Boletín de aludes

Flathead Range & Glacier NP

Flathead Avalanche Center
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Peligro de aludes

Wed
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
3 · Notable
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Desactualizado: este boletín caducó 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Shaded, low-angled slopes still offer the best and safest riding today. Wet snow avalanches pose a hazard on steep sunny slopes, and slabs that can break 3 to 6 feet deep lurk on steep shaded slopes. A compass and inclinometer can help you thread between these threats. Avoid traveling under glide cracks. Expect more widespread and larger wet snow avalanches if skies are sunnier than forecast. The avalanche danger will rise with the sunny skies and warm temperatures forecast for the next few days. 

Problemas de aludes

  • Loose Wet

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

    Triggered and natural wet snow avalanches pose a hazard on and below steep slopes today, mostly at mid and low elevations. Step back to low-angled or shaded slopes where you see trigger rollerballs that fan downslope. Once moving, point releases of wet snow can entrain surprising amounts of debris, particularly where they gouge below the surface. That's more likely at low elevations, where the snow surface didn't refreeze overnight. Slopes with continuously steep and straight tracks will produce the largest slides. If skies are less cloudy than forecast, expect more natural and triggered slides at mid and upper elevations. 

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    2–3

    The spring-like weather hasn't erased the lingering danger of triggering very dangerous slabs that break 3 feet or more below the snow surface, on weak layers that formed in late January. Yes, more than a month ago. That hazard is demonstrated by three large slides triggered this past weekend, one of which caught a snowmobiler above Skyland Road. These slabs are more sensitive to the weight of a person or snowmachine where winds have thickened and stiffened them. That's near ridges, on top-loaded or cross-loaded slopes, and near Marias Pass, where recent winds have been stronger. You'll need lots of luck to ride out of these slides. Or to survive getting caught and carried in one. Better to use that luck for lottery tickets and stick to slopes less than 30 degrees.

  • Glide Cracks

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    2–3

    An unusual number of glide cracks have formed on southerly slopes across the region this season. The latest - and largest - appeared in the last few days on the western side of the Swan Range. The likelihood of these failing is rising as warm air and sun produce meltwater that can lubricate the bed surface of these slabs. Unlike most avalanche problems, this hazard is obvious. Some cracks are hundreds of feet wide, and because they involve the full depth of the snowpack, they can produce very large avalanches that run well into valley bottoms. Stay out from under any brown frowns.

    (3/1/2021) The brown frown. Glide crack opened up 2/28 or 3/1

Avalanche Discussion

Sadly, a freezing rain crust deposited Sunday mars riding conditions in the northern half of the region.

Monday's partly sunny skies and spring-like temperatures warmed the near-surface snow enough to produce widespread rollerballs below about 6000 feet and a few natural sluffs that ran long distances.  Weather station data suggests a decent overnight refreeze at mid-elevations, though temperatures remain above freezing below about 4500 feet. Today's forecast calls for similar temperatures, more clouds, and continued moderate to strong winds at upper elevations.

Ah, the energy balance guesstimating game....shortwave in, longwave out, longwave reflected back in. Oof. The clouds, winds, and Sunday's freezing rain crust should inhibit energy input today. The near-surface snow will continue to warm, albeit slowly, with wet surface snow on sunny slopes at most elevations. Loose snow avalanches will pose a hazard where wet snow is more than boot-top or so deep: steep slopes at all aspects at low elevations, and on sunny aspects at mid elevations. Wet sluffs should be isolated at upper elevations. If the area sees less clouds than forecast, expect the hazard to be more widespread.

Three triggered slides this past weekend (Whitefish Range, Skyland remote, Skyland triggered) illustrate that it's still possible to trigger dangerous slabs that break on deeply buried weak layers. Indeed, it was only a week ago that mitigation in John F. Stevens Canyon triggered three very large avalanches that ran to valley floors (reports here and here). Warming may eventually sinter the facets and surface hoar that are widespread above the mid January crust. For now, though, they can still collapse and propagate the full width of start zones, producing avalanches with dangerous amounts of debris. It's March 2, not February 2, but this advice may sound like Groundhog Day nonetheless: stick to slopes less than 30 degrees that aren't below steeper start zones or above steeper rollovers. Steep slopes where recent drifting has thickened slabs are particularly sensitive. 

Brown Frowns! Some monsters have opened up on the west side of the Swan Range, and we've had numerous reports of glide cracks on southeasterly slopes in the Flathead Range since the start of the season, along with a few from the Whitefish Range. Warm temperatures increase the likelihood of the slabs below the cracks failing. It's a complicated set of processes, some delayed, so there's no direct correlation with maximum temperatures or direct sun. They're obvious, though, so it's easy to stay out from under them.

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