Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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1 · Debole
Limite del bosco
1 · Debole
Sotto il limite
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In sintesi

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 on middle and upper elevation south and west aspects. You are unlikely to see signs of instability before triggering an avalanche. Use terrain management to limit your risk and give yourself a wider safety buffer to handle the uncertainty. You can avoid the problems altogether by sticking to slopes less than 30 degrees.

You may encounter small wind slab formation on all aspects at upper elevations today. Red flags include cracking, collapsing, and recent slab avalanches. If you find any of these, avoid wind loaded avalanche terrain.

Problemi valanghivi

  • Wet Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    1.5–2.5

    Wet slab avalanches are highly unpredictable. They 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 extensive water percolation through upper elevations on south aspects. 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 throughout the snowpack. A wet slab avalanche could release without warning, run long distances, and be large and destructive. 

    The surface of the snow may be refrozen and supportable, 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, which can be squeezed into a tightly packed snowball. Rollerballs, wet surface snow, and recent wet snow 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. 

    (3/23/2024)

    Yesterday, in the Rattlesnake, we found hard and supportable surfaces as we skinned around. Digging a snowpit revealed a wet and weak snowpack lurking underneath.

    Photo: Payton Schiff

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    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 buried surface hoar existed midway down in the snowpack. Tragically, this avalanche resulted in a 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

All three zones received high-density new snow last night with gusty wind. We expect to see more high-density snow today. As with most storms, wind slab formation is a possibility, however high-density snow is not as easily transported by the wind. For this reason, we did not include wind slabs as an avalanche problem today but be on the lookout nonetheless. There are melt freeze crusts in many areas, which could act as slick sliding surfaces. Small fresh wind slabs could be found at upper elevations on both typical (north and east) and atypical (south and west) leeward slopes today. Older wind slabs in the Seeley Lake zone could still exist. If you see any cracking in new or recent snow, avoid wind loaded avalanche terrain.

This weekend's weather has been a far cry from the hot and sunny days we saw at the beginning of the week. We implore you to keep thinking about wet snow avalanches however. The snowpack has continued to hold onto warmth and moisture throughout. Rain was observed up to 7800 ft yesterday in the southern Bitterroot, which can rapidly decrease the strength of the snowpack as well. It is difficult to predict when and where a wet slab avalanche will release. You likely won't know until you're taking a ride in one. Use safe travel practices and terrain management to reduce your risk.

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. A load of new snow can increase sensitivity for a period of time as well. Find safe test slopes representative of the slope you plan to ski or sled before entering consequential terrain. 

Ski and sled safe!

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