Lawinenlagebericht

Swan Range

Flathead Avalanche Center
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Lawinengefahr

Fri
Hochgebirge
3 · Erheblich
Waldgrenze
3 · Erheblich
Unterhalb
2 · Mässig
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Veraltet — dieser Lagebericht ist 4 years ago abgelaufen. Du siehst vergangene Verhältnisse, nicht den aktuellen Bericht.

Kernpunkte

Winds have built reactive slabs in unusual locations. Large natural Persistent Slab 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
    1–3

    Recent north and east winds have drifted low-density surface snow onto atypical aspects. Wind-affected snow may have a scalloped surface or be deposited into pillowed dunes that feel stiff or hard. Blowing snow along ridgelines is a clue that winds may be actively loading start zones. Cracking beneath your machine or skis is a sign of instability. Stick to terrain sheltered from the wind for safer riding and skiing. Winds howling out of Badrock Canyon have drifted snow into hard slabs near the north end of the Flathead Valley; a wind slab avalanche hazard may exist on usually harmles slopes, like cutbanks above creeks and roads and sledding hills.

    (12/21/2022)

    Moderate to strong north-northeast winds transport snow from trees and forest openings southward along the west side of the Swan front. The majority of the transport occurred at low and mid-elevations.

    Photo: Mark Dundas

  • Persistent Slabs

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

    Natural avalanches have failed on buried weak layers within the last two days.  Recent snowfall has increased the load on weak snow near the ground. Whumpfing collapses signal that the snowpack is primed for human triggering. You are most likely to trigger a slide where the snowpack is thinnest, like rocky, wind-scoured slopes or convex roll-overs. The best method for managing this problem is to plan your route to avoid traveling on or under steep slopes - those about 35 degree or steeper, or, for a greater margin of safety, slopes of 30 degrees or more.

    (12/21/2022)

    A large natural avalanche (SS-N-D2-O) that likely failed on a persistent weak layer near the ground, likely the facets buried Nov. 22.

    Photo: A. Clark

  • Storm Slabs

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

    On wind-sheltered slopes, this week’s low-density snow is settling into soft slabs that can be triggered by the weight of a person or snowmachine on steep slopes. These can break one to two feet deep on density changes within the new snow or at the interface with the previous snow surface. Triggered avalanches will be larger and more widespread at mid and upper elevations, while isolated slabs may exist at low elevations.

    Unstable slabs will often – but not always – give themselves away with warning signs like shooting cracks. The most straightforward way to reduce your risk of this hazard is to stay off slopes that are steeper than about 35 degrees. On slopes between 30 and 35 degrees, avoid riding above terrain traps like trees and gullies.

    (12/1/2022)

    Localized cracking in the recent storm snow, 12/1/22. John F. Stevens Canyon. 

Avalanche Discussion

The winter solstice arrived Wednesday at 2:48 pm, and we welcomed the second winter day with a WIND CHILL WARNING for dangerously cold wind chill temperatures as low as 60 below zero. Ambient air temperatures across the area this morning are brutally cold, including readings of -44 in Polebridge and Marias Pass.

Not surprisingly, most observers avoided Wednesday’s frigid temperatures. This lack of feedback makes forecasting challenging. Thankfully, the BNSF Safety Team delivered my early Christmas present: an observation of three large avalanches in John F. Stevens Canyon!  These slides were on mid and upper-elevation southerly aspects and appeared to fail on the mid-November weak layer, which is responsible for our Persistent Slab problem. This is a likely confirmation—and a reminder,  that the PS problem is still with us. 

The BNSF team estimated these three slides likely ran Tuesday afternoon/evening, as the crowns were partially filled in with new and drifted snow Wednesday morning. Southerly aspects generally hold a thinner snowpack due to exposure to solar radiation and the prevailing southwest winds. Recent observations note the slab above the weak layer is thinner, and it is easier to affect buried weak layers than on the deeper northerly slopes. What gets me excited about learning about these three slides is that it illustrates the storm’s interplay of both wind and snow, and for this example, leads to tipping the balance, resulting in Persistent Slab avalanches.

Tuesday’s storm brought a lot of new, low-density snow to all three zones of our forecast area. Automatic weather stations sometimes underrecord new snow totals when we receive large quantities of low-density snow. Old-school manual measurements are generally more accurate. The BNSF team manually recorded 22 inches of new snow on the valley floor from the recent storm. We can assume that mid and upper elevations received more snow, which makes this a memorable storm for Northwest Montana.

Training: Now's a great time to hone your companion rescue skills. Transceiver parks are open at Whitefish Mountain Resort, one at the summit and one adjacent to Grey Wolf. Both are accessible to snowmachines. You can get more details about training and education opportunities events on our events calendar.

Observations - Thanks to everyone who's been utilizing our new observation submission page. Try it out and help your community by sharing what you see in the backcountry. The form requires you to fill out the location fields and put a dot on a map. Be as specific or vague as you want. Our new Observation View page highlights signs of instability and allows you to filter by date or zone to pull out reports relevant to your objectives.

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