Boletín de aludes

Whitefish Range

Flathead Avalanche Center
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Peligro de aludes

Sat
Alta montaña
2 · Moderado
Límite del bosque
1 · Débil
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 3 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

You can still trigger large avalanches that can ruin your barely-started riding season. These can break at the old snow/ new snow interface, or on older weak layers buried deeper in the snowpack. These hazards are more widespread and more sensitive to a person's weight at upper elevations. Snow showers today may drop enough snow to thicken slabs. Shooting cracks and whumpfing collapses are clear signs to avoid steep slopes. 

Problemas de aludes

  • Storm Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    Be wary of steep slopes with about 8 inches or more of new and drifted snow. You can trigger avalanches large enough to bury or injure a person. These slopes will be more widespread at upper elevations and in the northern Whitefish Range, which was favored by Thursday's snowfall. Cracks shooting from your sled or board are clear signs these freshly-formed slabs are unstable. You can test the sensitivity of the on small slopes with runouts free from terrain traps. 

    (3/6/2022)

    An example of a natural soft slab that on a steep slope near a ridge. Expect similar slabs today. 

    Photo: Mark Dundas

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    1–2

    On isolated slopes at upper elevations, the hazard of triggering slabs that break below the new snow/ old snow interface lingers.  These can break a foot or more deep, on surface hoar buried 12/1. Or near the base of the snowpack, on facets above a hard crust. You're more likely to trigger these slabs where they've been recently loaded by new and drifted snow. 

    Whumpfing collapse and shooting cracks are clear signs of this hazard. Avoid steep start zones and the runouts below them. Dig into the snow and look for a grey strip of feathery crystals generally about 10 to 18 inches below today's new snow and a layer of soft, loose grains above a hard crust near the ground. Sudden collapses or propagation in Extended Column Tests are clear evidence that slabs can be reactive to your weight.

    (12/7/2023)

    The 12/1 surface hoar is the primary layer of concern. It propagated in an one ECT on Thursday, though not in a second.

Avalanche Discussion

Weather stations are showing 2 to 11 inches of new snow above 6000 feet since early Thursday morning. The storm favored the northern Whitefish Range, the head of the McDonald Valley in Glacier National Park, the Flathead Range, and JFS Canyon. Storm totals in the Swan Range are uncertain because the Noisy Basin SNOTEL isn't reporting snow heights. Snowfall tapered off in the early evening hours.

Gusty winds accompanied the snowfall. They blew mostly from the southwest, though Tunnel Ridge saw west-northwesterly winds. I expect some redistribution of the fresh snow, mostly on the lee sides of steep ridges, the tops of chutes, and downwind of saddles. While you might find harder slabs in this terrain, the old snow/ new snow interface remains a concern on all steep slopes. YMMV, however.

Rob and Cam checked out conditions in Crown Bowl in the Swan Range Thursday. They verified one big hypothesis - two persistent weak layers remain a concern in areas that didn't receive much rain earlier this week. The 12/1 surface hoar was buried about 10 inches below the surface prior to Thursday's snow. And a layer of facets still exists above a melt-freeze crust near the base of the snowpack. While the results of the their large column tests were inconsistent, we still have enough data to know that a person can still trigger avalanches on these two layers in that area. And likely throughout the Swan and Flathead zones. 

It's hard to get more specific than that until we get more observations about how recent rain, a refreezing snowpack, and new snow have affected conditions at upper elevations. If I could launch snowpit drones this morning, I'd program them to figure out a few things for me:

  • How high did it rain on 12/4 and 12/5?
  • Has the 12/5 snow surface refrozen or was it insulated by Thursday's snow?
  • Has the 6-12 inches of new snow at upper elevations made the 12/1 surface hoar more sensitive to a person's weight?

Without them, I'm left... forecasting. While my confidence is higher than Thursday, I'd still call it low. 

If you venture out into the mountains today, especially into higher elevations, look for clues that can help answer those questions. I expect that you can get into trouble on steep slopes where the 12/1 surface hoar remains mostly upright and dry and is now loaded by a foot or more of new and drifted snow. That's probably limited to terrain above 7500 feet, though I'd start digging into the snowpack below that elevation. As always, whumpfing collapses, shooting cracks, and remotely triggered slides are public service announcements that steep slopes are dangerous. 

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