Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Sun
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Conditions are growing more complex, so it pays to stick to simpler terrain where you're less likely to be surprised: sheltered, mid-elevation slopes with no steep start zones overhead and less than 8 inches or so of recent snow above the mid-January crust. In higher, steeper terrain more exposed to recent winds, it's becoming easier for a person to trigger a large, dangerous slide as the recent snow consolidates into slabs. You can trigger these slabs from below and the resulting slides can run long distances. Shooting cracks, whumpfing collapses, and fresh avalanches clearly signal increased danger. If the sun and warming forecast for tomorrow arrive, expect a round of natural avalanches from steep terrain.

Problemi valanghivi

  • Storm Slabs

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

    You can find good riding with less avalanche hazard on slopes sheltered from the wind where the recent snow remains loose or is less than about 8 inches deep. These conditions are most widespread at mid elevations. Where the recent snow has consolidated into slabs more than about 8 inches thick, you can trigger large avalanches that break a foot or more deep and run dangerously fast and far. The failure plane for these slabs is a relatively uniform layers of weak facets and surface hoar above the 1/13 crust. The likelihood of triggering slides is highest near ridges at upper elevations, where winds earlier this week stiffened and thickened the slabs. You can trigger these slabs from below or from adjacent slopes, so avoid traveling or riding underneath steep start zones. Stay alert for shooting cracks and whumpfing collapses, which clearly signal the more dangerous conditions. 

    Looking up at the crown and runout of a remotely-triggered slide that occurred Friday on a leeward, upper-elevation slope in the Flathead Range.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2

    As you approach 7000 feet elevation, the 1/13 crust thins and peters out, so the potential lingers for triggering slides that break 1 to 3 feet feep on old weak layers, or for triggered slides in the recent snow to step down to these layers. You can distinguish the isolated slopes where this hazard remains by their shape and snow surface. Specifically, check for convex profiles (less steep to more steep) and signs that winds have left variable snow depths, like drifts alongside eroded patches or rocky outcrops. On these slopes, weak layers that have been dormant the past two weeks may have reactivated under the incremental loading of the past few days. In a snowpit, these layers appear as a gray stripe of surface hoar, or thin, soft layers of sugary snow around thin crusts. You can avoid this guessing game by sticking to slopes less than about 35 degrees that have a constant pitch throughout, or that get less steep lower on the slope. 

Avalanche Discussion

A few inches of new snow and light winds overnight, yet the forecast danger rises? And rises again Sunday, with little new snow and continued light winds? Today, the increase is because it's becoming easier for a person to trigger a large, dangerous slide as the recent snow consolidates into slabs. Sunday, the forecasted sun and warming should spark a round of natural avalanches that may run surprisingly far. While riding conditions have improved, avalanche conditions are growing trickier. Keep that enthusiasm dialed down and a relaxed thumb on the throttle. 

The culprit behind the tricky conditions is the slick, hard 1/13 crust. It provides a very distinct bed surface on which recently-developed weak layers can collapse. And it allows slabs and sluffs to run fast and surprisingly far. Sluffs observed Tuesday ran well over 1000 feet, and Friday's triggered slides (Flathead Range and near Fernie, north of our forecast region) ran over 1200 feet. The fast, long runouts increase the potential consequences for a person caught in the debris. Trauma is more likely during the ride, and as the slides entrain more snow, deeper burials grow more likely. The long runouts also increase the overhead hazard. While you may find just a few inches of recent snow on a mid-elevation slopes, slides can run down from start zones well above you, where there's more snow. 

All that said, you may see few signs of instability today. Steep test slopes and column tests may not produce propagation. You may not observe any shooting cracks or whumpfing. Expect, however, an abrupt transition into dangerous conditions, with the potential for surprises. Keep that in mind as you climb into higher terrain where winds accompanying the recent snow had more effects. You'll find more cornices and drifts nearby, and drifts up to several feet thick immediately below ridges, at the tops of chutes, and downwind of passes and saddles. These pockets are well-connected to slabs further downslope, and weak layers that developed during our late January dry spell are widespread. The potential for remote triggering and long running slides makes it important to stay out from under steep start zones, especially any near ridges where you see blowing snow. 

Today's conditions are reminiscent of the first week in April last year, when most of the season's close calls occurred. Then, as now, conditions abruptly changed for the worse as a slab consolidated above facets left by a long dry spell. People were enthusiastic about improved riding conditions, feedback indicating an increase in danger was rare, then, boom, triggered slides and near-misses as the loose snow settled into cohesive slabs.

At upper elevations where the 1/13 crust isn't present, triggered slides may step down to older weak layers. These layers have been dormant for most of our late-January dry spell. The incremental loading this weak may be waking them. Avoid steep terrain with likely trigger points. 

 

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