Flathead Range & Glacier NP

Update: We have raised the danger at mid-elevations due to an overproducing storm. The avalanche danger will rise today as new and drifted snow accumulates on steep slopes. You can trigger both slabs and loose snow avalanches; these will become larger and more hazardous as the day progresses. Choose less steep slopes with fewer consequences as snow accumulates. The danger may spike quickly during bursts of heavy snow accompanied by strong winds.  

Three ingredients in today's recipe for trouble: Wednesday's snow surface, today's snowfall, and today's winds. Stirred together, they make for an increasing danger of triggering small to large sluffs and slabs. Because upper elevations will see the most new snow and strongest winds, the hazard should be most widespread in that elevation band. 

Observations over the past few days have consistently described the snow surface as hard, mostly supportable melt-freeze crust. This crust extends to over 7000 feet on most aspects, though it seems to be thicker and more supportable on sunny aspects. A trace to 2 inches of low-density snow covers this crust, with some scattered patches of surface hoar and facets. The slick crust and thin, soft layer create a bed-surface and weak layer for any new snow. 

And new snow is forecast for today. Accumulations should be less than 6 inches by nightfall, with more in favored areas like the Swan front.  That's enough new snow to pose a hazard on steep slopes where slabs are thickened and stiffened by today's southwesterly winds. Plan to avoid crossing or riding in terrain likely to harbor this hazard - upper elevation slopes top-loaded or cross-loaded by southwesterly and westerly winds. 

Where today's snow remains loose and cohesionless, it will sluff easily. Most point releases will be harmless, because they won't involve enough debris to carry or bury a person. The exceptions will be slopes where more than 6 to 8 inches of new snow has accumulated, and slopes with long, continuously steep pitches that allow loose snow avalanches to run long distances and entrain more debris. Slope cuts on low-consequences terrain can tell you how easily new snow is releasing and how much debris it's entraining as it runs on the underlying melt-freeze crust.

Last week's warm, wet storm initiated a cycle of natural avalanches that failed on facets near or just above the ground. We've counted roughly a dozen of these slides so far. Not many, given the 1800 square mile forecast region. Enough, though, for FAC forecasters to quite literally dig into the question of whether basal weak layers pose a lurking threat. So far, it seems these layers are dormant until they get another shock - more warming, rain, or another heavy loading event. For now, I'm steering clear of steep slopes likely to harbor shallow snow where it's easier to trigger these deeply buried weak layers.