Apr 7, 2011 @ 10:34 pm

April 8 Avalanche Advisory

At all advisory area locations above 5000 feet the avalanche danger is MODERATE. Natural avalanches are unlikely but human-triggered avalanches are possible. Small to moderate size avalanches can be triggered on steep wind-loaded slopes.

Also above 5000 feet, pockets of CONSIDERABLE avalanche danger exist on wind-loaded terrain steeper than 35 degrees where wind slabs have formed during recent periods of heavy new snow and high wind.

Under these conditions natural avalanches are possible and human-triggered avalanches are likely. There can be many small avalanches over a large area or large avalanches on the more heavily loaded steeper slopes. Careful snow evaluation, cautious route-finding and conservative decision-making is essential. Move off of and out from underneath potential avalanche paths when you start seeing evidence of weakened conditions such as pinwheels or roller-balls, small point release avalanches, or sinking in or punching through the snow surface as you travel.

The avalanche danger is LOW at other mountain locations below 5000 feet.

Hello! This is Steve Karkanen with the West Central Montana Avalanche Center’s backcountry avalanche information report for Friday, April 8, 2011. Please note that this will be the last regular avalanche advisory for the 2010-2011 season.


Weather and Snowpack Analysis

A relentless display of winter weather characterizes nearly all mountain locations above 5000 feet in western Montana. Temperatures range from daytime highs in the mid-thirties to nighttime lows in the twenties. A series of fast moving systems passed through the area dropping several inches of snow and high winds with each turn.

Heavy snow favored higher elevation sites in the Bitterroot mountains with Hoodoo Basin and Twin Lakes SNOTEL each receiving close to 20 inches of snow and close to 4 inches of snow water. Other locations around the passes and the Rattlesnake mountains received about half as much precipitation.

Last Friday and Saturday it rained to 8000 feet and the resulting rain crust is evident in many places. Observers in the southern Bitterroot report that the newest foot of snow is not yet bonding to this layer and easily slides on slopes steeper than 35 degrees. They feel that the avalanche danger is elevated in this area.

Further north in the Rattlesnake, several melt-freeze crusts are evident in the upper 2 feet of the snowpack with the new snow failing during stability testing but strengthening with time. We still find isolated pockets of weakness that demands attention. This YouTube video shows Tim performing a stability test where he was able to get a fracture propagation with energetic failure. The slab failed on an old layer of mixed small facets and preserved graupel about 2 feet deep on a northwest aspect.

Although this feature is not widespread, is buried deeply and takes a lot of force to get it to rip out, it is worth noting as the weight of a sled, more than one person on a steep slope or disturbing a weak spot could trigger a slide with unsurvivable consequences.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting unstable conditions again Friday with light snow accumulations possible. Temperatures warm over the weekend as high pressure builds over the region. Another storm system is expected to arrive with an increased chance for more snow late Sunday.

Heavy new snowfall and high wind equals avalanches. We appear to be in a cycle of continued pacific storm systems rapidly moving through the area with significant precipitation embedded within these systems.

Expect avalanche conditions to remain similar until we come under the influence of more stable weather. During warm or sunny days, expect the wet snow avalanche danger to rapidly rise to CONSIDERABLE on any slope exposed to the sun.

No further avalanche advisories will be posted for the remainder of the 2010-2011 season. All our regular Forest Service observers have moved on to other commitments and the number of public observations has dropped off in the past 2 weeks.

We will continue to post information sent to the public observations page so if you get out and see avalanche activity or have information that would benefit other backcountry travelers, please use this feature. You can also send us a quick note at [email protected].




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.