Flathead Range & Glacier NP

Dangerous avalanche conditions exist at mid and upper elevations, thanks to several days of heavy snowfall, rain, warming temperatures, and wind. The higher you climb, the more likely you are to encounter dry snow avalanche hazards. Below the rain-snow line, the primary danger will be wet snow avalanches. Natural avalanches may run into lower elevations, so avoid the runouts of steep slopes above you. 

The warm, wet storm of the past few days has produced a cycle of large natural avalanches. Both slab and wet snow avalanches ran late Monday and Tuesday in and around John F. Stevens Canyon;  debris from these slides ran long distances, with one slide terminating 30 yards from Highway 2. Cam reported crowns and debris piles at low elevations in the Middle Fork.  Near Stryker Ridge, a natural avalanche covered 400 feet of a groomed snowmobile trail with debris up to 15 feet thick. That's a lot of slides for just a few observations. I expect we'll hear reports of many more debris piles and crowns once more people get into the moutains.

On Tuesday, the snowline rose to at least 6500 feet; rain below that line and above-freezing temps overnight mean that triggering wet snow avalanches is today's primary hazard. A person's weight is most likely to trigger these on slopes steeper than about 35 degrees where the top few inches of snow are wet; once running, they can entrain large amounts of dense debris that's very difficult to escape. At lower elevations, the near-surface snow is saturated much deeper. The natural avalanche cycle has peaked, though a person could still trigger wet snow avalanches on very steep slopes with 8 inches or more of wet snow near the surface. 

Above about 6000 feet, dry snow avalanches are the primary danger. The potential size of triggered slides increases as you climb in elevation; 2 to 3 feet of snow has accumulated above about 7000 feet since Monday. Moderate southwesterly winds Tuesday afternoon likely moved some of this snow around. And more snow last night and today is thickening slabs. Natural avalanches remain possible, and these can entrain large amounts of dense, destructive debris. Don't linger in avalanche path runouts as you travel, and be alert for shooting cracks and collapses once you reach dry snow. 

Before this week's storm, some large column tests propagated on surface hoar or facets buried 1 to 2 feet below the surface. We had no reports of avalanches breaking on these layers, and they seemed dormant. This week's dramatic loading may have re-activated these layers on isolated slopes. The debris from triggered slides may be enough mass to break on these deeply buried layers, creating avalanches that step down and break widely across start zones and terrain features. The best clue that this hazard exists is avalanches that have done just that. If you see avalanches that have broken recently, look for crowns that have stepped down, or slides that broke deeper as the debris traveled downslope. If you see these, avoid similar terrain.

If you want to learn more how the current natural avalanche cycle fits into longer record, check out our Topics class tonight with Dr. Erich Peitzsch. Erich is a USGS Research Scientist and former Director of the FAC. It's virtual, so you don't have to go out in the rain.