Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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

Be leery of steep and consequential slopes that face west, north, and east. These slopes harbor weak layers buried 8 to 16 inches deep. A whumpfing collapse indicates you have found an unstable slab. Choose lower-angle terrain or stick to south and southwest-facing slopes where the weak layer does not exist.

Expect rising avalanche danger tonight. Strong winds and new snow will build fresh slabs that may become reactive to the weight of person. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Reports of avalanches on buried weak layers have been scant since the 12/7-12/8 storm. In many locations, the overlying slab remains too soft to propagate. Exceptions to this are found in wind-loaded terrain and areas favored by snowfall where the slab is thicker and stiffer. The weak surface hoar layer is generally found 8-16 inches deep on shaded aspects at upper and mid-elevations. 

    Shooting cracks or a whumpf indicates you have located this problem and should move to a different slope or lower-angle terrain free of overhead hazards. It remains possible to trigger an avalanche from flat terrain above, below, or to the side of a steep slope.

    (12/9/2024) Crown profile on a northeast aspect at 6500 feet. This avalanche failed on surface hoar. Photo: Cameron Johnson

Avalanche Discussion

An atmospheric river will funnel moisture into the northern Rockies starting tonight. Precipitation numbers are not overly impressive (.5-1 inches of water), but wind speeds are with strong to gale force category winds. Expect the avalanche danger to rise with the arrival of this storm.

Daylight hours today will be fairly mild, offering us some safer riding before the storm. The most obvious problem to manage will be small slabs of drifted snow. These will be more common downwind of ridges and saddles. Slabs are a couple of days old, so they should be gaining strength and stubborn to trigger.

The less obvious problem is the persistent slab avalanche problem. A feathery layer of surface hoar or a layer of near-surface facets is buried 8 to 16 inches below the surface. It exists on shaded slopes at middle and upper elevations. We've found these layers in the Whitefish Range, Flathead Range, and Glacier National Park. Rain seems to have destroyed this weak layer before it was buried in the Swan Range. If you have not watched Mark's video, here it is. He does an excellent job explaining the distribution and evolution of the persistent avalanche problem. 

Today, the easiest way to avoid this problem is to ride on slopes that face south and southwest. Fortunately, we have not seen the sun in a few weeks, so those aspects are still riding well. If you choose to ride on other aspects, be aware of slope configuration. People often trigger slabs on these layers mid-slope, at roll-overs where a slope steepens. On Monday, we found more reactive results in our snowpit tests downslope from ridges, as shown in Monday's video (the first video in the gallery). Tomorrow, be alert for shooting cracks and whumpfing collapses, which can signal storm snow is overloading the buried surface hoar and facets.

Downloaded the app yet? You can now access our region's forecasts, observations, and weather data directly from the Avy App (IOS; Android). Many features are accessible offline, so you can doublecheck danger ratings and travel advice on the drive or in the backcountry. And submit an obs while driving home. 

Are you looking for avalanche education opportunities? Check out our calendar for upcoming events. A Partner Rescue Clinic for motorized Riders on January 4 would be a great way to start the new year.  

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