Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

At upper elevations, be suspicious of steep slopes where there's no supportable crust at the surface, as well as lower-angled slopes below avalanche start zones. In this terrain, it remains possible to trigger large avalanches that break on old, weak snow buried several feet down. This hazard is isolated at mid-elevations.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    It remains possible to trigger large avalanches on steep slopes that aren't capped by a supportable melt-freeze crust. The crusts are most robust on mid-elevation slopes that receive lots of sun. They become thinner and more variable as you climb higher in elevation or swing around to shady slopes. 

    The hazard is most widespread at upper elevations, and isolated at mid-elevations. Pay attention to whether your snowmachine's track is chewing through this crust as you sidehill or climb. If it is, be wary of steep slopes; ride one at a time and keep each person in your crew in sight. On skis or a board, the crust will support your weight in more places. Notice when it doesn't.

Avalanche Discussion

I respect Dry January resolutions. I might, however, have a word with Old Man Winter. He's not the life of the party this year. 

The upside of the stagnant weather conditions that have prevailed for the past week or so is that the weak, faceted snow buried right after Christmas is slowly strengthening, while the slab above it is losing the properties that enable it to collapse the weak layer and propagate a crack. Slowly is the operative word, however, thanks to cool temperatures. 

So triggering a large to very large avalanche on this layer remains the primary avalanche hazard in the mountains. This hazard appears isolated at mid elevations (below about 6500 feet), where a stout melt-freeze crust caps the snowpack on most slopes. Where this crust is supportable, a rider is less likely to collapse the problematic weak layer if they stumble into a spot where it's still intact. Snowmachines may break or chew through the crust in more places.

The hazard is more acute at upper elevations. Reports indicate the melt-freeze crust thins as you climb higher on all aspects, particularly on shady slopes. Observations during the first week of January, when this layer was most sensitive, showed noticeable spatial variability. No propagation in column tests on slopes adjacent to or near triggered slides. That means terrain selection is your primary defense against the hazard. Digging and testing may not give conclusive results about the initiation and propogation potential on a slope. 

A nuance that may be worth keeping in mind - the crust will be thinner and less supportable on lower-angled slopes. In a worse case scenario, a rider-triggered collapse in terrain like this could propagate upslope, remotely triggering a large slab above. 

So go for a sunny walk or ride in the mountains, as many people did over the weekend (Thank you for the observations!). Be alert for - and maybe flat out avoid - steep slopes that aren't capped by a stout supportable crust. Avoid traveling under steep slopes if the crust isn't robust. Get some Vitamin D and enjoy the views.

Education offerings:

Human-powered partner rescue clinic, January 24. Find out more details here. 

Ladies' human-powered Intro to Avalanches January 29 and 31. Get more details here.

Regioni coperte