Flathead Range & Glacier NP

The avalanche danger will increase through the day as snow and wind add weight to reactive weak layers 1-2' below the surface.  Triggered and natural slab avalanches have occurred over the past 4 days and are anticipated today. A more robust surface slab may allow slides to propagate further than expected and result in larger slides, especially on leeward aspects. Choosing lower-angled terrain free of convexities is the best way to manage this unpredictable problem.

This morning's avalanche danger is MODERATE, but it may rise to CONSIDERABLE by tonight due to new snow and wind. Forecasted precipitation totals are modest, so what gives with bumping the danger up at all elevations? Yesterday was the fourth consecutive day of avalanche activity in our area. During a storm cycle, that may not be all that big of a deal, but most of the slides occurred following our last snowfall. This is confirmation that we are dealing with a Persistent Slab problem in the top foot or two of the snowpack. The excitement in the first three days was confined to skier triggered slides in dry soft snow. The snow was cohesive enough to propagate but still light enough for great riding conditions. The slides' failure layer was weak snow, surface hoar or facets, above the 1/13 crust. In a few cases, the weak layer allowed for slides to be remotely triggered. Yesterday’s warming temperatures and solar input aided in forming a more robust surface slab.

In the Skyland area, Guy and I noted a natural slab avalanche that failed mid-day on an east-facing upper-elevation slope receiving direct sun. Small wet loose debris initiated the slide on a buried layer of surface hoar. That the trigger was such a puny pile of debris confirms the reactivity of the weak layer. A more cohesive surface slab will allow for propagation to occur over a larger area resulting in bigger, more dangerous slides. Today’s precipitation may initiate a natural activity, or it could "tip the balance" and allow for easier failure once you are on the slope. Dealing with a Persistent Slab problem is generally a bit complex and difficult. Not so much in this case. The weak layers that we are worried about rest on the 1/13 crust, which is only 1-2’ below the surface due to a dry second half of January. We saw 2 obvious stripes in our pit wall yesterday that confirmed the presence of weak layers. Without digging, the weak layers can be identified by hand pits in the top 1-2' of the pack. 

Many parties noted obvious feedback in the form of collapses or shooting cracks the past few days. Guy and I received no feedback on that form yesterday. Instead, we looked around and noted the numerous red flags which were telling us to stick to lower-angle terrain: Warming and surface rollerballs, recent avalanche activity, clean shears in hand pits, ECTP results. Today you can add loading from snow and wind to the list. Sticking to lower angle terrain free of convexities is the right choice to ride out our current Persistent Slab problem.