Bollettino valanghe

Seeley Lake

West Central Montana Avalanche Center
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Pericolo valanghe

Sun
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 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

The main hazard is windslab that will build throughout the day. This will be mostly isolated to upper elevations just below ridges. Lolo Pass and the central Bitterroot may harbor deeper windslab from the past 48 hours. Avoid wind loaded slopes greater than 35º. The faceted layer in the southern Bitterroot remains a concern. Avoid slopes greater than 30º on shaded slopes in upper elevations.

 

Problemi valanghivi

  • Wind Slabs

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

    Winds will be light to moderate with strong gusts out of the southwest. Wind slab will build throughout the day, and will be mostly isolated to upper elevations, just below ridgeline.

    Northwesterly slopes will harbor wind slab, but cross loading and shifting winds can create wind slab on multiple aspects.

    Stay aware of stiff hollow-sounding snow, wind lips, and cornices which indicate the presence of wind slab.

    Avoid wind loaded slopes steeper than 35º, and dial it back further if snowfall or wind intensifies.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–3

    The southern Bitterroot contains our main concern for persistent slab. Facets in the old snow remaining after the December thaw are more widespread in this area. They have been identified on northerly aspects above 7400 feet; however, they could exist outside of this as well. This is a low likelihood, high consequence layer. It’s best not to tease it; the right trigger point could cause avalanches up to 4 feet deep. Avoiding slopes over 30º where these facets are present is the best practice at this time. 

    Isolated basal facets exist in the Rattlesnake. These are present at formerly windswept sites where snow remained after the early December thaw and in wind-scoured areas around rocks and outcroppings. Spots like the top of Too Steep to Tele and the south face Burgandy are great places to find these facets. Avoiding these areas is wise until they get buried deeper and have time to round out. 

     

Avalanche Discussion

Merry Christmas and Happy Holidays. It’s a winter wonderland out there. Continued snowfall has improved riding conditions further. Early season hazards are a bit more covered, the trees are a bit snowier, and the powder is a bit deeper. 

Since our last forecast, up to 13" inches of snow has fallen in parts of the forecast area. Thursday’s snowfall favored the Rattlesnake and the central Bitterroot. Other places such as the southern Missions and the southern Bitterroots received less, with totals closer to six inches. The wind was less strong than forecasted. While there was some wind transport, it was not as extensive as predicted. 

Yesterday in the Rattlesnake, we found mostly stable conditions and about 12 inches of fresh snow. Signs of instability such as cracking were isolated to ridge top and minimal. The wind slab that had formed in the past 24hrs seemed well bonded. Storm slab was not an issue. Isolated pockets of basal facets exist in shaded spots like ridge tops and around shallowly buried rocks. There remains a slight possibility of triggering a slab in these areas. 

Lolo Pass area had more signs of instability yesterday. Wind slab was cracking and sliding up to 18 inches deep. A thin crust, likely created during the warming of the past few days, created an effective bed surface.  A public observation from south of Lolo Pass on Dec. 23rd noted widespread cracking in storm slab up to 6 inches deep. An observation from Gash on the 23rd noted significant wind transport, swirling winds, and variable snow conditions. Wind slab was cracking and moving on a bed surface up to 3 feet deep. These problems should be less touchy today, but exercising extra caution around Lolo Pass and the central Bitterroot is smart, especially as fresh windslab builds.

The southern Bitterroot continues to have a faceted layer in the old snow from before the early December thaw on shaded aspects above 7400 feet. A cornice fall on 12/21/2021 triggered a slide on this layer, up to 4 feet deep. Slides on this layer may take a large trigger such as cornice fall, or just the right shallow trigger point to fail, making them somewhat low likelihood. However, the consequences of a slide on this layer would be serious. Avoid slopes over 30º where this problem exists.

Light snow is in the forecast. Zero to one inch fell last night, one to three inches is forecast for today and up to two inches for tonight. These are not huge totals and are unlikely to tip the balance on the current layers or create storm slab. With the snow comes falling temperatures. So far this year, we’ve been lucky with mild temps accompanying our snowfall, so bonding has happened quickly. Colder tempuratures mean the snow will take a longer to bond. Loose dry avalanches, while small, could be a problem for longer after this storm cycle.

The biggest issue will be wind slab, with moderate winds and strong gusts in the forecast for the next 12hrs. The snow currently on the ground and the incoming snow are low density and easily transported. Upper elevations, mostly above 7000’, will be most susceptible to wind slab. However, any slope exposed to the wind or deposition may host wind slab down to lower elevations. 

As you travel in the backcountry today and tomorrow, stay alert for signs of changing conditions. If the snowfall picks up and more than 8” pile up, if the wind is moving considerable snow, if you move from soft powder into stiffer pillows and you see wind lips and cornices forming, it’s time to step back to lower angle slopes and avoid wind loaded terrain. 

Many thanks to the folks who have submitted observations. Please keep sending them in. You can submit observations here: https://missoulaavalanche.org/submit-an-observation/

Have a wonderful holiday.

Ski and ride safe

Regioni coperte