Posted:
Mar 12, 2013 @ 6:05 am

March 12, 2013 Avalanche Advisory

There is MODERATE avalanche danger in specific terrain in the west central Montana backcountry. Above 7000 feet in elevation, on slopes that are exposed,wind-loaded and 35 degrees or steeper, it is possible to trigger wind-slabs. Above 7000 feet in elevation, on slopes that are 35 degrees and steeper; and where the snow is shallow (5 feet or less) it may be possible to trigger slab avalanches. Elsewhere in our advisory area the avalanche danger is LOW.

Good Morning! This is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for Tuesday, March 12, 2013.

Weather and Snowpack Analysis

Mountain temperatures are mostly in the mid-twenties this morning. Above 7000 feet, winds are blowing at 20mph, generally out of the west and northwest. Wind, warm temperatures and light accumulations of snow were the weather story over the weekend. Yesterday, weather stations recorded winds steady in the 20mph range, with gusts to 40mph at 7000-8000 feet. Temperatures reached 38 degrees F at 7900 feet in the Bitterroots on Saturday.

While the warm temperatures have helped settle the snow; the relentless winds have moved what snow was available to the leeward slopes and created unstable pockets with wind-slabs. On Saturday we had a report of a skier-triggered wind-slab on a northeast aspect at 8000 feet in the southern Missions. This wind-slab was 12 inches deep and ran a 100 feet. Yesterday, in the Rattlesnakes, Steve and I found small, sensitive wind-slabs on an east aspect at 7100 feet. We could observe snow being transported on more open, exposed ridges.

The other avalanche problem is a persistent weak layer that has been noted in observations from the Rattlesnakes to Lost Trail Pass. This weak layer formed in February and continues to show some energy when it fails in our stability tests (VIDEO) . This is a classic “low probability/high consequence” avalanche problem. This situation seems to be confined to high elevations where the snowpack is 5 feet or less in depth.

The Bottom Line

Our snowpack is mostly stable, although the warm weather has limited opportunities to ride soft powder. Wind-slabs are a potential problem on steep, open, exposed slopes above 7000 feet. The persistent layer (mentioned above) deserves consideration if riding or skiing steep terrain above 7000 feet; particularly if the total snow depth is less than 5 feet. During the warmest parts of the day or if it rains at elevation (yes, spring is coming), steer clear of the large cornices that have developed.

Weather Forecast and Avalanche Outlook

Accumulations of snow should be on the light side this morning. High pressure is expected to build this evening with” warmer than normal temperatures”.  Some precipitation is expected Wednesday with rain up above 6000 feet by Thursday.

If the winds abate, and with the warmer temperatures predicted, I would expect the snow to settle. However, possible rain above 6000 feet has my attention. The immediate effect of the rain will destabilize the snow and the large cornices.

If you would like to report on avalanche or snow conditions use our public observations form or send us a note at [email protected] .

I will issue the next advisory on Friday, March 15.

Ski and ride safe and have a great week.

READ FULL ADVISORY  

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.