Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

A lurking weak layer in the snowpack has resulted in recent large avalanches. Traveling in the mountains today requires the knowledge that there is still a possibility of triggering a 1-2 foot thick slab avalanche. Choose terrain free of convexities to limit your chance of triggering a slide. 

Lawinenprobleme

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2.5

    Saturday's avalanche activity verified that it is still possible to trigger a slab that breaks 18 to 24 inches deep on thin layers of weak snow above the very hard, early February ice and crust layer. Notably, these layers are better developed on shady, upper-elevation slopes but are also found on isolated sunny and mid-elevation slopes. 

    The weekend's warm temperatures and recent winds have stiffened the snow surface, forming a slab in many locations. You may not get warning signs of unstable snow, such as cracking beneath your boards or machine. Choose terrain free of convexities to limit the chance of triggering a slide. Steer around thin areas, such as those scoured or peppered with rocks, in favor of a deeper, more consistent snowpack. 

    (2/17/2024) Photo: Zach Miller

    This is an example of an avalanche that was skier-triggered on 2/17/2024 demonstrating characteristics of a Persistent Slab problem.

Avalanche Discussion

On Saturday, three separate skier-triggered avalanches in the Marion Lake area were symptoms of a Persistent Slab problem.  These slides resulted in one incident which required a helicopter evacuation, as well as two close calls. As we investigated the characteristics of these avalanches, some themes emerged.

Temperature: Friday’s solar input began the initial warming of cold surface snow, which accelerated with Saturday's warmer temperatures. This transition from a loose surface to a consolidated slab was the missing ingredient for the Persistent Slab problem. An impressive temperature change is a hallmark of increasing instability within the snow. Regarding the three avalanches occurring on February 17th, all three slides occurred in the afternoon, around or slightly after the day’s peak warming.

Terrain: Skiers triggered slides on steep, northerly upper-elevation slopes with convex rollovers in all three avalanches.  These slopes have been the target of FAC’s messaging about the current Persistent Slab problem. The snowpack’s structure of weak faceted snow above a stout crust has been unpredictable, and multiple people have skied similar or adjacent slopes before triggering an avalanche. The lack of obvious signs of instability increases the need to pay attention to terrain features. Thin parts of the snowpack and convexities increase the probability of propagating an avalanche.

Community: All three parties involved in Saturday’s events were transparent and forthcoming with details of each incident. For the skier caught and carried, it was extremely fortunate to have ski partners who acted quickly and thoughtfully. Two Bear Air and ALERT Air Ambulance were invaluable resources that evacuated and transported the victim to the hospital for overnight care. 

Practicing safe travel habits can stack the odds in your favor. These habits include communicating and being aware of your partner’s intended travel.  Get in the habit of identifying potential avalanche paths and positioning yourself in a safe spot within the terrain. Unpredictable avalanches, such as the current Persistent Slab problem, should garner your respect. If you head out into the mountains today, be mindful of changing conditions, the slope aspects you ride, and where you and your partners are positioned in the environment.

Upcoming event: Flathead Avalanche Friends Fest, February 21-24. We have four days of events on tap for you: Beacons & Brews, Name That Tune/Name That Avalanche, Workshop Night with State of the Snowpack and USGS, Hot Dog Days, and the Ultimate Après with Backcountry Partner Speed Dating and the Big A$$ Raffle. More info at flatheadavalanche.org/event/friendsfest

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