Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 5 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Expect to find a mix of wet and dry snow problems today. Wet snow problems increase in size and likelihood as the day warms and last night's crusts break down. Small surficial wet loose avalanches are the most likely, but should be easy to identify and avoid. Much larger avalanches failing as wet slabs are much less likely, but continue to drive our terrain selection. If you head up high, look for fresh wind slabs sitting on slick crusts. The size and sensitivity of these slabs will depend on the amount of new snow you find. It feels like spring but it's not quite time to turn your avalanche brain off.

Problemi valanghivi

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    You may encounter a range of wet snow problems today. Small wet loose avalanches will be the most likely and the easiest to trigger, but larger wet slab, glide, and cornice-fall avalanches should remain at the forefront of your mind. The likelihood of encountering these issues increases through the day as the sun heats the snow. 

    WET LOOSE: These start at a point (rocks, trees, your skis) and fan out and gain mass as they travel downhill. You're most likey to find these on slopes facing the sun after last night's crusts break down and on shaded middle elevation slopes that remain soggy from Monday night's rain.

    WET SLAB: Last week's heatwave produced a number of very large, destructive wet slabs. In places, these gouged to the ground, picked up rocks and dirt, and snapped large areas of trees. Similar slides are much less likely now, but these beasts are unpredictable, and we've still got a good bit of water hanging in our snowpack.

    GLIDE: The Bitterroot Mountains have many smooth slabs of granite that produce glide avalanches. This type of slide occurs when the entire season's snowpack slides down these smooth slabs. Relatively fresh glide cracks and glide avalanches were observed in the Bitterroot as recently as yesterday.

    CORNICE: Windy March storms built many very large cornices along ridgelines. Many of these gargoyles peeled away during last week's heat, but some remain. Look for them and give them a wide berth.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–1.5

    Most of the recent precipitation fell as rain, but there is some fresh, dry snow up high. The size and sensitivity of today's wind slabs directly relate to how much new snow you are finding. Most places only tallied an inch or three out of this storm, but if you head back to the crest of the Bitterroot you may find significantly more. Pay attention to the amount of new snow and how the snow you are moving through behaves. If the snow you are moving through quickly become deeper, denser, and is producing shooting cracks, you've found yourself a reactive wind slab. If put into motion, these may travel quickly over the icy crusts left behind during last week's heatwave.

Avalanche Discussion

It has been a wild ride the past two weeks, here is a brief summary: Mid-March atmospheric rivers delivered feet and feet of snow to the mountains, resulting in a widespread natural avalanche cycle. Many slides failed on our January drought layer, breaking widely and mowing down swaths of forest in places. Coming close on the heels of this action, an impressive heatwave produced days of wet avalanche problems (loose, slab, and glide). Many slides ran thousands of feet and left deep debris piles. Some of the meltwater produced during the heat is still trickling its way down through the snowpack. A quick-hitting warm storm on Monday night into Tuesday brought rain to near or above summit level before temperatures cooled and precipitation turned to snow on Wednesday evening. 

Our snowpack has begun its transition to a springtime snowpack, but we are in the middle of this process. Cooling temperatures and clearing skies last night allowed the snow surface to refreeze, but underneath this crust is a stack of warm, in places wet snow. Let's take a look from top to bottom:

Up high on shaded slopes, the snowpack remains dry and it feels like winter. Here, you'll encounter fresh wind slabs that may be reactive to your weight. 3-5 feet below the snow surface, our weak layers formed in January and February remain. Triggering a slide on these layers is unlikely, but worth thinking about if you are heading into very steep, rocky terrain on the high peaks. 

The snowpack in the middle elevations has been whiplashed back and forth from summer to winter several times. High rain lines on Monday night/Tuesday added liquid water that is gradually working its way down through the snowpack. Temperatures cooled overnight and a crust formed on top of this warm, moist snow. This crust will break down as the day warms and wet snow problems will become increasingly likely. If you're sinking into a pile of mush its time to find colder slopes to play on, or pack it up and head home.

Lower elevations are free of snow in most places. You'll start your day on frozen dirt and finish on mud. If you're headed to Lolo Pass today you'll find more snow in the lower elevation band. Be on the lookout for wet snow problems here as the day warms. 

As we see avalanche danger and the likelihood of triggering slides decreasing it is easy to get lulled into a sense of complacency. We encourage you to continue to pay attention and be curious about the snow as you move through the mountains. Look for signs of instability (cracking in fresh wind slabs, and wet, soggy snow) and heed their warnings. 

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