Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
1 · Gering
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Veraltet — dieser Lagebericht ist 3 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Sheltered terrain is where you will find powder riding and skiing. Varying generations of wind-drifted snow may still be reactive to human weight. Continue to monitor changes in the snow surface, as you travel through different aspects and gain elevation. Buried weak layers within the snowpack are still a concern, as a small slide could initiate a much larger avalanche.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    North and east winds strong enough to transport snow have buffeted our area for the past 36 hours. Speeds are tapering this morning but still capable of forming slabs and cornices in locations favored by these atypical winds. These features may develop on a firm sliding surface such as a sun crust or previously scoured slope, adding to their instability. Older slabs formed from intense southwest winds Thursday and Friday are lurking on the opposite side of the compass. Today, expect to find a patchwork of soft and stiff features in all aspects. At lower elevations, north and east winds are notorious for wreaking havoc on and just above the valley floor. 

    If the snow you are moving through becomes deeper and denser, you are likely moving through a wind slab. Snowpack collapsing or cracks shooting out from your skis or sled indicate you are traveling on an unstable slab.

    Riders report excellent snow on slopes sheltered from the wind and sun. Seek out these areas for a safer and higher quality experience.

     

    (3/4/2023)

    A wind slab avalanche (top of photo) triggered a very large hard slab avalanche on Nyack Mountain. This slide occurred during an intense wind storm on March 2.

    Photo: md

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    2–3

    Thursday's intense wind storm triggered a very large avalanche on a buried weak layer in the Flathead Range (photo at right/above). This slide confirms that triggering 2-4'+ thick avalanches on buried weak layers (sugary facets or crust + facet combinations) remains possible in this range. Recent snowpit tests show signs of improving stability as the weak layers strengthen. This layer is becoming buried to a depth that a rider or machine is unlikely to affect. Most observed avalanches on these layers occurred on NW-NE-SE aspects at the mid and upper elevations where these grains are weakest. 

    Identifying and testing this layer is a chore, as it involves lots of digging and time spent in snowpits. Test results may be difficult to decipher. Until we can feel comfortable removing this problem, a preferred method for traveling safely is to focus on selecting the appropriate terrain. Ask yourself, "Where can my weight affect this layer the most? " The answer is thin windswept or rocky areas where the weak snow is closer to the surface. "Which slope shape is most susceptible to triggering?" Convex slopes or rollovers where stress is concentrated in the snowpack. "If I trigger a slide, will the terrain below me amplify the consequences of getting caught?" Choose slopes with clean runouts free of surface obstacles that could produce trauma or terrain traps where the snow piles up deep. Enjoy lower-angle slopes away from overhead hazards to increase your safety margin further.

    (3/4/2023)

    A very large D3 hard slab avalanche on Nyack Mountain. This slide was triggered during an intense wind storm on March 2.

    Photo: md

Avalanche Discussion

An arctic air mass has settled into the Flathead, with mountain temperatures hovering on either side of zero Fahrenheit this morning. Fortunately, the cold north and east winds have moderated from yesterday's readings which included 19 hours of strong sustained and extreme or gale-force gusts, including a reading of 73 mph! There remain up to 12 inches of loose, dry snow available for transport in many locations facing north through east. Winds from the northeast affect vast areas, such as the front side of WMR and the Swan front, while leaving many narrow drainages in the Middle Fork corridor and elsewhere virtually unscathed. They often intensely affect the valley floor as the cold, dense air sinks and spills out to the west. 

Yesterday, observers found great riding conditions on north aspects from ridgeline elevations down to the valley floor. In areas sheltered from the wind, blower pow was identified by several parties. Saturday's sunshine left a thin surface crust on solar aspects up to 7000 feet in the Flathead. The FAC secret agent observer in the Swan noted substantial blowing snow from east to west along the Swan Crest throughout the day.

Sunday, we dropped the Persistent Slab problem from the Swan Range, where we have not observed slides on the buried mid-January weak layers for the past 12 days. In most locations, the weak layer is buried well over 3 feet. This is at a depth where the weight of a rider or their machine is less likely to affect. Also, a strengthening process occurs as the overlying weight compresses this weak layer. This problem remains in the Flathead Range, where the likelihood of triggering remains POSSIBLE, and in the Whitefish Range, where the possibility is UNLIKELY.

Avalanche Encyclopedia: The National Avalanche Center has issued a new and updated encyclopedia containing 257 avalanche-related terms, a new graphic design, a new hierarchical term structure and navigation, and a new user interface. The US Forest Service funded the project. The writing and content were produced by Zach Guy, former Director of the FAC. You can find it here.

State of the Snowpack and Fireside Chats. Join us at 6 pm on March 9 for a virtual evening combining two popular community ed workshops. New this season, State of the Snowpack is a presentation from FAC on recent weather and avalanches to help you digest the forecast and plan your weekend. Fireside Chats are an intimate, conversational forum for open dialogue about human and avalanche factors. With skier/psychologist Dr. Sara Boilen as our facilitator, we dig into the tough decisions, tricky snow, and lessons learned from local near misses. No registration is needed; click here for a Zoom invite.

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