Bulletin d’avalanche

Whitefish Range

Flathead Avalanche Center
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Danger d’avalanche

Fri
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

New and drifted snow will keep the danger elevated today and Friday. Very large natural and triggered avalanches the past two days demonstrate the lingering potential for triggering dangerous slabs that break 2 to 4 feet deep, on weak snow buried in January. These slabs will remain sensitive to a person's weight on steep slopes. Freshly-formed wind slabs will develop into a hazard near ridges and other terrain that collects wind-blown snow. If triggered, these may step down to the January weak layers. Stick to slopes less than 30 degrees that aren't below steep start zones, particularly those with active wind loading. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Weak layers near the mid-January crust can still fail and propagate the full distance across steep starting zones. The steeper the slope, the more likely the failures will produce avalanches. The bigger the start zone, the bigger the resulting avalanche. Gambling that you'll miss lurking trigger points isn't a sustainable safety strategy. Sticking to slopes that are far less likely to release is more reliable.  Find slopes that are less than 30 degrees from top to bottom and aren't connected to steeper terrain above. Stick to these. In current conditions, even short, steep slopes can be dangerous, particularly if they're above trees, gullies, flat benches, and other terrain where trauma and deep burials are more likely.

    (2/24/2021) The crown of this triggered slide shows why slope angles and convexities are so important.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Gusty westerly winds and light snowfall will combine to create reactive wind slabs at mid and upper elevations today. Look for pillows of drifted snow on the lee sides of ridges and chutes and downwind of saddles and trees. These will be thicker and can break wider at upper elevations. Cracking around your board, skis, or snowmachine is clear evidence of this hazard. Once triggered, these dense slabs can step down to weak layers near the mid-January crust, producing much more dangerous avalanches. Hightail it out from under steep start zones near ridges where you see blowing snow. 

    (2/23/2021) Blowing snow

Avalanche Discussion

Sometimes it's not the number, it's the scale. Only a few reports of natural avalanches since Monday, but they were doozies: a large slide on a southwest - west facing slope in the Middle Fork, a very large natural avalanche that crossed McDonald Creek, and another that ran over a snowshed in John F. Stevens Canyon. The crown for that one was up to 15 feet deep. 

Triggered avalanches in the past week have also been disturbingly large. These include a crown 1000 feet across in the northern Swan Range that left a snow biker partially buried and injured, and 2-3 foot thick crown on a slide triggered by a snowboarder in Canyon Creek. A group in near Tranquil Basin remotely triggered a slide 3 feet deep. And then there were the slides triggered Wednesday in a rare test of our snowpack with more than body weight. See the videos and images, and this summary. 

This avalanche activity has some very critical take home messages for those of us that like to ride in the backcountry. First, the weak snow buried near the mid-January crust can still fail and propagate long distances. Hundreds to thousands of feet, if a start zone is that wide. Second, it's not taking very large loads to trigger these slabs, even if many slopes aren't failing naturally. The explosives used Wednesday weren't large. They may be slightly more effective at triggering slides than a person. Slightly. Betting one's life on the difference isn't going to work for long; all it will take is hitting one shallow spot where your weight starts a collapse. The slab will do the rest. Third, most of the avalanche activity this past week occurred on slopes that face southeast to south to west. Not the aspects where we saw activity earlier in the cycle. So we can no longer use aspect to reduce our exposure to this hazard.

That leaves one simple but reliable tool. Slope angle. Slabs on slopes less than 30 degrees aren't likely to release even if you start a collapse. That number, once again: 30 degrees. Not 35. Not "it isn't as steep as what I normally ride." In case you missed it earlier: 30 degrees. Anything more than that substantially increases the risk of triggering an avalanche that breaks across the terrain feature your on. Those avalanches are likely to be serious even on small terrain featuers, and unsurvivable in large bowls and chutes. Measure the angle of slopes you plan to ride. They may be steeper than they appear.

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