Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

Wed
Alpin
2 · Modéré
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Reduce your chances of triggering a large slab avalanche by riding in areas where a thick, supportable rain crust exists below the surface snow. Uncertainty grows above roughly 7200 feet where this crust becomes less supportable and fewer observations exist. Select slopes with deep, uniform snow cover, and hedge your bets by selecting slopes with smooth, clean runouts.

Problèmes avalancheux

  • Persistent Slabs

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

    Five days have passed with no warning signs of instability. A hard rain crust and a deep snowpack reduces the chance of triggering a large, destructive avalanche. Common trigger points on a slope are where the snowpack is thin, is rocky, and from convexities where the slope breaks from less steep, to steeper. There is a higher degree of uncertainty above 7200 feet where we have less observations, and where you do not find a supportable crust beneath the snow surface. Seek out slopes that are planar in shape with relatively uniform snow depths. Expose only one person at a time if you choose to step out into avalanche terrain. 

Avalanche Discussion

I’ll start off by discussing locations where you may find trouble and where we have more uncertainty. There is a higher level of uncertainty above roughly 7200 feet due to a lack of observations. The uncertainty here is the strength, or weakness, of the November facets. This elevation is also where the rain crust becomes unsupportable to the weight of a human. Areas where you may find trouble are where the snowpack is thin and shallow. Thin spots develop where winds scour away snow and drift it elsewhere. One thing to note is we’ve had long stretches of both northeasterly and southwesterly winds. In areas where the snowpack is thinner will be where the November facets are larger and weaker, and where the weight of your body or machine can collapse this layer of faceted snow. Your primary tool to assess these two factors will be probing in the snow, feeling for both the depth of snow and crust hardness.

There is less uncertainty where the rain crust exists and is supportable. There is also less uncertainty about collapsing that weak layer where it is buried more than a meter below the snow surface. Snowpack heights between 6000 and 6500 feet vary between 1.5 meters to 3 meters throughout the forecast area. Thankfully, many options exist for us below 7000 feet. Remember, because it is unlikely, does not mean it is impossible.

If your decisions take you onto steeper slopes, here are a few pointers/reminders. Travel one at a time and position your partner out of harm's way where they can see you. Break the slope into numerous pitches to maintain visual contact with one another. Select slopes that fan out at the bottom vs. slopes that terminate above cliff bands, trees, or steep gullies. Terrain traps compound the consequences of being caught in even a small avalanche.

It is quite a relief as a forecaster to look back at 5 days worth of observations (roughly 43) with no signs of whumpfing or recent avalanches. This trend spun a 180 on us compared to the beginning of December when nearly every observation reported a thundering collapse. In large part we can thank the Christmas rain event for this shift. Mark does an excellent job describing the processes at play in Monday’s forecast discussion (found here).

Régions couvertes