Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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

In sintesi

New snow and strong winds formed drifts sensitive to the weight of a human. These will be more common below ridgelines and on cross-loaded gullies further down the slope. You are less likely to trigger an avalanche on slopes sheltered from the wind. 

Problemi valanghivi

  • Wind Slabs

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

    Sustained winds formed stiff slabs of drifted snow. Winds were transporting snow along upper ridgelines yesterday (Monday). Remote weather stations throughout the forecast area picked up an additional one to four inches of new snow. These will be more common the higher you climb where recent precipitation fell in the form of snow vs. rain. 

    Utilize small and inconsequential test slopes with a spotter to assess the sensitivity of recently formed drifts. Shooting cracks are signs to seek out slopes sheltered from the wind, or to reduce your slope angle. Steer away from pillowy-looking drifts during your hunt for wind-protected snow. 

    (3/12/2023)

    This slab avalanche failed deeper than expected. The slab involved wind-drifted snow. 

    Photo: Brody Pate

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    Persistent Slab avalanche activity since mid-January is confined to aspects that face northwest-northeast-southeast. The last human-triggered avalanche was six days ago in the northern Whitefish Range. Snowpack tests show mixed signs of instability. The chances of triggering are low, but the consequences are severe. 

    The simple way to avoid the problem is to stick to low-angle slopes not connected to steeper ones where this problem exists. Avoiding common trigger points, such as convex rollovers, can reduce your chance of triggering. A person is more likely to trigger one of these deadly beasts in thin, rocky start zones where weak layers are buried closer to the surface. This type of terrain exposes weak layers closer to the surface, making them more susceptible to triggering. A cornice fall or triggered wind slab may step down to a much larger, more dangerous avalanche. 

    (2/26/2023)

    This Hard Slab failed on faceted (weak snow) below a crust. This crown spanned roughly 1600+ feet and took out trees. 

    Photo: Cam

Avalanche Discussion

Yesterday's (Monday) and last night's storm can be characterized by warm and windy. Temperatures climbed above freezing to roughly 6500 feet. Precipitation favored the Flathead Range, where the Tunnel Ridge weather station picked up four inches of new snow. Weather stations in the Swan and Whitefish Range saw no measurable change in snowpack height, despite getting a small amount of precipitation. The rain/snow line likely reached above 5500 feet. Winds cranked gusts above 50 mph in all three forecast areas. 

As of 5 AM this morning, temperatures are dropping below freezing at 4000 feet. This will help lock the snow in place where it got wet from rain and warm temperatures. Temperatures will continue to drop throughout the morning, which will thicken a surface crust. However, if you don’t feel a surface crust this morning, expect loose wet avalanches are still possible. For this unlikely scenario, stay out from underneath steep terrain at low elevations. This will likely only be the case during the morning hours due to dropping temps. 

On Sunday, a rider triggered a noteworthy slab avalanche in the southern Whitefish Range. This slab failed roughly a foot thick. This slide may have failed on weak sugary facets or a layer of feathery surface hoar that was buried beneath last week's storm. Both of these layers are persistent weak layers and can produce avalanches long after they get buried. Warm temperatures settled recent low-density snow, possibly forming a slab on wind-sheltered slopes. I'd be wary of slopes that got loaded by recent strong winds. Slides on persistent weak layers can propagate wide distances and be difficult to escape. Whumpfing collapses and shooting cracks are signs this unstable structure exists.

Fireside/Snowpack Wednesday, March 15. Join us for a virtual evening combining two favorite community ed workshops. FAC Forecaster Cam Johnson will kick things off with a State of the Snowpack presentation to help you digest the current conditions and plan your next trip. Then we'll shift to Fireside Chats for open dialogue about human and avalanche factors. A recreationist, forecaster, and avalanche educator all experienced a near miss in the first three weeks of February. Together, Kat, Cam, and Meg will explore the tough decisions, tricky snow, and lessons learned from their experiences. No registration is needed; click here for Zoom. The event will begin at 6 p.m. 

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