Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 5 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

While today's list of potential avalanche problems is long, the individual hazards are not widespread, natural avalanches are unlikely, and except on isolated slopes, it will be tough to trigger a slide. However, any avalanches will likely have serious consequences, and the complexity of the problems is keeping the danger elevated. Because many of the hazards involve overhead threats, assess dangers at your feet and on steep slopes well above you. Sticking to slope angles less than 35 degrees is the most reliable way of dealing with this complexity.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    The monsters under the bed. Not under every bed, but under enough steep slopes that it's critical to pay close attention to the steepness and shape of terrain above you and at your feet or track. Slopes steeper than about 35 degrees with variable snow depths and convex rollovers are where you're most likely to trigger one of these large, hard slabs. Pick lines that avoid stairstep-shaped slopes with visible rocks that suggest there might be thin, weak spots. On sunny slopes where you or your machine sink into wet snow, meltwater may be destabilizng weak layers. Retreat to less steep slopes with thicker snow. 

  • Glide Cracks

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2–3

    What looks like brown frowns are glide cracks. The entire snowpack is slumping and slowly sliding down the hill on smooth ground underneath - like bear grass or bedrock. They give a rumpled look to the snow surface down slope. They can fail all at once resulting in very large and dangerous avalanches. That’s more likely to happen when rain showers lubricate the bed surface with water. Cracks are scattered on sunny aspects throughout the region, though outliers exist. 

    (3/17/2021) Glide cracks on a southeast-facing slope. The rumpled snow and apparent moguls below the cracks are signs the snowpack has deformed as it's moved downhill.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Most mid and upper elevation slopes have gone one night without a refreeze, and some have gone two. Rain showers this morning will also add water to near surface layers of the snowpack. Triggered sluffs of wet snow will be possible on steep slopes where the snow surface consists of unsupportable wet snow more than 6 inches or so deep. Stick to slopes where the slope angle is less than about 40 degrees or the snow surface supports your weight or the weight of your snowmachine. 

    (3/7/2021) Natural wet loose running through not so obvious avalanche terrain.

Avalanche Discussion

I could use some new icons for avalanche problems today. One might be a graphic for a kitchen sink, because we have a host of potential but isolated avalanche hazards today. Another might be a graphic of a piano falling on a running person, because many of those potential problems involve overhead hazard. 

One difficulty in determining the primary hazard today is a change in the weather pattern. Colorado wants its sunny days and clear, cold nights back. Washington filled the void early this morning with clouds and showers, both rain and snow. It doesn't look like accumulations will amount to more than a few inches in most locations, though moderate to strong winds might be enough to blow some snow around.

Let's sort through the dishes in the sink. 

Persistent Slab: Yep, still a lingering concern about triggering large, hard slabs that break on weak layers buried in late January.  The structure exists but they've been unreactive. It's been nearly a week since the last reported hard slab. The weather today and tomorrow shouldn't increase the likelihood of triggering these Yetis. They'd be unsurvivable if you did, so it's been the primary driver in how I pick routes and move through terrain.  

Cornice Fall: Large brows and waves of wind-packed snow still loom over many north through east facing slopes. They've sagged in recent warm temperatures, as evidenced by gaping holes  along ridges. Gravity is still pulling these blobs towards the Pacific, and the change in temperature isn't likely to speed or slow that inexorable process today.  A falling cornice can pose a danger in itself - that's some haaaard snow - or act as a trigger for the large, hard slabs that break on persistent weak layers. 

Glide Slabs: A rumpled, bumpy snow surface marks where the entire snowpack has been sliding downslope. Often there are clear cracks at the top of these deformed areas that look like brown frowns from below. These don't tend to break very wide, but because they involve the entire season's snowpack, they can pummel you from above. Rain showers might add liquid water to slick bed surfaces, speeding up glide. This hazard may be more likely with today's showery weather. 

Loose Wet Snow: Despite sun and warm temperatures, we've had few reports of natural or triggered sluffs of saturated surface snow in the past few days. The surface snow is draining any meltwater produced during the day. Moderate to strong winds should keep the snow cool at upper elevations. The rain forecast for today - 0.1 to 0.3" or so - is unlikely to overwhelm the drainage capacity on most slopes. The excaptions will be very steep slopes where the snow surface hasn't refrozen in the past two nights. I don't expect most triggered or natural wet loose slides to be large enough to injure or bury a person.

Wind Slabs: Isn't that a dry snow avalanche problem? Yes, and on shaded slopes above about 5500 feet the snow surface remained dry until late Thursday. The snow on those slopes is faceted and weak, and thin pockets of drifted snow will be sensitive to a person's weight. I just don't think they'll be enough snow for these to form in more than the most isolated spots up high.

We definitely need the kitchen sink icon. Keep in mind, however, that those concerns are isolated; the slopes where you could trigger each of these share common characteristics, but you're unlikely to trigger an avalanche in most of that terrain. Careful, slope-specific assessment of conditions and consequences can help sort out the complexity.

And the falling piano icon - nearly all of those hazards involve avalanches that pose a threat by falling from steep slopes above you, as well as steep start zones and convexities where you are.

Regions covered