Lawinenlagebericht

Rattlesnake

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

Small human triggered wind slab avalanches are possible today at upper elevations. Old wind slabs can be found on north and east aspects. A storm impacting the forecast region tonight will create fresh wind slabs on upper elevation west and south aspects. If you see cracking in new snow or recent slab avalanches, avoid steep, wind loaded terrain.

Although unlikely, you could trigger a large persistent slab or wet slab avalanche today. Weak layers buried 2 to 4 feet deep linger on upper elevation north aspects. Wet snow within the snowpack can still be found at middle and upper elevations on southerly aspects. You are unlikely to see signs of instability before triggering an avalanche. Give yourself a wide safety buffer to handle the uncertainty, and use terrain management to limit your risk. You can avoid the problems altogether by sticking to slopes less than 30 degrees.

Lawinenprobleme

  • Wind Slabs

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

    Small old and new wind slab avalanches are possible at upper elevations. 3-4 inches of snow fell yesterday with wind from the north and west in the Seeley Lake zone, creating small wind slab formation on leeward (east and south) slopes. 4-5 inches are forecasted for tonight, with wind predominantly from the east, which will create fresh wind slab formation on the other side of the compass rose (north and west). Although small, wind slabs could easily slide on slick crusts below. Expect to find wind loaded terrain on all aspects at upper elevations by tomorrow morning. 

    Wind slabs can be found on or just below the downwind side of exposed ridgelines, behind rocky outcrops, on rolls or convexities, or in cross loaded terrain features such as gullies. They can look smooth, rounded, and pillowy.  Snow will feel more dense and consolidated underneath your skis or sled than surface snow that has not been wind affected. It may even be hard and sound hollow, like a drum. Be on the lookout for rippled, uneven snow surfaces, indicating that wind has transported snow, and wind slabs may be lurking nearby.

    Red flags include cracking, collapsing, and recent slab avalanches. Cracks shooting out from your skis or machine clearly indicate that you've found an unstable slab; however, if the snow is very hard, you may not see cracking before it's too late. A wind slab avalanche could step down and trigger a larger persistent slab or wet slab avalanche. Stick to wind sheltered slopes less than 35 degrees if you find signs of instability.

    (3/3/2024)

    Shooting cracks triggered in wind slabs on northeast upper elevation aspects.

    Photo: Payton Schiff

  • Wet Slabs

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

    Wet slab avalanches are highly unpredictable but occur due to water percolating (flowing) down through the snowpack and pooling on crusts and weak layers, decreasing their strength. Recent observations have found evidence of water percolation through upper elevations on south aspects after a long period of warm and sunny weather. Observers reported large natural wet slab avalanches in the Swan Range earlier this week.  Although the last few days have been cooler, layers of wet snow can still be found. A wet slab avalanche could release without warning, run long distances, and be large and destructive. 

    The surface of the snow may feel hard and be refrozen, but that doesn't mean the snowpack is fully frozen and stable underneath. You can identify this by digging down and looking for wet snow. Wet snow can be squeezed into a tightly packed snowball. Rollerballs (pinwheels), wet surface snow, and recent wet loose or wet slab avalanches are indicators that the snowpack is losing strength. If you find any of these signs or wet snow buried within the snowpack, consider stepping back to lower angle terrain. Limit your risk by going out earlier in the day and choosing cooler and shadier slopes. 

    (3/22/2024) Photo: Payton Schiff

    Lingering moisture and new crust formation were found yesterday in the Northern Bitterroot.

  • Persistent Slabs

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

    Triggering a persistent slab avalanche is a low-likelihood, high-consequence scenario. This problem has been unreactive and had not produced avalanches in weeks, until last week when a group of skiers triggered a persistent slab avalanche in the southern Bitterroot. This avalanche occurred on a slope where a layer of especially weak surface hoar existed above a crust midway down in the snowpack. Tragically, this resulted in an avalanche fatality. You can read the preliminary report here. 

    Recognize the continued potential for large and dangerous persistent slab avalanches. Use your individual or group risk tolerance to dictate how steep of a slope you are willing to expose yourself to. You can reduce your risk by defaulting to shorter, planar slopes without convexities or rollovers and with runouts free of surface obstacles and terrain traps. Use slope angle to avoid the problem altogether by sticking to slopes less than 30 degrees.

Avalanche Discussion

The avalanche problems that exist within our snowpack right now are difficult to predict and manage. It is important to have a solid understanding of your personal risk tolerance and to use the tools in your toolbox to make good decisions in the backcountry.

There is still poor snowpack structure of facets above and below a melt freeze crust buried midway down in the snowpack. For the most part, these layers have become well bonded and the persistent slab problem is largely unreactive. However, if you find an isolated pocket where particularly weak buried surface hoar still exists, the likelihood of triggering an avalanche increases quickly. Find safe test slopes representative of the slope you plan to ski or sled before entering consequential terrain.

Wet slab avalanches create a lot of uncertainty as these types of avalanches can occur at any time during periods of extended warm weather. We are no longer in a period of mild weather, but heat and moisture can remain with the snowpack for a long time. Temperatures that continue to hover around freezing at night and cloud coverage, which can create a greenhouse effect during the day and night, have kept the snowpack from fully refreezing. There is potential for rain at middle elevations today, which will rapidly decrease stability on all aspects. Be especially cautious if you are out during the warmest daylight hours, and don't be fooled into thinking the snow is completely stable because the surface has a hard and supportable crust.

Avalanche danger will increase as a storm impacts the northern end of the forecast region tonight. Significant snow accumulation is unlikely in the Bitterroot, so we have not included a wind slab problem for this zone. However, small wind slab formation could be possible, so be alert for signs of instability and changing conditions. The avalanche danger will be at its peak during the overnight hours and into early morning tomorrow.

Ski and sled safe!

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