February 21, 2011 Avalanche Advisory

The avalanche danger is MODERATE above 5000 feet in west central Montana. Natural avalanches are unlikely; human-triggered avalanches possible.

The avalanche danger is LOW below 5000 feet in west central Montana. Natural and human-triggered avalanches unlikely.

This is Steve Karkanen with the backcountry avalanche advisory for President’s Day, February 21, 2011.

MODERATE avalanche danger means that there are heightened avalanche conditions on some terrain features. The main concern today is with wind-slabs that have formed the past few days.

Weather and Snowpack Analysis

Mountain temperatures have cooled and the wind has calmed down but is still moving a lot of snow around. The wind Saturday and early Sunday was from the east in the Rattlesnake and Seeley Lake areas so many of the slopes that were skiing well prior to the weekend are now wind-scoured or at least wind-packed making for challenging skiing. High elevation open areas now have a wind-crust evident on all aspects.

It is possible to find wind-slabs on many terrain features and the more dangerous of these may be lurking in areas that are cross-loaded several yards below ridgetops.

The snow we’ve been getting the past few days is cold and dry therefore easily blown around. In places where it has a chance to accumulate it is not forming a slab and is very good skiing and riding. Really not much of an avalanche concern.

Where ever this dry snow is exposed to wind it is getting packed into solid hard-slabs that appear to be quite strong. You can easily walk around on them and they are resistant to the ordinary forces that the weight of new snowfall or a skier adds to a steep slope.

The problem with slabs of this nature is the degree of difficulty in getting out of one if you do get caught

We are finding these slabs on nearly all aspects which is making stability assessments tricky. Fortunately, the overall stability is very good, you just have to be thinking about where the wind has moved all the snow we got last week.

Weather Forecast and Avalanche Outlook

The Missoula Office of the National Weather Service is forecasting the next weather system to enter our area from British Columbia on Monday. This system is expected to stall and produce accumulating snowfall along the frontal boundary for a 48 hour period. On Wednesday, cold arctic air will cross the Continental Divide bringing strong and gusty NE winds to the Kalispell region. Well below normal temperatures will settle over the area Thursday.

Little precipitation is expected during the next few days but the snow to water ratios will be very high; at 40:1, meaning that .10 of water will produce 4 inches of snow. That is what we want to see!

Expect the avalanche danger to remain similar until significant snowfall occurs.

Tim Laroche will issue the next advisory on Friday, February 25, 2011.