Bulletin d’avalanche

Flathead Range & Glacier NP

Flathead Avalanche Center
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Danger d’avalanche

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

Points saillants

Unseasonably warm temperatures will make for dangerous avalanche conditions today. Very large persistent slab avalanches failed naturally in the past two days on various aspects at upper elevations. Plan your day to stay off of and out from underneath terrain steeper than 30 degrees and give southerly facing avalanche paths an extra-wide margin for another day.

Problèmes avalancheux

  • Persistent Slabs

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

    Very large and destructive avalanches failed Wednesday and Thursday naturally on aspects that face southwest to northeast at upper elevations. One of these propagated over 500 feet wide and ran approximately 2500 vertical feet. I’d expect this kind of activity to continue today with warming temperatures. There is a high degree of uncertainty of how widespread this activity will be today. The only way to manage this problem is by avoiding avalanche terrain.

    (3/4/2021) A very large Persistent Slab avalanche on an SW aspect in southern Glacier Park.

  • Loose Wet

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

    Today will be the warmest day of the season. Thankfully snow surfaces have been thoroughly worked over by wind and sun over the past week, which can reduce the amount of loose wet activity. I still suspect that sun-baked terrain can produce wet avalanches large enough to bury, injure, or kill you in isolated areas. Rollerballs or pinwheels are signs that conditions are ripe for wet loose avalanches. To reduce your risk of this problem, seek out shaded, northerly-facing slopes. Plan your day to avoid crossing below south and west-facing avalanche paths, especially later in the day.

    (3/4/2021) Size D2 wet loose avalanches on Mt Stanton. These occurred at 2:30 on the afternoon of 3/4/21.

  • Glide Cracks

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

    What looks like brown frowns on solar aspects are glide cracks. The entire snowpack is slumping and slowly sliding down the hill on the smooth ground underneath - like bear grass or bedrock. They can fail all at once, resulting in very large and dangerous avalanches. That’s more likely to happen when sun and warming temperatures lubricate the bed surface with fresh meltwater. Avoid traveling on slopes beneath these large cracks in the snow.

    (3/1/2021) Glide crack opened up 2/28 or 3/1

Avalanche Discussion

Another unseasonably warm day will make for dangerous avalanche conditions. As forecasters, we base a lot of decisions on uncertainty and confidence. These thought processes come after figuring out how the weather, snowpack, and terrain all intermingle on a given day. This helps us develop accurate forecasts and travel advice for you to use, or at least that’s the hope.

Today I have good confidence that warm temperatures and solar radiation will cause more avalanche activity. It almost feels inevitable, given today is warmer than yesterday. The past two days have produced large and very large avalanches with warming temperatures (observation 1, observations 2, observation 3). With that said, I have a higher than usual degree of uncertainty about how widespread this activity will be. Slopes may be primed for natural triggers today, or the snow will continue to sit precariously on top of weak snow above the mid-January crust. This type of persistent weak layer we are dealing with does not come around very often. 

Many things come into play today. Three days of unseasonably warm temperatures are further weakening the snowpack making it more unstable. Increasing winds out of the southwest and numerous freeze-thaws of the snow surface may make windward slopes less prone to natural avalanches. Subtle changes in atmospheric conditions today can tip the scales in either direction. This is uncertainty at its finest, and with uncertainty comes conservative terrain choices.

What we can do is take what we know and apply it to today. We know that in the past 48 hours, there have been numerous large and very large persistent slab avalanches that failed naturally. We know that the sun will warm the surface and will spur on more avalanche activity. Look at avalanche terrain today as unruly suspects. For today I’ll be staying far away from south-facing avalanche paths and keeping slope angles less than 30 degrees on all slopes. A useful saying has repeatedly come to mind the past few days: Weird weather makes weird avalanches.

Pro tip: Apply the acronym ALPTRUTH to your daily travels in the mountains. In one study, 92% of studied accidents happened when 3 or more of these things are present. How many are present today? If you want to learn more, listen to this podcast on your way to the trailhead today.

Avalanches: Were there recent avalanches in the last 48 hours?

Loading: Is there loading by snow, wind, or rain in the past 48 hours?

Paths: Are you in an avalanche path?

Terrain traps: If you get caught, what kind of terrain will you get dragged into?

Rating: Is the current avalanche advisory considerable or higher?

Unstable snow: Are you getting collapsing or cracking?

Thaw instability: Is there recent warming of the snow surface?

Régions couvertes