Lawinenlagebericht

Whitefish Range

Flathead Avalanche Center
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Lawinengefahr

Thu
Hochgebirge
3 · Erheblich
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

Recent snow and Arctic northeast winds have loaded slopes in unusual locations. Large natural avalanches occurred Tuesday. Keep it simple today, as bitterly cold temperatures will complicate any mishap.

Lawinenprobleme

  • Wind Slabs

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

    The combination of new storm snow and increasing strong winds throughout the day can build wind-drifted slabs several feet thick. Strong northerly winds have been building drifts in unusual locations, which can catch you by surprise.  Expect larger and more reactive drifts on slopes where you see active wind transport today. You may find stubborn drifts of snow from earlier southwest winds earlier in typical leeward aspects. Slabs of drifted snow can be found below corniced ridgelines, in cross-loaded gullies, or behind wind-loaded rollovers. Seek out soft, wind-protected powder and watch for shooting cracks or collapses that warn you of unstable snow. 

    Loose Dry: The surface snow remains loose in many areas sheltered from the wind. Anticipate triggering a Loose Dry (sluff) on slopes steeper than 35 degrees. Fresh snow is resting on various surfaces, including crusts which may inhibit bonding. Manage your sluff and limit time spent above terrain traps or in long-running gullies.

    (2/20/2023)

    The crown of a large avalanche on a wind-loaded upper elevation slope.

    Photo: sw

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    1–3

    The possibility of triggering a slide on a buried weak layer has increased with the recent atmospheric river. Rapid loading can cause these layers to fail, producing large deadly avalanches. Although triggering one of these deadly beasts may be more difficult, the consequences will be severe. If triggered, slabs may step down to a deeply buried weak layer, causing destructive avalanches. Peristent slabs are notorious for wrapping around terrain features and being triggered remotely. The only way to manage this problem today is through effective terrain management - avoid traveling on and below steep slopes.

    (2/19/2023)

    This small slab failed on a buried layer of surface hoar. This weak layer can linger and cause avalanches for weeks to months after a loading event. 

    Photo: Cam

Avalanche Discussion

Observers yesterday reported a cycle of natural avalanche activity in the Flathead Range. Due to the storm conditions and limited visibility, the extent of the cycle is unknown. However, given the recent snow and wind, we anticipate that there will be a widespread avalanche cycle, which we are eager to report on. 

For context, the settled storm totals from our weather stations (snow water equivalent and new snow measurements) are listed below. The change in weather arrived late in the evening on February 17th and continued through the morning of February 21st.

  • Whitefish Range
    • Stahl: SWE: 4.0 inches, 22 inches of snow
  • Central Glacier National Park
    • Flattop: SWE 2.9 inches, 16 inches of snow
  • Northern Swan Range
    • Noisy Basin: 5.6 inches, 25 inches of snow
  • Flathead Range
    • Tunnel Ridge: 22 inches of snow
    • Shed 7: 18 inches of snow
    • Marias Pass: 17 inches of snow

In addition to new snow, the arctic front following the storm brought cold northerly winds and frigid temperatures. In the media of this discussion, I’ve attached an excellent graphic created by LeeAnn Allegretto, Meteorologist at the National Weather Service in Missoula visualizing the cold airmass moving at Marias Pass towards Essex. Even without wind chills, the temperature change in less than 24 hours, from the northern front, is impressive.

  • Marias Pass—-32 F to -10 F = drop of 42 F
  • Snowslip—-23 F to -19 = drop of 42 F 
  • Shed 11—25 F to -16 = drop of 41 F
  • Java East—-32 F to -4 = drop of 36 F
  • Essex—-34 F to -2 = drop of 36 F
  • Polebridge—-20 F to -2 = drop of 22 F
  • West Glacier—-30 F to 3 = drop of 33 F
  • Hungry Horse—36 F to 3 = drop of 39 F

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