Flathead Range & Glacier NP

Riders may trigger slides up to 1 foot thick on a buried weak layer on slopes facing NW-NE-SE. Wind-loaded slopes are the most likely place for avalanches, but sheltered locations may also prove troublesome. If you recreate on the shady side of the compass, choose lower-angle terrain for a safer experience. The weak layer is generally not present on sunny and windward aspects where the danger is LOW. 

We transitioned to a Persistent Slab problem in the Swan Range yesterday, the Flathead Range, southern Glacier Park, and the Whitefish Range today. A widespread weak layer in the upper snowpack is responsible for natural and triggered slides along with propagation in snowpit tests. You can find this layer across shady, sheltered slopes (NW-NE-SE), where it survived the effects of recent wind and sun. 

Swan Range: Over the past nine days, this area has been favored by recent storms, with 3.2 inches of SWE and over a foot of settled snow at the Noisy Basin SNOTEL. The buried weak layer is generally twice as deep here as in the other ranges. Above about 5500 feet, this layer may be found 16-24” below the surface. It consists of a thin layer of feathery surface hoar and faceted grains, which may appear as a grey stripe in the pit wall. Cam and Guy found this layer to propagate in snowpit tests yesterday on the east side of the Swan Crest (Observation). Triggering an avalanche on this layer in sheltered or wind-loaded terrain may result in a slide large enough to bury, injure, or kill a rider. Rob was in the Mid-Swan yesterday and could not find surface hoar in a dozen hand pits on an exposed westerly ridge (Observation). He noted large surface hoar in the neighborhood last week but surmised that the wind and sun destroyed this pesky layer where he traveled.

The other ranges: The remainder of the advisory area has received far less recent snow, and the layer is found 6-12” below the surface above about 6500’. The weak snow layer is also located below that elevation, but the lack of a slab limits the problem currently. Areas where you could initiate a slide on the weak layer appear to be limited to wind-loaded slopes, as evidenced by this observation from the northern Whitefish Range. Down the road: Additional snowfall may form a more widespread surface slab prompting us to expand this problem to the mid-elevation band. 

Christmas Crust weak snow: In late December, light snow fell on top of the Christmas Atmospheric River rain crust. The first week of January was cool and dry, contributing to faceting in this snow. Incremental loading occurred over the past ten days. We now have a 2-3 foot thick slab resting on this weak layer. The BNSF Avalanche Safety Team reported propagation on this deep layer earlier this week. Cam noted natural slides, which may have failed on this layer during last Friday’s warmup. We are more concerned about the weak layer closer to the surface, but as the surface slab thickens, this crust-facet combination may come into play and become an additional Persistent Slab. 

Managing the Persistent Slab Problem: Avalanches failing on surface hoar or facets may break wider than expected or higher on the slope above you. Remotely triggered slides occurred this past week in the Swan, and today you could start a slide from gentler terrain above, below, or to the side of steeper slopes. Triggered slides in wind-drifted snow occurred this week in the Swan and northern Whitefish. If you plan on recreating NW-NE-SE slopes in the near future, choose lower-angled pitches free of overhead hazards to mitigate this problem. If this layer is present in a hand pit or snowpit, choose a different aspect or stick to lower-angle terrain. Buried surface hoar and facets are not to be trusted and should be given a wide berth. Slopes that face the sun or wind do not appear to harbor this weak layer and will offer a safer alternative.