Whitefish Range

You can trigger soft slabs of drifted or wind-stiffened snow on steep slopes at upper elevations. They can bury or injure you where the slabs are a foot or more thick, or above gullies, cliffs, or trees. On shaded slopes, with no supportable crust below the new snow, you may still be able to trigger slabs that break on weak layers buried one to two feet deep. Avoiding slopes steeper than about 35 degrees is a straightforward way to enjoy the fresh snow while minimizing your exposure to these hazards.

Wednesday I concluded "Not much snow and not much wind - but enough to form fresh drifts or stiffen the new snow into slabs in upper elevation terrain." And indeed, that proved out, in the Whitefish and Flathead zones. It was, however, much windier-er in the Swan zone than forecast. I expect a more widespread wind slab hazard today, thanks to wind speeds increasing overnight, winds shifting to the southwest, and a decent amount of loose, dry snow available for transport.

First, a roundup from Wednesday. Near Lake McDonald, a rider was surprised by a small, soft slab of wind-stiffened snow on an east-southeast slope at about 7600 feet. He skied out of the moving debris. Elsewhere in the Flathead zone, Cam reported no results ski cutting numerous pockets of drifted snow at mid and upper elevations near Essex, though he did see numerous small soft slabs that released naturally and likely failed on a mid-storm graupel layer. Another rider nearby reported minimal wind effects from overnight easterly winds. In the Whitefish Range, a party triggered a small soft slab on an easterly slope near a ridge line; it didn't propagate far but entrained a fair amount of snow. They noted that the wind-affected snow didn't extend far below the ridge crest, while The Patrol Funds Level 1 class didn't see much effect from the wind east of WMR. 

It was a different story in the Swan Range, where winds were more easterly and stronger than forecast until about early afternoon. The winds drifted snow onto westerly aspects, creating reactive slabs. Clancy was able to trigger one on a mid-elevation slope, and a large natural slab ran on an upper elevation, westerly slope near Broken Leg mountain. 

While Wednesday's wind slabs are stabilizing, I expect a new crop of soft, reactive slabs to have formed overnight and continue growing today. You'll find these in more typical locations - steep slopes facing north through southeast, the tops of chutes and couloirs, the shaded sides of gullies, and downwind of passes and saddles on ridges. They'll be widespread at upper elevations in all zones, with some at mid elevations in the Swan and Flathead ranges. While these may be easily triggered by a rider or snowmachine, many will not be large enough to bury or injure a person. Many, but not all. The exceptions will be above terrain traps like gullies, trees, rocks, and cliffs. 

Weak, collapsible layers buried one to two feet below the snow surface remain a concern. Where these layers sit under a stout melt-freeze crust, a person is unlikely to collapse them. Those crusts exist up to 6500 feet or so in the Swan and Flathead zones, so the concern is limited to upper elevation, northerly slopes in those zone. In the Whitefish zone, the crust has a spottier distribution, and Cam found the layers to be a concern at mid-elevations as recently as Tuesday.