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 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Winter-like weather and new snow avalanche problems have made a return appearance to the Flathead. Thin slabs of snow are developing above 5000 feet and will be most dangerous on leeward aspects where the wind has drifted them eight inches or thicker. Look for blowing snow and monitor new snow depths as you gain elevation.  It is still possible to trigger a slide on buried weak layers on slopes without a supportable surface crust. Choose terrain 35 degrees or less in steepness to minimize this hazard.

Problemas de aludes

  • Storm Slabs

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

    Snow has returned to our area, albeit slightly less than forecast or as widespread as we hoped.  Regardless, 2-5" of low-density snow is reported at mid-elevation weather stations this morning, with light to moderate accumulations expected throughout the day. As snow accumulates, we will transition from a wind slab problem confined to leeward aspects to a storm slab problem with a broader distribution. Thin slabs will form on a slippery melt-freeze crust or a layer of weak faceted snow on shaded upper-elevation slopes. Both scenarios will inhibit bonding and possibly allow the hazard associated with this problem to linger longer than normal. Utilize hand pits to gauge how well the new snow is adhering to the underlying layer. Terrain sheltered from the wind will provide for the best chance of finding cohesionless snow.

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    2–3

    Last week's warm sunny weather wetted the snow surface on all aspects and elevations. Above 5000 feet, recent cool temperatures formed a stout supportable surface crust on sunny aspects and a thinner, generally unsupportable crust on shaded aspects at upper-elevations. In areas where you or your machine break through the crust, there remains the chance that you can trigger a slide on weak layers buried 2 to 6 feet below the surface. These locations are where you need to identify terrain features to minimize your risk of initiating a large dangerous avalanche; slopes 35 degrees or less in steepness from top to bottom with runouts free of surface obstacles which could magnify the consequences of getting caught.

    (3/17/2021) The crown from a hard slab avalanche that's more than a few days old.

Avalanche Discussion

Far from a snowpocalypse, or even a big powder day, but we'll take it. After a lengthy absence, the snow has returned to the Flathead. Accumulations aren't much, two to five inches at mid-elevation stations, but enough to start spring off on the right foot. The summit of the Big Mountain and the Tunnel Ridge station in the Flathead Range came in on the higher end, with the northern Whitefish Range mostly skunked. Yesterday, an observation from the Essex area reported slightly higher totals with no wind. It sounded like they enjoyed some mid-winter blower powder on a northerly aspect. Cam and I were on the eastern side of the Swan Crest, where we found 2 to 3 inches of low-density snow, which sluffed easily on the underlying crust. We experienced terrain-driven winds from the northeast with light to moderate drifting snow. We had the option to ski steep northerly terrain but opted to avoid the Persistent Slab problem and chose a southerly aspect with a supportable crust which allowed for surprisingly fun dust-on crust riding.

Today, if you're in areas unaffected by the wind, you may only find cohesionless powder with no evidence of a slab; if that is the case, congratulations. On these slopes, you may need to manage your sluff. Dry loose sluffs will be a concern on steep slopes, particularly those that harbor a surface crust, which is many. Due to the thin layer of new snow, these sluffs will start small but can run long distances on the slippery crust. Fortunately, there is little chance that they will break through the crust and entrain old snow. Be on your toes if recreating directly above a terrain trap where debris can pile up surprisingly deep.

You may have noticed that we trimmed the distribution of the Persistent Slab problem from the mid-elevation band and on sunny aspects at upper-elevations. This is due to the recent cool temperatures locking up the surface snowpack on aspects that went through a melt-freeze cycle last week. Southerly slopes hold a stout surface crust that should support you or the weight of your machine. Northerly aspects at mid-elevations hold a mostly supportable surface crust. Northerly aspects at upper-elevations barely got wet last week, and therefore the crust is thin and unsupportable. In areas without a stout surface crust, we recommend choosing terrain to minimize your chances of triggering a large unsurvivable slide.

Last week's summary of weather and avalanche conditions can be found in the media gallery or here.

 

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