Bollettino valanghe

Rattlesnake

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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In sintesi

You are likely to trigger a wind slab avalanche at upper elevations in the Rattlesnake. Persistent slab avalanches are also possible at upper elevations on north and east aspects. If you encounter shooting cracks, collapsing, whumpfing, or recent avalanches, avoid slopes steeper than 30 degrees.

The avalanche danger will rise throughout the day as today’s storm progresses. If we get storm totals in the upper end of the forecast, we anticipate the southern and central Bitterroot to be close to a high danger rating and the Rattlesnake to be close to considerable.

Problemi valanghivi

  • Wind Slabs

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

    Human triggered wind slab avalanches are likely in the Rattlesnake. As this zone has received less snow than the other two zones, wind slab formation will be smaller but not insignificant. New slabs will build on existing wind slabs formed earlier in the week on north and east aspects. Slabs will be deeper and more widespread, and heightened sensitivity will be expected in these areas. 

    Moderate to strong wind from the west will carry new snow onto north and east facing aspects throughout the day. 40-55 mph gusts will build slabs lower on slopes than usual. A major wind shift is scheduled to arrive in the evening and will begin to carry snow onto south and west aspects.

    Wind slabs look pillowy and rounded and can be soft or sound hollow like a drum. Wind deposited snow can be found on or just below ridgelines, on the lee side of ridges within cirques or couloirs, and behind high or convex parts of slopes. Wind slabs may rest on persistent weak layers and act similarly to persistent slab avalanches. Be aware of terrain above and around you, as you can trigger an avalanche from a distance (remote triggering). Signs of instability include whumpfing, collapsing, shooting cracks, and recent avalanches. If you see any of these red flags, avoid slopes steeper than 30 degrees.

  • Persistent Slabs

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

    Persistent slab avalanches are possible and may increase to likely by the end of the day. North and east aspects at  upper elevations all have some form of buried weak layers, including surface hoar, facets, and depth hoar.  Now that a slab has formed on top and continues to grow with new snow falling, these weak layers are beginning to fail. A persistent slab problem may exist in isolated areas at middle elevations.

    Red flags include whumpfing, collapsing, shooting cracks, and recent avalanches. Be aware of terrain above and around you, as you can trigger an avalanche from a distance (remote triggering). It is worth your time to dig down into the snowpack to look for poor snowpack structure (harder snow over soft) but don’t be fooled by a lack of unstable results in snowpit tests. If you see signs of instability, stick to lower angle slopes.

Avalanche Discussion

BIG NEWS! We are excited to announce that we are changing our forecasting schedule. Starting with today’s forecast, we will publish consecutive forecasts on Thursdays, Fridays, Saturdays, and Sundays. Check out this blog post for further information.

It is exciting to see our forecast region get new snow! The Bitterroot and Seeley Lake zones have faired the best since stormy weather began this month.

24-hour totals:

North Fork Jocko SNOTEL (Seeley Lake): 4 inches of snow and .40 inches of SWE (snow water equivalent)

Stuart SNOTEL (Rattlesnake): 5 inches of snow and .40 inches of SWE

Twin Lakes SNOTEL (Bitterroot): 5 inches of snow and .60 inches of SWE

Storm totals (since January 7th):

North Fork Jocko SNOTEL (Seeley Lake): 12 inches of snow and 1.60 inches of SWE

Stuart SNOTEL (Rattlesnake): 10 inches of snow and 1 inch of SWE

Twin Lakes SNOTEL (Bitterroot): 28 inches of snow and 2.70 inches of SWE

Much of the new 24-hour snow fell overnight. An additional 5-8 is expected today, favoring the southern Bitterroot. Strong wind will blow from the southwest with gusts up to 55 mph. An Arctic front will cause winds to shift easterly at 20-30 mph with a sudden drop in temperatures in the evening. More snow is expected overnight, and the southern Bitterroot will continue to receive the most, up to six inches.

With all this new snow comes new and dangerous avalanche conditions. Weak faceted snow, buried surface hoar, and depth hoar are buried throughout the snowpack in many areas. Before our weather pattern change to stormy weather, melt-freeze crusts formed on the snow surface on south aspects. Now, a new slab of relatively dense snow has formed on top, and especially in wind loaded areas, these weak layers have not yet adjusted to this new load. The new slab is dense and cohesive in many areas, making it possible to trigger an avalanche from a distance (remote trigger). Slabs can propagate a wide distance and run far down slopes.

Lower elevation slopes still have a very thin snowpack and lack snow altogether in many areas. There are not currently persistent weak layers with a slab sitting on top. For this reason, low elevations are at a low danger for the day.

Continue to watch out for buried hazards such as downed logs, stumps, and rocks throughout elevation bands, as the snowpack in our region is still relatively thin. Remember that the Rattlesnake zone also has an even thinner snowpack than the other two zones.

Now is not the time to step out into the backcountry to start skiing and sledding big objectives you’ve been eyeing all season. Keep it dialed back and stick to lower angle slopes today.

The Arctic front is moving into our region Thursday evening, bringing strong wind from the east and historic low temperatures. Most of western Montana will have wind chills capable of causing frostbite in under 10 minutes. Extreme weather can make an avalanche incident go from bad to worse. Be prepared and always carry your backcountry essentials. 

Ski and ride safe!

Regioni coperte