Swan Range

Very dangerous conditions are developing in the Swan Range. If heavy snow and gusty winds arrive this afternoon and tonight as forecast, large and very large natural avalanches are likely at upper elevations. Triggered and remotely-triggered avalanches also pose a very serious hazard. The potential size and likelihood will increase as you gain elevation and as new and drifted snow accumulate. Avoid traveling on or below steep slopes. 

Conditions are unusually dangerous. Not tree-snapping, take-out-forest dangerous, like we saw last January. Not yet. But the incremental loading of the past week or so - an inch or two here, 4 more there - has built thick, stiff slabs above relatively uniform weak layers. These slabs are now 1 to 3 feet thick on mid and upper elevation slopes, and probably thicker on the higher peaks of the Flathead Range and Glacier National Park. They are not as thick or reactive in the Whitefish Range. However, they've proven reactive to the weight of a person. We've had reports of natural, remotely-triggered, and triggered avalanches nearly every day for the past week. Many of these slides have surprised people, several have been near-misses, and in at least one incident, a person was caught, carried and slightly injured. The snow and winds forecast for today and tonight will only increase the likelihood and size of potential slides. 

Now for the unusual part. Most big avalanche cycles here occur when the weather is foul and backcountry travel is difficult or unpleasant. The danger has diminished by the time people can or want to get into avalanche terrain. We're in the opposite situation now. The danger is rising gradually while backcountry riding conditions - motorized and non-motorized - stay straightforward, or improve. And after several weeks of less than desirable conditions, people are hungry to ride.

Complicating the situation is that the hazard has been concentrated - though not confined - to particular terrain. Specifically, steep slopes immediately below ridgelines that face north, northeast, and east-southeast. That's terrain where winds have thickened and stiffened slabs sitting on persistent weak layers that are not healing quickly. On windward, sunnier slopes, evidence for the same hazard is less clear - propagation in column tests but few, if any, collapses or triggered slides. 

All that leaves us with lots of opportunities for accidents. You can climb a sunny, windward slope and trigger a lethal slide on the first steep, shaded slope you encounter, with little warning before the slide releases. All under partly cloudy skies, with easy skinnning or riding. That's a situation more more common to Idaho and Colorado than western Montana. 

The most effective way of dealing with this kind of complexity is to stick to simple terrain. Those are slopes that are less than 30 degrees and not under steep start zones, even those way above you. It's easy to misjudge slope steepness, and underestimate slope angles because a slope doesn't feel "steep" when you're riding it. So use an inclinometer, and measure slope angles. Use a map. Or just stay out of avalanche terrain.