March 19 Avalanche Advisory

Hello! This is Steve Karkanen with backcountry avalanche information from the West Central Montana Avalanche Center for Friday, March 19, 2010.

Current Avalanche Danger

Above 6000 feet within the advisory area and on terrain steeper than 35 degrees, the avalanche danger is MODERATE. Natural avalanches are unlikely, human triggered avalanches are possible. On all other terrain above 6000 feet and at the lower elevations, the avalanche danger is LOW. Natural and human triggered avalanches are unlikely.

During warm afternoons, or whenever it starts raining, the avalanche danger will quickly rise to CONSIDERABLE, when it is probable that you can trigger an avalanche.

Weather and Snowpack Analysis

Our advisory area received very little snowfall this past week (average of 2 inches at all SNOTEL sites since 3/12) but we have been been through several melt-freeze cycles at the higher elevations which has certainly helped settle and strengthen the weak layers in the upper snowpack. All SNOTEL sites in our area reported temperatures in the mid-fifties Tuesday and Wednesday with nighttime temperatures dropping below freezing at night on all but Wednesday. East and south facing aspects are close to isothermal and should be avoided on warm sunny days.

All observers reported stable conditions keeping in mind that the southerly facing aspects weaken quickly when the sun is out. The melt freeze crust is impenetrable early in the day but weakens once melt water begins moving down into the buried weak layers we have been watching for several weeks now.

North and heavily shaded aspects are skiing well but preserving the surface hoar layer responsible for so many incidents this past month. This layer continues to gain strength but on a 35 degree north slope in the Rattlesnake I had consistent clean energetic failures 16 inches deep (CTH23, Q1 shears) all involving the buried surface hoar we have been describing for over a month now. Other observers are describing similar conditions but highly variable. It’s ability to propagate has diminished greatly over the past week. If you plan on skiing anything steep, wind loaded and open, it would be worth your time to dig a quick pit to see if it exists on the slope you want to ride.

The variability of the snowpack structure in Western Montana this late in the season when during normal years, our snowpack is strong, is a bit disconcerting. I’ve been monitoring the snowpack in this area for many years now and have completely lost sight of what normal means. These mountains and the weather and snow conditions that make up the mountain environment have evolved over millions of years. We really have NO idea what normal is. We just need to be prepared for anything.

Weather Forecast and Avalanche Outlook

The National Weather Service Office in Missoula is forecasting scattered showers along the Continental Divide into SW Montana through Friday morning. Light to moderate accumulations are expected. Clearing skies are expected through Sunday morning. Beyond Sunday, weather models are having difficulties timing the arrival of the next upper level trough which is the next best chance for snowfall accumulation.

Expect avalanche danger conditions to remain similar. Cooler air with no new snow will give us very safe conditions. A warm air mass with rain in the mountains will deteriorate avalanche conditions.

Recent Avalanche Accidents

Last Saturday, March 13, two separate avalanches took the lives of 3 backcountry riders. One near Priest Lake in the Idaho Panhandle Forest and the other 2 near Revelstoke, British Columbia. Although these incidents were a long distance from our area (Revelstoke is about 350 air miles NW of Missoula), initial reports indicate that buried surface hoar layers that have persisted for several weeks may have been the weak layer involved in these accidents. The snowpack in these areas is similar in many ways to ours.

The Canadian Avalanche Center has an excellent preliminary report on this accident at this website: Revelstoke Accident report.

Information about the avalanche in northern Idaho can be found on avalanche.org.

Our thoughts are with the families and friends of those who were lost in these accidents.

The accident near Revelstoke B.C. is noteworthy in that many, many more people may have died if not for the quick thinking and well executed companion rescue that took place immediately following the avalanche. Canadian and other news media reported that more than 200 individuals were on the site when the avalanche released, many of whom were in the direct path of the avalanche. The same reports indicated that more that 30 people were injured, many of whom were buried.

This is a time to reflect on the decisions we make when we travel in the backcountry. Not to assign blame. If we spend any amount of time in the backcountry in the winter, we will be exposed to avalanche terrain. Many of us seek out the steeper more exposed terrain because it offers a higher level of personal reward than the groomed trails and crowded, expensive terrain ski resorts offer.

But you owe it to your friends and family, not to mention the many dedicated people who volunteer their time to search and rescue organizations who put themselves at risk when the call goes out, to educate yourself.

The last avalanche advisory of the season will be issued on Friday, March 26, 2010. Have a safe weekend!