Av danger rated 'high' in spots

SANDPOINT — Substantial snowfall, strong winds and some very reactive weak snowpack layers are creating a high avalanche danger above and below the tree line in the Selkirk and Cabinet mountain ranges, according to the U.S. Forest Service’s Idaho Panhandle Avalanche Center.

The avalanche center is counseling backcountry travelers to avoid navigating slopes steeper than 30 degrees in areas at or below the tree line, according to an advisory published online Wednesday.

The avalanche danger is rated “considerable” above the alpine tree line and it’s rated “high” at or below the tree line. When the slide risk is categorized as considerable, backcountry travelers are urged to conduct careful snowpack evaluation, cautious route finding and careful decision making. In areas ranked high, dangerous avalanche conditions are present and travel in that terrain is not recommended.

The snowpack in Bonner and Boundary counties is problematic in at least three respects, according to the avalanche center. Building storm slabs are resting on a very weak surface of hoar crystals, crust layer and facets.

“The most reactive slopes will be at elevations below 5,500 feet, where surface hoar crystals are growing on top of a firm crust. Expect slopes in valley bottoms that you normally wouldn’t recognize as avalanche terrain to become sensitive to human triggers and avoid all avalanche terrain. These slabs will vary in thickness across the area but it is likely that we may see a cycle of natural avalanches through Thursday,” Wednesday’s advisory said.

Also problematic are wind slabs and persistent slabs.

On Tuesday, there were persistent 20-mph wind that clocked around the Cabinet Mountains. The avalanche center received reports from skiers who were able to trigger some small, “pillowy” pockets in wind-loaded terrain.

“With fresh snow available for transport overnight, it is likely that these wind slabs became thicker and more cohesive through the evening at tree line and above,” the advisory said.

There are some deeply buried facets and depth hoar layers across the area that have been sleeping and showing very little reactivity in stability tests. Additional weight from the recent storm may tip the scales for this weakness in areas where the snowpack is thin and lacking in strength.

“Triggering a storm slab in steep terrain may also cause a ‘step-down’ to these deeper weak layers resulting in deeper burials,” the advisory said.

Info: idahopanhandleavalanche.org

Keith Kinnaird can be reached by email at kkinnaird@bonnercountydailybee.com and follow him on Twitter @KeithDailyBee.