Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
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Lawinengefahr

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

Avalanches failing on buried weak layers and crusts in the top 1 to 2 feet is today's primary concern. Warmth and compaction turned the overlying snow into a slab, and the potential for avalanching has increased, but the grains have healed in many places. Today's warm temperatures will cause some concern of loose wet avalanches at mid and lower elevations, and small wind slabs at ridgelines on north and east aspects are also possible. Choose dry, non-loaded, planner slopes with clean runouts free of steep convexities over 35° to avoid today's hazards. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    It will be possible to find fresh wind slabs along ridgelines on east and northerly aspects at upper elevations today. Snow showers brought a few inches of fresh snow, and we observed productive westerly winds in the Rattlesnake yesterday. These small slabs could be touchy today with near-surface facets and melt-freeze crusts as bed surfaces. It’s unlikely but plausible that larger lingering wind slabs exist. These have shown signs of bonding in recent days. 

    Continue to look for clues like wind textured snow, drifts, and wind lips to indicate wind direction. Hollow-sounding snow and shooting cracks show that wind slabs are nearby and reactive. 

    Avoid wind-loaded slopes with angles over 35 degrees.

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2.5

    It is still possible to trigger avalanches on near-surface facets sitting over the slick rain crust formed by the atmospheric river event in early March. They are buried 1 to 2 feet deep. Elevations between 6,000 and 8,000 feet where the crust provides a bed surface and temperatures remaining cold enough for facets to linger are most suspect. Recently formed melt-freeze crusts higher in the snowpack may also harbor near-surface faceted layers. 

    The weight of you or your machine may trigger avalanches on persistent grains and the potential exists for a small wind slab or cornice fall to step down to these layers. We’ve been finding it on multiple aspects with varying degrees of reactivity throughout this week. As warmth, more snow, and compaction consolidate the overlying snow into a slab its potential for avalanching has increased. However, in some areas, this same process has allowed the facet grains to heal and bond together. There is no way of knowing exactly where these layers exist and to what degree they are capable of producing an avalanche. Our best bet for reducing the risk of triggering is to consider the terrain we choose to travel.  You’re most likely to trigger it on steep convexities over 35°. It’s possible to run on any slope over 30°. Choose planner slopes with clean runouts if you find it necessary to step into steeper terrain.

  • Loose Wet

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–1.5

    Rather than direct sunlight (though it could poke through the clouds in some places), elevated temperatures will be the driving factor in loose wet instabilities forming today. Expect melting snow throughout the day. Temperatures will rise into the mid-50s for the Bitterroot and mid to upper-40s for Lolo, the Rattlesnake, and Seeley Zone. Thunderstorms will bring rain up to 5,500 feet tonight. 

    Rollerballs and sinking past mid-boot in wet snow are clear signs that loose wet avalanches are forming nearby. Staying off steep slopes with wet saturated snow and staying alert for overhead hazards to avoid getting caught in a loose wet slide.

Avalanche Discussion

Warm temperatures on Thursday left a thin zipper crust below approximately 7,600 feet in the Rattlesnake and below 6,400 in the Bitterroot. Snow showers deposited 1 to 3 inches with snow lines near 4,500 feet Thursday night and yesterday morning. The showers dissipated, and temperatures climbed above freezing in the afternoon for elevations below 7,500 in the Rattlesnake.

The warm temperatures over the past few days and light snow showers have aided slab formation and weight over the March 1st rain crust layer. The thin melt-freeze crust formed on Thursday has near-surface facets atop showing signs of reactivity in the Rattlesnake yesterday. While the deeper facets over the rain layer were less reactive in the area we dug, 7,800 feet on a north aspect, fresh slabs forming over these top layers could be touchy today, and there is the possibility of them stepping down to the rain crust in some areas. 

The snowpack has a complex picture right now with wet snow, dry snow, and numerous crusts, some with facets, some not. I will keep it simple and use terrain to dictate my likelihood of triggering an avalanche. Steeps with convexities over 35° will throw red flags in my mind. Slopes above cliffs, gullies that funnel avalanche debris, abrupt benches in the terrain are all places to avoid. There are plenty good safe turns out there and this week provided some of the best of the season. 

Ski and ride safe. 

 

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