Lawinenlagebericht

Bitterroot

West Central Montana Avalanche Center
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Lawinengefahr

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

Today it will be possible to trigger fresh wind slabs along ridgelines. These slabs will grow through the day as winds increase ahead of an incoming storm arriving tomorrow. Watch for red flags like shooting cracks, recent avalanches, and freshly formed cornices. 

A lower possibility of triggering a persistent slab avalanche on a variety of persistent weak layers remains. Dig in the snow, and use slope angle to avoid the problem if you find weak, sugary snow above a firm crust. These avalanches are more unlikely on slopes under 35 degrees in slope angle.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    Wind speeds yesterday were higher than forecasted along ridgelines, and those increased winds continued through last night. As a result, forecasters found fresh wind slabs in their fieldwork yesterday and were able to trigger small wind slabs in the Bitterroot and Rattlesnake on northeast aspects.

    D1 wind slabs are likely today, and D2 wind slabs are possible. The slabs will continue to grow today as light snow falls ahead of the incoming storm, increasing the snow available for transport.

    Wind the past 24 hours was in the upper end of the moderate category out of the southwest, with averages in the 20mph range along ridgelines. Wind speeds will increase today with gusts into the low 40mph range from the west.

    Wind slabs can often be identified by their smooth, rounded appearance. If you encounter firm snow that feels hollow underneath or see shooting cracks around your skis or snowmobile, step out of avalanche terrain and stick to slopes under 30 degrees. 

    (2/23/2024)

    A small intentionally triggered wind slab on a wind loaded northeast aspect at 8100 ft on Sweeney Peak's east ridge. It was around 50 ft wide and up to a foot deep. 

    Photo: Arden Feldman

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Unlikely
    Größe
    1.5–2.5

    It is unlikely to trigger a persistent slab avalanche. Still, there is concern that as the top portion of the snowpack begins to round and become a cohesive slab, the sensitivity will increase, and we will see persistent slab avalanches become possible. 

    There are a variety of persistent weak layers in the snowpack with exact distributions still uncertain, including rounding facets near the ground, small facets above a crust in the middle of the snowpack, a thicker layer of buried near-surface facets above a crust, and buried surface hoar. Read the forecast discussion for a deeper dive into these layers.

    It has been nine days since a human triggered avalanche on persistent weak layers despite people skiing and riding slopes with known poor snowpack structure. If you do trigger an avalanche on these layers, it is unlikely that you will see obvious signs of instability like whumfing, collapsing, or cracking before the avalanche occurs. Snowpit tests have shown inconsistent results on these lingering persistent weak layers as well. 

    Digging in the snowpack is the best way to deal with this problem to look for loose, sugary snow with a slab above. If you want to avoid the problem, these avalanches become more unlikely on slopes under 35 degrees. Otherwise, it is a question of risk tolerance. 

    (2/23/2024) Photo: Payton Schiff

Avalanche Discussion

Today will be the last day of relatively benign conditions before a strong winter storm impacts the region with up to 2 feet of snow and strong winds. This storm will trickle in late tonight and increase in strength late in the day on Sunday. 

Temperatures today will be mild, in the low 30s, but moderate winds and increasing cloud cover should keep the snow from becoming wet. 

So how did we get where we are with the persistent slab problem with so many potential problem layers but no recent avalanches? We have to look at how these layers likely formed to get an understanding. In the Seeley zone, there was a close call with a skier-triggered avalanche on February 15th. On the 5th through 7th of February, this area saw warm temperatures and crust formation before 6 inches of snow fell on the 8th and 9th. On the 10th and 11th, clear conditions lead to near surface faceting in this 6 inches of recent snow. On the 12th through 14th, these near surface facets were buried when a larger storm dropped 18+ inches of snow. On the 15th, the avalanche was remotely triggered. Although the avalanche had all the characteristics of a storm slab, when WCMAC forecasters investigated the slide, they discovered it actually failed a bit deeper on the buried near surface facets just above the crust.

As we travel south through the forecast zones, these buried near-surface facets still exist, but the layer is less pronounced. In the southern Bitterroot, this layer is nonexistent, and the primary layer of concern becomes small facets directly above the crust, formed through a different process. When a crust is buried, it often has a strong temperature gradient surrounding the crust. This can create a thin layer of small facets directly above and below the crust.

There has not been a skier or snowmobile triggered avalanche in nine days on these layers. The snow above the persistent weak layers is beginning to become a more cohesive slab as it rounds. There is uncertainty whether this will lead to avalanches becoming more likely.  For now, unreactive persistent slabs are a matter of risk tolerance. You can stack the odds in your favor by recognizing and avoiding areas more prone to avalanches, like convexities, rollovers, and shallow areas around rocks, and looking for slopes with a smooth runout free of terrain traps.

Finally, some large facets remain lingering in the bottom of the snowpack near the ground. These formed early season when the snowpack was shallower and a strong temperature gradient existed within the entire snowpack. Since the snowpack depths increased, they have been rounding, but are still have not gained enough strength to be totally trustworthy. Triggering an avalanche on these is unlikely, but a smaller avalanche could step down and release a full depth avalanche in isolated areas.

Let us know what you see in the mountains this weekend by submitting an observation.

Ski and ride safe.

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