Boletín de aludes

Flathead Range & Glacier NP

Flathead Avalanche Center
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Peligro de aludes

Tue
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Numerous slides failing 1 to 2 feet deep occurred as recently as Saturday. Be wary of slopes that face north and east since they were the hot spot for human-triggered avalanches. Slabs of wind drifted snow remains our primary concern. For safer riding, or if you want to ride in steeper terrain, choose windward aspects or aspects capped with a sun crust.

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1–2

    A few inches of snow and moderate to strong winds Friday and Saturday drifted snow into reactive slabs. These slabs rest on buried persistent weak layers that may still catch you off guard. Upside down snow and cracking are signs you have found a slab. This kind of structure will be more common if you venture onto north and east-facing aspects. Slabs may break a foot thick and run surprisingly far and fast thanks to a slick rain crust. Choose windward aspects for safer riding.

    (3/12/2022)

    A skier triggered a soft wind slab on the leeward side of the Swan crest. The debris pile was large enough to bury a person. 

    Photo: Nathan Basford

Avalanche Discussion

On Saturday, we received reports of roughly eight human-triggered avalanches. Three of those avalanches were large enough to bury and injure a person. Thankfully no one was carried or injured in any of these reported avalanches.

Some characteristics these triggered slides had in common.

  1. Most avalanches failed on east and north-facing slopes. Most slid mid-slope at around 6000 feet, and only a couple failed ridgetop at about 7000 feet.
  2. Blowing snow was the primary red flag. Only one group observed shooting cracks, and only one group had an audible collapse.
  3. Avalanches from all three forecast zones likely failed on a similar layer of near-surface facets.

The sensitivity of slides on Saturday surprised us. It only took .3 to .6 inches of snow water equivalent (SWE) and moderate wind speeds to tip the scales and overload buried weak layers. We typically don't see this kind of activity with that little amount of precipitation. A warming trend also contributed to slab formation. Warming may be why some slabs failed much further downslope.

So what can we expect looking ahead? Slabs are resting on one or even two layers of near-surface facets. In most locations, these layers are less than a foot below the surface, a depth you are likely to impact with the weight of your body. I anticipate slabs will be stubborn to trigger today with 48 hours since the loading event. We are looking at 1 to 2 inches of water tonight and tomorrow. This amount of precipitation will put these layers to the test again. There is a fair amount of uncertainty with buried weak layers and how long they will persist.

If you decide to step out onto steeper terrain on the north and east side of the compass, first acknowledge the number of recent human-triggered avalanches. If you choose to proceed, hedge your bets on terrain with clean and smooth runouts. Have exit strategies in case the unexpected happens. Be extra diligent with keeping your eyes on your partner riding. If your uncertainty grows, back off and choose simpler terrain. Windward slopes or slopes with a sun crust are unlikely to slide.

Here is a short article on near-surface faceting for those who want to learn more about the process. In northwest Montana, we deal with Diurnal Recrystallization and Melt-layer Recrystallization. Both of these processes occurred since the begining of March. 

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