Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

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

Weak snow buried 2 to 4+ feet deep remains our primary concern. If triggered, slides may break wide distances, be triggered from gentler terrain, and be large enough to bury and injure a human. Choose lower angle slopes or carefully select terrain that does not terminate in a stand of trees, gully, or cliff, where even a small slide can have a devastating outcome.

Lawinenprobleme

  • Persistent Slabs

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

    Soft and hard slabs of snow rest on weak layers above roughly 5000 feet. Yesterday (Monday), my partner and I triggered 4 audible collapses on an east-facing ridge. These are warning signs of unstable snow. Meanwhile, another experienced observer had no propagating results in snowpit tests on this weak layer, where it was buried deep in the snowpack. This type of inconsistent feedback is a prime example of the struggle with forecasting a Persistent Slab problem.

    The easiest way to avoid the problem is to stick to low-angle slopes not connected to steeper ones. If you decide to step out into steeper terrain, choose slopes free of terrain traps, like the ones mentioned in the Bottom Line. Expose only one person at a time, and avoid convex rollovers where the chances of collapsing a buried weak layer are higher.

    (2/26/2023)

    This Hard Slab failed on faceted (weak snow) below a crust. This crown spanned roughly 1600+ feet and took out trees. 

    Photo: Cam

Avalanche Discussion

A quick shot of moisture is in store for us. Expect anywhere from 2 to 8 inches of snow by nightfall. Believe it or not, the Whitefish Range is forecast to see the upper end of those snow totals. The Swan Range should also pick up 4 to 8 inches, and the Flathead Range/southern Glacier National Park is forecast to get 2 to 5 inches. This shot of snow should improve riding conditions, without increasing the avalanche danger. If you do see 6 or more inches of snow pile up by the end of the day, watch for loose dry sluffs and steer around small drifts on the downwind side of ridges and gullies.

On February 20/21st, an Atmospheric River dropped enough precipitation to cause a widespread avalanche cycle. A poor snowpack structure lead to some impressively wide and destructive avalanches above 5000 feet, especially in the Flathead Range. Large hard slabs were just observed in the northern Whitefish Range. Since the atmospheric river, strong winds from varying directions have thickened and stiffened slabs above persistent weak layers.

There are challenges to managing a Persistent Slab problem. We are in a period where signs of instability are inconsistent and becoming less common. The further away from the loading event we get, and the further down the weak layer gets buried, the harder it is for a human to trigger an avalanche. This kind of avalanche problem is notorious for catching the 3rd, 4th, or 8th person down the slope, thus making tracks untrustworthy. Slabs, especially where they are hardened by wind, can break far above you and be nearly impossible to escape. We are on a trend toward this problem becoming unlikely to trigger, but recent signs of instability are making us leary of that transition just yet. Although we suspect this problem no longer exists in the Swan Range, a lack of data limits our ability to remove the problem without a high degree of uncertainty.

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