Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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

Recent snowfall has improved the riding, but developing winds are drifting snow into slabs, which may be reactive to your weight. At upper elevations, you could trigger a slab in the surface snow or on a buried weak layer large enough to bury you. Shooting cracks and whumpfing collapses indicate that you should seek out lower-angle terrain. 

 

Problemi valanghivi

  • Wind Slabs

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

    Variable S-SW-W winds increased overnight and began drifting abundant low-density surface snow. Breezy conditions today should continue forming slabs that may be sensitive to your weight. These slabs will be thicker and more widespread as you climb in elevation, where recent snow totals increase and the wind is strongest. Look for and avoid fresh dunes, drifts, and pillows commonly found near ridgelines and the sides of cross-loaded gullies. Cracking in stiff drifts indicates that you've found a sensitive slab.  

     

    (12/3/2023)

    Recent west/southwest winds have formed thick snow drifts on leeward slopes. These slopes are dangerous due to the fresh slab resting on a weak basal snowpack.

    Photo: Mark Dundas

  • Persistent Slabs

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

    Rain earlier in the week saturated the snowpack and narrowed the locations where you can trigger an avalanche 2-3 feet deep on surface hoar or in facets near the base of the snowpack. Areas where these weak layers survived, are limited to upper-elevation slopes that remained above the rainline or mostly missed out on the precipitation. You're more likely to trigger these slabs where today's winds are actively loading them.

    To confirm if either weak layer is present, dig into the snow and look for a grey stripe of feathery crystals 2-3 feet below the snow surface or a layer of soft, loose grains above a hard crust near the ground. Collapsing or whumphing are clear indicators that you have found a buried weak layer. These signs should direct you to seek out lower-angle terrain without overhead hazards.

    (12/4/2023) Northern Swan, 6300', N aspect Surface Hoar standing tall below 20cm of new snow

Avalanche Discussion

Today's forecast calls for snow to favor the Swan Range, possibly an additional 8 inches or more by tomorrow morning. Much smaller amounts are expected in the other ranges. The storm slab problem continues for the Swan, but we dropped it from the problem list for the Whitefish and Flathead Range, along with Glacier Park. Observers note the 1-2 feet of low-density snow that fell on Thursday and Friday was cohesionless and did not possess slab characteristics. Storm slabs generally strengthen within a couple of days of forming, and I assume lingering instabilities are mostly healed. 

The two recent pulses of precipitation arrived with light winds, which left moderate amounts of snow available for transport. Winds increased overnight and will continue throughout Sunday. The speeds are not overly impressive, but drifting the soft, fluffy snow won't take much. Fresh slabs will be most dangerous and abundant at upper elevations. A good example is the Aeneas Ridge weather station (7186 feet), which has reported average hourly wind speeds above 30 mph for 8 of the last 12 hours with a maximum gust of 62.  If you're familiar with that area, we can expect the west-facing slopes to be scoured and interspersed with whales of drifted snow in the lee sides of trees. On the east side of the ridge, the steep chutes leading down to the Picnic Lakes are actively loading and forming slabs of drifted snow. 

The warm rain on Tuesday melted snow across the area, especially at low and mid-elevations. The silver lining to this depressing storm is that rain percolated through the snowpack in many locations. Before this storm, we were concerned about a buried surface hoar layer and a facet-crust combination near the ground. Recent observations note a saturated snowpack in many mid-elevation locations and some upper-elevation sites. We feel confident that the weak mid-elevation layers did not survive and removed the mid-elevation band from the Persistent Slab problem. We still need more observations to determine where this layer survived in the upper elevations.

 

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