Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Sat
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

Avalanche danger rises from LOW to MODERATE as the day warms and the sun melts the icy crusts at the snow surface. Pay attention to the snow you are moving through; if you are sinking into a pile of wet mush, it's time to find colder snow to play on. Up high, wind slabs formed during Wednesday's storm may still be reactive. Look for these along ridgelines and mid-slope features that slow the wind down. Cracking in fresh drifts is an indicator that the slab you've found may be unstable. It feels like spring, but it's not quite time to turn your avalanche brain off.

Problemi valanghivi

  • Wind Slabs

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

    Wednesday's storm brought a dusting of fresh snow to the mountains, accompanied by strong westerly winds. This combination built a round of dense, generally thin, wind slabs in exposed, upper elevation terrain.  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.

    Are you heading for a peak today? Pay attention to the amount of new snow you are moving through as you gain elevation. If the snow you are moving through quickly becomes 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.

  • Loose Wet

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

    We start the day with a stout, supportable crust on the snowpack. As the day warms and the sun heats the snow, this crust will gradually unlock and wet snow problems will become increasingly likely. Small wet loose avalanches will be (by far) the most likely and the easiest to trigger, but larger wet slab, glide, and cornice-fall avalanches remain a concern.

    LOOSE: These start at a point (rocks, trees, your skis) and fan out and gain mass as they travel downhill. This problem will be at its worst on steep, rocky or treed slopes baking in the strong March sunshine. Up high, where more snow has fallen, small loose snow avalanches may fail above the crust before the crust breaks down (video at right/above).

    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 swaths 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 Wednesday.

    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.

Avalanche Discussion

We've got a beautiful spring day on tap for us. It's a cold and icy world out there this morning, with a stout crust on the surface of the snow and ice-locked puddles everywhere you look. The greatest hazard you'll face initially is keeping your sled cool and slipping and sliding on icy surfaces as you climb. Our avalanche problems today fall into two categories: wet and dry.

WET: The surface crust will gradually break down as temperatures rise and the sun goes to work. Our snowpack has seen some melt-freeze cycles already and today's wet snow problems should generally be small and easy to identify and manage. There is some lingering uncertainty about larger wet snow avalanches failing as slabs. More on this below.

DRY: Wednesday's storm brought a coat of fresh snow to the mountains, primarily above about 7,000'. The crest of the Bitterroot seemed to be favored, but our observations from back there are limited. Nata was further out towards the range front yesterday and found 4-6" of dry snow above 8,000'. If you are headed for a peak today, pay attention to the amount of fresh snow you are finding, and look for signs that the wind has been at work. Expect that small triggered slabs could travel quickly on the icy surfaces beneath.

The greatest sources of uncertainty with our snowpack are related to the prominent weak layers that formed in January and February. These layers produced a widespread avalanche cycle when they were heavily loaded in our major atmospheric river two weeks ago. Avalanches broke thousands of feet wide, with crowns in the 4-6+ foot range. Then, during last week's heatwave, these layers produced another round of avalanches, this time failing as destructive wet slabs. Triggering avalanches that fail on these layers, either as wet slabs or dry persistent slabs, is unlikely. However, if you're heading into very steep, open, rocky terrain, these much larger avalanches should be on your mind. 

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. Take the time to look for signs of instability (cracking in fresh wind slabs, and wet, soggy snow) and heed their warnings. 

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