Flathead Range & Glacier NP

Wind-sheltered slopes less than about 35 degrees offer the best and safest riding today. Steer clear of pillows of wind-drifted snow near ridges and cross-loaded gullies. A lack of feedback from the snowpack (cracks, collapses, or whumpfs) does not mean that a slope is safe.

The first day of winter is still two weeks away, yet conditions warranted an early season start for FAC on Saturday. While the avalanche danger has slowly decreased since then, we are still dealing with both Persistent Slab and Wind Slab problems.

Let’s start with the Wind Slab problem, which is the most immediate. We know from our weather stations that winds Tuesday and Wednesday were strong enough to transport snow and l build slabs. In the last 48 hours, Aeneas Ridge in the Swan Range has recorded southwest winds averaging 24 mph with gusts as high as 44 mph. Other stations have reported lower average wind speeds, though these winds were still capable of transporting snow. The Snowslip station in southern Glacier National Park reported winds from the southeast averaging 11 mph, with gusts up to 28 mph, while the Link weather station in the Whitefish Range had southwest winds averaging 8 mph with strong gusts of 27 mph.

With those wind speeds, we have all the ingredients for building wind slabs along the lee side of ridgelines and gullies. The BNSF avalanche safety team found slabs like these Wednesday in John F. Stevens Canyon. However, their ski cuts on test slopes produced no slides, suggesting these are stubborn. Although we have limited observations at upper elevations, we expect perhaps large and more reactive slabs. Pay attention to changes in the density and the surface texture of the snow to avoid exposing yourself to being caught or triggering a wind slab avalanche.

Now, back to our regularly scheduled programming, the Persistent Slab problem.  

On Tuesday my partner and I toured east of Whitefish Mountain Resort, with the goal of collecting data on the distribution of the Persistent Slab problem. At mid-elevations, we found that the weak layer was about 18 to 20 inches below the surface and widespread across various aspects. Additionally, we investigated the crown of a triggered avalanche that failed in weak facets resting on a thin crust. The avalanche occurred on a steep slope (40 degrees) with a convex starting zone. Notably, the adjacent slopes that flanked the avalanche path cracked but didn’t slide because they were less steep. Slope angles matter. The most clear-cut approach to keep yourself and your friends safe is terrain selection, particularly slope angles. The nature of our Persistent Slab problem is that it is difficult to predict and should be managed with a wide margin of error. The takeaway from this unpredictable avalanche problem is captured in the BNSF avalanche safety teams observation: “the poor snowpack structure appears to be widespread with a consolidating slab 1-3+ feet thick resting on top of a sometimes-reactive layer of facets formed back in November.”

Education - Our next Avalanche Awareness presentation is Tuesday, December 13, at Whistling Andy Distillery, Bigfork, and on Wednesday, December 14, at Penco Power Products, Kalispell. You can get more details about this free event on our events calendar.

Observations - Thanks to everyone who's been utilizing our new observation submission page. Try it out and help your community by sharing what you see in the backcountry. The form requires you to fill out the location fields and put a dot on a map. Be as vague or specific as you want. Our new Observation View page highlights signs of instability and allows you to filter by date or zone to pull out reports relevant to your objectives.