Feb 20, 2009 @ 12:00 am

February 20 Avalanche Advisory

Posted Friday February 20th, 2009 at 0600.

Good morning! This is Steve Karkanen at the West Central Montana Avalanche Center with the avalanche advisory for February 20th, 2009. 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. Expect avalanche conditions to change as weather conditions change. Our advisory area includes the Bitterroot Mountains from Lost Trail Pass North to near Lookout Pass, the Rattlesnake Mountains and the Southern Swan and Mission Mountains near Seeley Lake. Avalanche information about the St. Regis Basin can be found on the Idaho Panhandle Forest Avalanche Center website.

Weather and Snowpack Analysis

Most mountain locations received a trace of precipitation since Sunday with the Northern portion of the advisory area receiving the most snow. North Fork Jocko SNOTEL received .80 inches of snow water equivalent or about 6-8 inches of snow. Most of the other sites received 3-4 inches since Sunday. Mountain temperatures were in the mid thirties for highs and in the teens for lows.

In Monday’s advisory I discussed a persistent weak layer that hasn’t been much of a problem because it did not have a slab above it. Wind, warmer temperatures and a bit more snow have formed a slab above this weak layer and it fails on steeper slopes with the right kind of force applied. We’ve received excellent reports from several different locations in the Bitterroot Mountains, Sheep Mountain and the Rattlesnake which described similar weakness and potentially dangerous avalanche conditions with the next big storm. This is perhaps best illustrated by the YouTube clip from our tour in the Rattlesnake Wilderness on Thursday. This appears to be a widespread condition above 7000′ on East to North aspects in West Central Montana.

February 19 Rattlesnake Rutschblock stability test.

Current Avalanche Danger

Above 6000′ on wind loaded terrain steeper than 35°, the avalanche danger is MODERATE. Natural avalanches are unlikely but human triggered avalanches are possible. All other slopes have a LOW avalanche danger where natural avalanches are very unlikely and human triggered avalanches are unlikely. You can almost always find isolated pockets where there are dangerous avalanche conditions on some terrain features.

Weather Forecast and Avalanche Outlook

A ridge of high pressure will build over the region Friday bringing a warming and drying trend to the mountains with valley inversions and fog setting up over the lower terrain. A moist Pacific system will approach our area Sunday night with warm air and widespread precipitation. Snow levels are expected to be between 5000′ and 6000′ on Monday.

It doesn’t appear that we’ll see significant snow at least for the next few days. The predicted warm weather (into the mid-forties at high elevations Saturday) should help stabilize the persistent weak layers we’re worried about. Expect avalanche conditions to improve for the next few days. Wet snow avalanches may be a concern on the Southern aspects during sunny days. These will initially involve the surface snow but once wet and heavy snow starts moving it can overload more deeply buried weaknesses all the way to the ground. Remember, we still have mature facets (depth hoar) at the ground in most locations.

If you have any information you’d like to share or have questions about anything related to snow safety, please contact us at [email protected].

Dudley Improta will issue the next avalanche advisory Monday February 23rd.


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.