Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Thu
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Dangerous conditions exist at upper elevations, where a complex yet localized set of avalanche hazards exists. On steep slopes, you can trigger dense slabs of drifted snow or softer slabs sitting on weak snow buried in the past few days. At low and mid elevations, smaller slabs exist and will still be dangerous above terrain traps. In today's cold temperatures, even small mishaps can have exaggerated consequences.

Problemi valanghivi

  • Wind Slabs

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

    Avoid steep slopes with freshly-formed slabs of drifted snow more than about 8 inches thick. These will be largest and most widespread at upper elevations. They may also exist in atypical locations, like gully walls and cutbanks near valley bottoms. Be alert for hard, hollow-sounding snow and shooting cracks, which show that these slabs are hollow. Distinguish between wind-affected snow surfaces and dangerous slabs

    (1/29/2022)

    Shooting cracks in hollow, stiff snow are a sign that wind slabs are unstable.

    Photo: Clancy Nelson

  • Persistent Slabs

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

    Beware of convex rollovers well below ridges and steep, open glades on easterly and northerly slopes above 6000 feet. This is the terrain where you're most likely to trigger a slide that breaks on recently buried surface hoar or other collapsible layers. These layers are 10 to 18 inches below the snow surface. They can break widely across a slope or be triggered from below or from adjacent slopes. Whumpfing collapses are clear signs that these layers exist and can be triggered by your weight or the weight of your snowmachine. 

    (2/1/2022)

    This snowboarder skinned near a convexity to test the slope and triggered an avalanche that failed on a layer of buried surface hoar. North aspect, mid-elevation.

    Photo: Zachary Dugan

Avalanche Discussion

Complexity and uncertainty are the words of the day. 

Avalanche and riding conditions will vary markedly in short distances today, thanks to Tuesday's dramatic weather shifts. Roughly 3 to 12 inches of low density snow accumulated across the region prior  to a blast of northerly and easterly winds. The winds arrived late morning near Essex, and mid to late afternoon in the Whitefish and Swan Ranges. Temperatures plummeted below zero following the winds. Wind speeds diminished at mid elevations overnight.

Today, the trick will be distinguishing dangerous wind slabs from wind-affected snow. Pay attention to slope angle and the thickness of the drifted snow; steep slopes with more than 8 inches of dense or hollow feeling snow at the surface are where you're most likely to trigger a slab large enough to bury or injure you. This combination will be most widespread at upper elevations, though you may find smaller slabs at mid elevations in drainages that channel these winds, or all the way down to valley floors. It's important to distinguish these from wind-hardened snow surfaces, or wind crusts, which may only be several inches thick. Regular probing with a ski pole or hand pits are a great way of determining when the drifted snow is thick enough to be dangerous.

While sheltered slopes that retain soft snow offer better riding conditions, they may harbor a different hazard - collapsible persistent weak layers. The storm snow of the past two days has buried soft, weak layers of surface hoar and facets around crusts. So far, only the surface hoar has been reactive. Riders have reported an intentionally triggered slide on this layer in the southern Whitefish Range Tuesday, and audible collapses in near Essex both Monday and Tuesday. Ski cuts at WMR have also produced several slides on this layer. So far, it seems to exist above 6000 feet on northerly and easterly slopes and is buried about a foot deep. Based on the observatons Monday and Tuesday, it does not seem to be present in John F. Stevens Canyon. No reports from the Swan Range yet. The layer of facets buried at about the same depth has not yet been reactive. My confidence in this pattern is low this morning; we just don't yet have a lot of reports, and upslope winds on easterly and northerly slopes - where these collapsible layers are best developed - may have stiffened the snow surface enough to make these collapsible layers more reactive.

We've removed the Deep Persistent Slab avalanche problem from the Flathead Range, because available evidence indicates it's currently dormant.

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