Bollettino valanghe

Flathead Range & Glacier NP

Flathead Avalanche Center
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Pericolo valanghe

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1 · Debole
Limite del bosco
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Sotto il limite
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In sintesi

Enjoy today's mild weather and generally stable snow while recognizing that not all slopes have green light conditions. You can still trigger a slide on a buried weak layer in terrain without a surface crust. Maximize your fun and minimize your risk by choosing planar slopes free of convexities. 

Problemi valanghivi

  • Persistent Slabs

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

    An extended period of mild weather continues to strengthen weak layers 6-18 inches below the surface.   Buried surface hoar and facets may still be affected by the weight of a rider or their snowmachine in locations where a supportable surface crust does not exist. Saturday's propagation in snowpit tests confirms we are not out of the woods quite yet. If you choose to recreate in terrain on the north half of the compass, select planar slopes free of convexities to minimize your chances of triggering a slide. Choose areas with clean runouts free of surface obstacles that may limit the extent of trauma and burial if you are unlucky enough to awaken this layer.

    (2/12/2022)

    Riders opted out of skiing a steep northeast slope after noting propagation in their snowpit tests. ECTP18@25cm below the surface (x2) on a thin well-preserved graupel and facet layer at the interface between a 1F wind slab (above) with 4F rounding facets layer (below).

    Photo: Zach Miller

Avalanche Discussion

Thursday's tropical weather moistened the snow surface to at least 6000 feet on all aspects and much higher on sunny slopes. Several nights of a hard freeze at all elevations have left a stout crust on top of this once mushy surface. Shady terrain above 6000 feet is where you can escape the crust and the loud, annoying scraping noise of riding on top of it. Unfortunately, these are the areas where you can still trigger a slide on a buried weak layer. We dropped the avalanche danger to LOW Saturday and trimmed the distribution of the Persistent Slab problem to the slopes on the northern side of the compass.

The forecast map is green for LOW, but that doesn't necessarily mean that all slopes have green light conditions. An avalanche professional turned away from a steep upper elevation slope on a NE aspect Saturday in the Flathead Range. This party did not notice any signs of instability during their tour but decided to dig a pit before committing to a more consequential line. They had propagating results in snowpit tests and decided to continue their tour elsewhere. A seasoned backcountry rider ventured to the west face of the Great Northern Saturday. Pit results were unremarkable, and they noted no signs of instability but decided to bail due to uncertainty about the buried wind effect and the numerous thin trigger spots they would pass on the upper face. A FOFAC Intro to Avalanches class at the summit of WMR found buried surface hoar 14 inches below the surface with pit results that varied from ECTX to ECTP5.

Following an extended period of dry, mild weather, a pattern change begins Sunday evening. Thoughts of corn skiing will dissipate as clouds, cooler weather, and light snow return. The initial round of precipitation delivers very light snow Monday afternoon into Tuesday. Most of the energy will head south and deliver much-needed moisture to areas struggling with drought much worse than NW Montana. We have a bit better chance of accumulating snowfall Wednesday, with northwest flow developing and scattered snow showers through the end of the workweek. New snow will fall onto either a slick crust, windboard, or old settled snow - all potential sliding surfaces. Snow totals look modest, but we may return to a new snow avalanche problem if enough accumulates. 

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