Lawinenlagebericht

Flathead Range & Glacier NP

Flathead Avalanche Center
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Lawinengefahr

Sun
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

Today's increasing temperatures may lead to a rising avalanche hazard. Surface slabs and loose snow may be easier to trigger than over the past few days. Small surface slides can cause a larger, more destructive avalanche on a buried weak layer. When in doubt, default to lower-angle terrain free of overhead hazards.

Lawinenprobleme

  • Wind Slabs

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

    Light southwesterly winds have drifted this week’s snowfall into slabs at the upper elevation. These features may form on surfaces of low-density snow or facets atop hard slabs, resulting in avalanches that can break easily and wider than expected.  

    Look for wind-affected snow along ridges, gullies, and other prominent terrain features. Ripples, dunes, and fresh cornices, are all signs the wind has been at work. You've found a slab if the snow surface becomes stiffer than you've been traveling through. Cracks shooting out from your skis, board, or machine indicate instability.

    Previous northeasterly winds drifted snow in unusual locations at all elevations, including on or near the valley floor. These slabs, however, have had adequate time to heal. Natural or triggered avalanches at the surface have the potential to initiate the larger Persistent Slab avalanche described below. 

    (1/18/2024)

    Wind-driffted snow was confined to ridgelines at this location. 

    Photo: Cam Johnson

  • Persistent Slabs

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

    You can still trigger a hard slab on weak layers buried 2-4+ feet deep. Avalanches failing on these layers occurred during each of the last two storms; however, the distribution of this problem remains unclear.

    You may or may not observe obvious warning signs like snowpack cracking or collapsing before triggering a slide. Avalanches could be triggered remotely, from gentler terrain below, above, or to the sides of steep slopes. Persistent Slab avalanches can break wider and higher on slopes than expected. Small slides in recent and wind-drifted snow can step down to these layers and initiate a large avalanche. 

    The only way to identify this problem is to dig into the snow. Look for weak snow resting on a crust formed in late December. Below 5500-6000’ look for depth hoar near the ground.

    This problem may exist on isolated, open, unvegetated slopes at low elevations, like steep cut banks and road cuts.

    (1/19/2024) January 19th, 2024: Looking across and down the slope at the debris from the avalanche in Shed 9.

  • Loose Wet

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

    Copious cold, low-density snow fell at all elevations Wednesday. Temperatures skyrocketed overnight and are hovering around freezing at mid and upper elevations. This rapid temperature change may cause natural wet loose slides and increase a rider's chance of triggering one. Slides can entrain substantial amounts of loose surface snow.

    Snow falling from tree branches and rollerballs are precursors to a wet loose slide.  Stick to lower-angle slopes without overhead hazards to increase your safety.

    (3/24/2022)

    Recent warm temperatures and solar input initiated slides large enough to bury, injure, or kill a person.

    Photo: Mark Dundas

Avalanche Discussion

Temperatures this morning are 20-30 degrees warmer than this time yesterday. A rapid increase in temperature is a red flag as this can cause rapid settlement in the recent snow and increase sensitivity. Furthermore, a couple vigilant observers already reported freezing rain at low elevations this morning. Rain on snow is a surefire way to cause an avalanche, but today’s incoming precipitation should be minimal.  Regardless, be on the lookout for signs that the snow surface is becoming wet like roller balls and pinwheels.  There’s a lot of recent snow out there and it could get messy as it warms up.

More tolerable temperatures also allowed folks to return to the mountains yesterday (Friday). A handful of observations continued to document widespread soft slab activity during peak snowfall Wednesday evening. They did not note any lingering signs of instability.

BNSF Avalanche Safety returned to the alpine as well and offered this synopsis of the Persistent Slab problem from their program area in John F Stevens Canyon:

  • Below 5500-6000’, where there was little to no snow before the New Year, there is a layer of depth hoar at the base of the snowpack that has been buried by 2-3’ of snow from the January storms. Ski penetration is often still to the ground.
  • Above 6000’, where a more developed snowpack exists, the 2-4’ of snow from the January storms is resting on top of a weak layer of facets that rests on top of a stout melt-freeze crust from late-December.

They also added “Both weak layers are producing audible collapses and shooting cracks. I lost count of the number of audible collapses, but I have not experienced that many in one day for quite some time.”

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