength; but when it does fail,  it fails very clean with lots of energy. This layer is 80cm to 1 meter (2.5 to 3 feet) from the surface of the snowpack. Observers near Lookout Pass, Lost Trail Pass and Hoodoo Pass also noted this layer and the possibility of a failure. As you can see in this photo it is a significant slab.

We found the slab to be most reactive during the heat of the afternoon. I would be extra cautious in the afternoon and take note of when the surface snow starts to move (about 1pm yesterday). It’s great to get out in the mountains during the warm weather; pick your terrain carefully during the warmest part of the day.

A secondary concern would be steep wind-loaded slopes above 7000 feet. We got a report from Downing Mountain Lodge of skier triggered slabs on 40 degree slopes this week. The winds have been pretty fierce over the week and steep leeward slopes above treeline may harbor wind slabs.

The accident near Seeley Lake was a bad deal; it looks like a sledder from another party triggered the slide that buried two people (fortunately, they were dug out unscathed) and it appears to be a “hit  and run”. The third sledder may not have known or looked for people on the slope below him; either way he didn’t stick around. There are a lot of folks out there recreating; I will quote from Steve’s accident report:

“In this era of powerful machines and lightweight efficient skis there are many more people recreating in remote, often dangerous, terrain. It is paramount that skiers and riders have a high level of situation awareness and be aware of not only the terrain and snow conditions, but also of the location of other people in the area. The actions of a single person can affect the safety of many.”

Weather Forecast and Avalanche Outlook

A ridge of high pressure is expected to continue over the region today. Then the weather will come from the southwest and warm temperatures will continue on Saturday with possible snow. A cold front will arrive Sunday causing a return to winter and possible significant snowfall; particularly over the Idaho border.

I expect the avalanche danger to remain the same with caution advised during the heat of the day on Saturday. The attention will then shift to the predicted new snowfall for Sunday.

I would like to thank the folks who are sending in public observations again. It’s great for all of us to have the extra information.

If you get out and see avalanche activity or want to send us quick snow observations please use our public observations form link on our home page or write us at [email protected] with any observations or questions.

Have a great weekend and ski and ride safe!


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 the information can help you make a more informed decision regarding travel in avalanche terrain for the next few days.

Our advisory area includes National Forest System lands in the Bitterroot Mountains from Lost Trail Pass north to Granite Pass, the Rattlesnake Mountains north of Missoula and the Southern Swan and Mission Mountains near Seeley Lake, MT. Avalanche information for the Lookout Pass/St. Regis Basin area is available from the Idaho Panhandle Avalanche Center.