Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Dangerous avalanche conditions exist and will continue to develop throughout the day. It is likely to trigger a slab avalanche breaking up to 2 feet thick that will be large enough to bury, injure, and kill you. Winds are drifting snow into thicker, more dangerous slabs on leeward terrain features. Stick to terrain sheltered from the wind and keep slope angles less than 35 degrees if you see 8 or more inches of new snow stack up. 

Problemi valanghivi

  • Wind Slabs

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

    Overnight wind speeds were sustained at 15 to 20 mph with gusts as high as 92 mph out of the southwest. Human triggered and natural wind slab avalanches were reported on Saturday and Sunday, with intense blowing snow observed Sunday afternoon. Expect wind slabs to be more dangerous and larger today. Slabs can be up to 2  feet thick and will continue to grow in size throughout the day. Winds will shift to the northwest by late morning, making wind slabs more widely distributed across the landscape. Your primary objective should be to avoid being on or under wind-loaded slopes. Obvious signs of blowing snow and pillowy-looking surfaces are the kind of features to avoid. Slabs are forming on top of weak faceted snow and crusts, resulting in avalanches breaking wider distances and in surprising ways. 

    (3/27/2021) Increasing winds drifted snow into slabs that were reactive to the weight of a human.

  • Storm Slabs

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

    If the precipitation forecast verifies, expect a more widespread, soft slab avalanche problem to develop throughout the day. Pay close attention as you seek out terrain sheltered from the wind and observe conditions changing throughout the day. Look for cracking in the new snow and utilize small test slopes to see if you can initiate shooting cracks or small avalanches. Stick to slopes less than 35 degrees if you begin to see any of these signs.

    (3/23/2021) Natural storm slab avalanches on Nyack Mountain.

  • Persistent Slabs

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

    Weak faceted snow and surface hoar can still be found above the January 13th crust anywhere from 2 to 6 feet below the surface. We are more concerned about this layer's possibility of failing and producing a very large avalanche on slopes that lack a hard melt-freeze crust under the new and recent snow. Aspects that face northwest to northeast to east at upper elevation lack this crust and are currently getting loaded by wind and snow. Give this kind of terrain a wide berth today since being involved with an avalanche of this size would be very difficult to survive. 

    (3/16/2021) A hard slab of snow sitting over buried persistent weak layers.

Avalanche Discussion

So far, it’s been a lackluster storm as far as precipitation goes. Snow totals did not reach forecast amounts overnight, yet the storm remains on track throughout the day. Snowfall rates could reach in excess of an inch an hour, with totals reaching 6 to 10 inches in the Swan Range, Flathead Range, and Glacier National Park. The Whitefish Range may see similar totals in the northern part of the range and likely less in the range's southern part. 

On Sunday, winds picked up to strong speeds by mid-afternoon. A report came in from the Swan Range of intense blowing snow, which spurred on a slab avalanche that appeared to be large enough to bury and injure someone. I also observed this kind of blowing snow in the northern Whitefish Range and saw impressive transport on the drive home on peaks in the Flathead Range. On Saturday, Mark could trigger small wind slabs that were up to 8 inches thick in Glacier National Park. New snow will add fuel to winds which will further thicken slabs and create dangerous conditions.

New snow and wind-transported snow are landing on an array of weak interfaces. Crusts on southerly aspects and weak, faceted snow on upper elevation, northerly aspects. These faceted layers have been observed throughout the forecast area and most commonly above 6500’ on shaded aspects. These are the same slopes that are being loaded by snow and wind. Facets and faceted crusts were found roughly 1 to 2 feet below the surface on Sunday and are getting buried deeper by the hour today. The primary concern is that avalanches might propagate wider distances and be triggered remotely, making a run-of-the-mill storm slab or wind slab avalanche much more dangerous. It is best to give slopes steeper than 35 degrees at middle and upper elevation a wide berth today to see how these layers react to this loading event.

Weak layers from late January still show their colors in snowpits. On Saturday, we were able to find large grains of broken surface hoar and facets roughly 3 to 4 feet below the surface. I’d give upper elevation, leeward terrain, an extra-wide margin for error today. Not just to avoid the wind slab problem, but the chance of a smaller avalanche to trigger a very large, hard slab avalanche failing 2 to 6 feet deep. Hopefully, we don’t see any avalanche activity on this layer from this loading event, and we can take it off the list… at least for a little bit.

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