Boletín de aludes

Swan Range

Flathead Avalanche Center
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Peligro de aludes

Sat
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 3 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Slabs of wind-drifted snow reactive to the weight of a rider may be found in unusual locations. You could trigger a slide on a buried weak layer, resulting in a larger, more destructive avalanche. Dangerously cold conditions require a simple, straightforward field plan.

Problemas de aludes

  • Wind Slabs

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

    Atypical north through east winds are drifting recent low-density snow at all elevations. Slabs reactive to the weight of a rider can now be found on slopes that are usually windward. These features may be downwind of saddles, ridgelines, and in cross-loaded gullies. Unusual winds may form slabs in unique places at low and mid elevations, including on or just above the valley floor.

    Evidence of these brutal winds includes blowing and drifting snow, pine needles littered slopes, and scoured or scalloped surfaces. Cracking in dense dune features indicates instability, directing you to lower-angle sheltered terrain.

    (1/6/2024)

    The blue arrow denotes the direction of the prevailing westerly wind. This wind top-loads an avalanche path facing east and cross-loads another facing southeast.

    Photo: Mark Dundas

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    1.5–2.5

    This week's bountiful snowfall has buried weak snow and crust combinations 1- 3+ feet. Natural and remotely triggered slides have occurred on these layers since Sunday. We do not have the distribution of these layers pinpointed, and all steep slopes at mid and upper elevations should be considered suspect until proven otherwise. Collapsing or shooting cracks may give you a warning of their location. Otherwise, the only way to identify if a slope harbors these concerning layers is to dig into the pack and perform snowpack tests. 

    (1/11/2024)

    This 3-foot fracture line is the deepest part of a recent avalanche just east of the WMR ski resort boundary. This avalanche is in a popular side-country location and is out of bounds.

    Photo: Mark Dundas

Avalanche Discussion

Bitterly cold arctic air has settled over the Flathead Valley. Ambient air temperature readings are generally 20 to 30 below zero, with several stations at negative 38! Snowslip stopped updating at 2300 last night with a negative 38 reading and is probably colder than that now. Winds shifted to the north or east yesterday, producing dangerously cold wind chill readings. The National Weather Service has issued a wind-chill warning until 1100 on Saturday. Tunnel Ridge in the Flathead is the outlier station, where the wind remains southerly.

Snow and precipitation totals for the past 24 hours are minimal except for Noisy Basin in the Swan Range, where 10 inches of settled snow accumulated with 0.6 inches of water. If measured on a 24-hour storm board, this would equate to 6% density. However, due to settlement, we assume more snow than 10 inches fell, making the density even less. An observer in Ferndale reported 9.5 inches new with a mere 0.31 inches of water. This registers around 3% density, or as he stated, "blower indeed." Ferndale fluff?

Thursday's atypical winds were forming wind slabs on my tour yesterday in the southern Whitefish Range at all elevations. Our exit was challenging, with numerous fresh drifts and fallen tree branches adding a bit of excitement to warm us up. BNSF snow safety reported wind slabs forming at all John F. Stevens Canyon elevations. North and east winds are known for haphazard destruction. They often ruin the snow surface on the west side of the Swan Range and the southern aspects of WMR, while the narrow valleys of the Middle Fork and sheltered locations on the east side of the Swan go unscathed. Often, they do as much, if not more, damage at lower elevations in the valley floor.

We have added Persistent Slab to our list of problems this morning. We initially discussed including this problem last weekend, but for clear messaging, we utilized wind and storm slab to cover this past week's stormy weather. With the precipitation ending, our attention focuses on weak layers below the surface. Yesterday (Thursday), Cam and I visited the site of a Persistent Slab avalanche in a popular side-country location east of the WMR ski area. This slide was near a well-used skin trail and occurred either naturally or remotely triggered. The crown was up to 3 feet thick and failed on a thin layer of facets just above the 12/22 crust. On Six Mile in the Swan Wednesday, Rob noted propagation in snowpit tests on buried facets and surface hoar. In Lost Johnny Monday, Cam found facets resting on a crust that initiated a collapse and triggered a small 2-foot-thick, soft slab. An experienced rider remotely triggered slides on the 12/22 crust/facet combination Sunday on Six Mile.

 

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