Flathead Range & Glacier NP

You're unlikely to trigger an avalanche on most slopes. There are few reactive slabs near the snow surface. Weak layers near the ground are strengthening and are unlikely to collapse where they're capped by stout ice lense or melt-freeze crusts. Carry a beacon, shovel and probe, ride one at a time, keep your partner in sight, and avoid riding above terrain traps.

I've added several images to this forecast that illustrate two things:

• A boom-bust cycle of reports of instability, with most reports prior to Christmas including signs of instability (orange or red dots). And only 5 reports of instability in the past week. Most of these are minor (a cornice fall avalanche in the Flathead Range, isolated cracking in the northern Whitefish Range, cracking cornices in southern Glacier NP). The most significant was Cam's report of loud, booming collapses in the northern Swan Range on Dec. 28. 
• The value of submitting observations. We've got a pretty clear picture of conditions across the region at all elevations thanks to all the riders who've taken the time to fill out the observation form. 

Based on those observations, we're dropping the danger to Low at all elevations across the region. There are few reports of slabs and failure planes in the near-surface snowpack. It's unlikely the weight of a rider or snowmachine will trigger an avalanche that breaks on weak snow buried near the base of the snowpack. That's largely due to the ice lenses and melt-freeze crusts near the surface of the snowpack. These layers are hard and supportable and extend to at least 7400 feet, with one report of these ice layers up to 7700 feet. They limit the likelihood of a rider or snowmachine collapsing the basal layers. It's also because the weak layers themselves are rounding and sintering as the snowpack deepens.

There are, of course, isolated exceptions. These are slopes where the ice or crusts are weak and don't support the weight of a rider, or where they're thin and the track of a snowmachine at high RPM can slice through them. The persistent slab structure seems to have been most developed on mid-elevation, sunny slopes, because of sun that created hard crusts and strong temperature gradients above that promoted facet and depth hoar growth. The few confirmed reports of avalanches failing on the basal weak layer released on slopes like these. And the only reports (here and here) of this layer propagating in the past week are slopes like these, both east of WMR. Thanks to an unusually cold early season, upper elevations slopes saw little warming and the weak snow doesn't seem to be as developed, with no ice crust. 

After light winds for the most of the past week, there have been few reports of wind-drifted snow and slabs. You might find a few unstable drifts in the terrain most prone to collecting wind-blown snow, like the tops of chutes and couloirs. 

We have received multiple reports of open glide cracks. They've been sighted in the southern Whitefish Range, the Swan Range, and the Flathead Range. They likely exist because the ground was still warm when first buried by snow in late October. They tend to fail randomly, so it's best to stay out from under them. 

Monday's snow left a thin but rideable crust at the surface on many slopes, with some reports of thin rime crusts. These crusts are thinner on shadier slopes.