Jan 20, 2012 @ 6:32 am

Avalanche Warning 1/20/2012

On slopes that are steeper than 35 degrees there is HIGH AVALANCHE DANGER in the West Central Montana backcountry. The advisory area has received more than a third of the total mountain snowpack this week. This big load, accumulated in such a short time, is causing widespread instabilities on steep slopes. Strong winds accompanying this storm have made leeward slopes particularly dangerous.

Good morning, this is Dudley Improta with the West Central Montana Avalanche Center’s avalanche advisory for January 20, 2012.  We are continuing an avalanche warning for West Central Montana. Keep in mind that we have had avalanche accidents in gullies and on steep slopes below 5000 feet during and after heavy snowfall events associated with strong winds.

Weather and Snowpack Analysis

The storm that started Tuesday has put down some impressive amounts of snow. The storm came in cold and then went warm. This density change that occurred during the storm is the weakest part of the snowpack right now. All observers are getting very low scores in stability tests on this weakness. I’m sure many skiers at the local ski areas will tell you the most recent snow is a bit thicker and heavier than the “blower powder” they waited in line for yesterday.

On our tour in the Rattlesnakes yesterday we remotely triggered the slab that sits on this weak layer when we approached a north slope. There was also a large fracture reported on a northeast slope that ran very deep (about 2 feet) and slid on a rime crust that formed on January 7. Using explosives, the Snowbowl patrol triggered deeper slabs Thursday morning than they did on Wednesday morning.

This video we shot yesterday shows the slab formation.

I got a couple of calls from Carl up at Yurtski in the southern Swan Mountains. They measured 34 inches of snowfall in two days with continuing snow. At one point they measured 6 inches of snow in 2 hours. They are seeing surface movement and getting collapse noises and very low scores with their stability testing. Their weak layer is at that temperature change in the storm we mentioned previously. Needless to say they are sticking to very low angle terrain and doing some serious shoveling around the yurt!

The Lookout Pass and St. Regis Basin area was also looking at over 30 inches of new snow. Observer Dan Frigard was also getting low stability test scores in the storm temperature change. Dan also noted very strong winds over the past couple of days which have loaded lee slopes.

Reports from Lolo Pass and Lost Trail Pass both indicated over 2 feet of snow in the last 3 days with strong winds.

Weather Forecast and Avalanche Outlook

We should see steady precipitation for the next several days. Snow levels may rise Friday causing the potential of rain up to 5000 feet. Any rain on a steep slope will immediately cause the snow to become unstable.

Gusty west and southwest winds will load leeward slopes on Saturday.

This most recent warmer, higher density snow is the type of snow that should settle quickly. Can it settle quicker than the instability of the new loading that is expected? That is the million dollar question. In the next 48 hours I would expect the avalanche danger to drop to CONSIDERABLE. But no promises; I would exercise extreme caution for the immediate future.

I will issue the next advisory Monday, January 23.

A BIG THANK YOU! to those of you who braved the Missoula roads to attend community unite benefiting Missoulaavalanche at the Kettlehouse last Wednesday.

For those of interested in looking at snow profiles from this season we are posting them in a folder of 2012 snow profiles in our photo section.

If you would like send us quick snow observations please use our public observations form on the home page or write us at [email protected] 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 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.