Dec 21, 2012 @ 6:11 am

December 21, 2012 Avalanche Advisory

Avalanche Danger in the west central Montana backcountry is rated as MODERATE. It is possible to trigger avalanches on wind-loaded slopes 35 degrees and steeper.

Good Mayan morning! This Dudley Improta with the West Central Montana Avalanche Center’s advisory for December 21, 2012.


Weather and Snowpack Analysis

During this last week the advisory area received decent amounts of snow. Over the last few days storms dumped up to 15-20 inches along the Idaho border near Lookout pass to Lolo Pass. The Rattlesnakes, southern Missions and southern Swans received about 10-15 inches of new snow. The most recent event from the last 48 hours appears to have skirted the southern end of the area. Although our overall rating is MODERATE; potentially triggered avalanches will be larger in the Lookout / Hoodoo region because that area received more snow.

The last storm came in with high winds across the area. All observers mentioned wind-scouring and wind deposits. Our main problem is wind slabs that may have formed on high exposed terrain. Any leeward aspect is suspect; as winds shifted from west / southwest to east / southeast during the last 48 hours.

A secondary problem is weakness in the top part of the snowpack. That weakness may be 10 inches from the surface (Southern Missions, Rattlesnakes) to 24 inches from the surface (Lookout Pass, Hoodoo Pass). Generally, these weaknesses are associated with surface hoar formation from last week. Snowpit tests showed this weakness to be relatively touchy near Hoodoo Pass; and yesterday, the Snowbowl Patrol was able to trigger small soft slabs on Southwest slopes while ski-cutting.

At Lolo Pass the observers noted an overall strong snowpack below 7000 feet but were concerned about the aforementioned wind-loading. Observer Dave Williams provided this video from Lolo: ECT Lolo Pass Dec. 12, 2012 .

There is some good riding and skiing to be found out there for sure; but steep slopes should be suspect and approached cautiously. Take time to check out the snowpack carefully if you’re considering recreating on steep terrain.

On the bright side; snotel sites in the area indicate the snow is settling rapidly, a sign of a stabilizing snowpack.

You can view current season snowpit profiles here: 2012/2013 SnowProfiles .

Weather Forecast and Avalanche Outlook

A southwesterly flow has resumed and will persist through Saturday producing snow showers. A weak disturbance Saturday night may enhance snowfall. A cold arctic air mass is predicted to move in and produce lower temperatures on Sunday.

With cooling temperatures and some additional amounts of snow I would expect the avalanche danger to remain at MODERATE. Wind slabs and weaknesses in the upper snowpack will remain the main problems.

Happy Holidays! If you head into the backcountry this weekend, ski and ride safe!

Steve (Santa Claus) Karkanen will issue the next advisory Tuesday, Christmas Morning, 2012.

As we enter into another skiing & riding season, I would like to extend thanks to folks who post public observations or sent us reports and information. Quite a few people in the area get out a lot and we have a large forecast area; it helps to hear if we missed something or get a confirmation of the advisory. I believe everyone who uses the advisory appreciates your contributions.


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.