Boletín de aludes

Swan Range

Flathead Avalanche Center
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Peligro de aludes

Sun
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

 Riders can trigger slabs 2 feet thick in wind-affected terrain and slopes sheltered from the wind. The danger is most significant in locations facing NW-NE-SE. Choose lower-angle terrain free of overhead hazards if you recreate on shady slopes. Sunny and windward aspects offer a generally safer experience. A storm overnight may raise the danger by tomorrow morning.

Problemas de aludes

  • Persistent Slabs

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

    The snowpack has had an opportunity to adjust to Thursday's storm snow which dropped over an inch of water at Noisy Basin. The chance of a rider triggering an avalanche on a weak layer buried around 2 feet deep has decreased slightly over the past two days. However, buried surface hoar is a tricky layer not to be trusted. Continue to dial back terrain choices in areas this layer exists, primarily NW-NE-SE at the mid and upper elevations. Slabs will be thicker and more dangerous as you climb in elevation. Choosing lower-angle terrain free of overhead hazards is wise until we can rule this problem out. There is limited evidence these weak layers exist in the windward and sunny aspects. On these aspects, utilize hand pits to identify the layer and assess the surface for the sign of a slab. Without both of those ingredients, there is no Persistent Slab problem. 

    (1/20/2023)

    One of two buried surface hoar layers in the top 50 cm of the Snowpack.

  • Wind Slabs

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

    Winds will increase this afternoon and blow at speeds capable of transporting snow. Plenty of snow is available for transport from the recent storms that have favored the Swan Range, and southwest winds may form thin slabs on leeward slopes by sunset. These slabs will become thicker and more widespread overnight and tomorrow. An avalanche in the wind-drifted snow can step down to a buried weak layer resulting in a much larger slide. Look for blowing snow as you climb in elevation and note the density change in the snow you are traveling through to identify a slab. Cracking in the wind-drifted snow is a sign of instability, directing you to seek safer conditions in sheltered terrain.

    (1/1/2023)

    An excellent example of cross-loading on Elk Mountain, Glacier Park.

    Photo: Mark Dundas

Avalanche Discussion

10-day precipitation totals across the forecast area can be divided into haves and have-nots. The Swan Range has taken home the lion's share with 4.4 inches of SWE since January 9 at the Noisy Basin SNOTEL. Meanwhile, mid-elevation weather stations in the northern and southern Whitefish Range, interior Glacier Park, and John F. Stevens Canyon have picked up a mere 1.2 to 1.9 inches of SWE. Distribution of buried weak layers formed around January 9 can, to a lesser extent, be lumped into the same have and have-not groups. In the Swan Range, a well-documented surface hoar and a faceted layer are found at mid and upper elevations, primarily on NW-NE-SE aspects. There is evidence of these weak layers in the other ranges, but they do not seem as widespread or well-developed. The presence or absence of buried weak layers combined with the amount of recently fallen snow is responsible for the discrepancy in our avalanche danger ratings. 

Swan Range: Yesterday's dry weather and light winds, combined with today's dry daylight forecast, allow us to drop the danger rating at upper elevations to MODERATE on NW-NE-SE aspects. The hazard on slopes facing S-SW-W has been lowered to LOW.

  • In the Swan Range yesterday, Rob found two layers of buried surface hoar in the top 50 cm of the snowpack in hand/test pits on W, S, and E aspects at middle and upper elevations. Small and large column tests failed with medium and heavy force, including a propagating result in a south-facing test pit. There was little sign of wind effect in an area that typically funnels SW winds.

Whitefish Range, Flathead Range, southern Glacier Park: Continued mild weather consisting of light winds and light precipitation keep the avalanche danger steady in the Whitefish Range, southern Glacier Park, and the Flathead Range. In these areas, slabs resting on weak snow remain thin and are less dangerous than the current condition in the Swan. 

  • An experienced rider visited the side country east of the WMR ski resort. He skied trees and open lines on east, south, and west aspects 30-40+ degrees in steepness. Aside from sluffing of the top 10 cm of soft snow on steep rollovers, no signs of instability were observed. The 1/19 rime crust was covered with 6-10 cm mid-density snow, and 10 cm below that, there was another breakable rime crust. 

This evening, a cold front will enter the Flathead bringing snow and moderate to strong winds lasting through the day Sunday. The avalanche danger may rise overnight and most likely during the day tomorrow. Precipitation totals look modest, but the real story might be the winds. New snow and wind may add thickness to existing slabs and build fresh slabs in locations currently lacking a slab. Leeward aspects are the locations where the near-surface weak layers are most prevalent, and wind-drifted snow will test the strength of our upper snowpack.

Education: Motorized Partner Rescue - Join us for an eight-hour field day on Saturday, January 28, 2023. More details are here.

Motorized Level 1 - Instructed by Eric Knoff and Six Points Avalanche Education from Tuesday, February 7, 2023, to Thursday, February 9, 2023. More details are here.

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