Swan Range

Very dangerous avalanche conditions are developing thanks to unseasonably warm air and the potential for partially sunny skies today. Avoid traveling on or below all steep slopes; large natural wet snow avalanches are likely. Also rising is the danger posed by large to very large avalanches that release on deeply buried weak layers and run long distances. The debris from both hazards will be dense, heavy, and almost impossible to escape. 

While people often talk about a "January thaw" in the Flathead, this is something. Nearly all stations at mid elevations have recorded air temperatures above freezing for the past 48 hours, with 0.2 to 0.5 inches of rain. Freezing levels are forecast to climb higher today, possibly to near 10,000 feet. Although the rain looks over, it'll be replaced by partially clearing skies. So solar radiation will add to the energy that's producing meltwater on upper elevation, sunny slopes. 

The wet snow and meltwater are creating very dangerous avalanche conditions. Large to very large natural wet snow avalanches are likely on steep slopes at mid and upper elevations. Debris from these slides may reach low elevation runouts. Even slides that stop after short distances can involve large amounts of debris, as seen in the one observation we received Sunday. A rider reported multiple slides that hit or almost hit the snowmobile route across Red Meadow Pass in the central Whitefish Range. These released at mid elevations and only ran several hundred feet, but piled up more than enough debris to bury a person. I expect more of the same today, with most activity on upper elevation sunny aspects. The hazard is isolated at low elevations, because slides won't involve dangerous amounts of debris. There just as likely; I ran across two small debris piles on the Whitefish Trail near Lion Mountain Sunday. 

The danger posed by the lingering Persistent Slab avalanche problem is also increasing, as water percolates into the snowpack. The snowpack structure is weakest on sunny aspects at mid and upper elevations; that coincides with slopes where solar radiation will produce more meltwater today. This water will reach old, faceted snow above crusts soonest in terrain where these layers are closer to the surface. That describes the southeast corner of the forecast region - JF Stevens Canyon, Marias Pass, and the Skyland area, where observers have reported audible collapses and propagation in stabilty tests the past few days (Elk Mountain, 6996 Friday and Saturday, Challenge Friday). While cornices have been smaller than usual this season, there's still plenty of snow that can fail in these warm temperatures, producing debris that can tirgger deeply bured weak layers on slopes where a person might not. Leeward start zones near the crest of the Flathead, Swan, and Whitefish Ranges are prime candidates for these conditions. 

It's important to note that our confidence in today's forecast is moderate. We have no direct measures of the amount of meltwater at the snow surface or how deep it's percolating in the snowpack. The natural avalanche cycle may have peaked late last night. I wouldn't use that uncertainty to justify riding steep slopes.