Bollettino valanghe

Bitterroot

West Central Montana Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 4 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Below freezing temperatures have refrozen the snowpack and lowered the avalanche hazard. Triggering a wind slab at upper elevations is likely in wind loaded terrain over 35º. Large persistent slab avalanches remain possible on upper elevation north slopes. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–3

    The assumption, and the wish, are that the rain and warmth have fixed the persistent problem. Unfortunately, this isn’t the case. The moisture did not reach far enough into the snowpack to bond these layers. We continue to find persistent layers and poor structure. 

    Very weak basal facets sit under the snowpack in the Gash area. Mid-storm avalanches failed at ground in this zone. In the Rattlesnake, buried surface hoar and weak basal facets are still propagating in stability tests. In general, you can assume that the areas that were of greatest concern for persistent weak layers before the storm, the southern Bitterroot, the Northern Bitterroot, the Rattlesnake, and the Southern Swans still contain persistent layers capable of producing an avalanche. 

    Avalanches up to five feet in depth are possible. This is a low likelihood/high consequence scenario. If you do trigger these, you will be dealing with 2’ to 5’ of hard slab, not a good scenario. 

    Choosing slopes less than 35º, under 30º if you want to be certain, is the best way to manage these layers. Avoid convex rolls and areas where the snowpack is thinner, such as around rocks and windswept ridges.

  • Wind Slabs

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

    Abundant new snow and moderate overnight winds out of the west have built reactive wind slabs. These are mostly isolated to areas below ridge line and along the edges of gullies at upper elevations. 

    Watch for stiff, hollow-sounding snow with a scalloped appearance that indicates the presence of wind slab. Wind lips and cornices indicate wind direction and loading. 

    Avoid wind-loaded slopes over 35º.

     

Avalanche Discussion

It’s been quite a week for the weather and the snowpack. Conditions have gone from heavy snowfall to frigid cold, to rain, to seasonal temperatures. It’s been an uncommon weather pattern. 

Temperatures swung as much as 72º in 4 days from -32 to 41º at the North Fork of the Jocko. Starting Christmas Eve, the Lost Horse SNOTEL stayed above freezing for 72hrs and received 3.2” snow water equivalent (SWE), 2.3” of it as rain. Similar weather occurred throughout the forecast area. That amount of warmth and water added a huge weight to the snowpack. While widespread wet loose and wet slab avalanches were reported at lower elevations. Avalanches at middle and upper elevations were not as extensive as anticipated, although observations are still limited. It may be that the extreme cold of the previous week, which was still held in the snowpack, acted as a buffer against the warmth and moisture. As we get up deeper drainages and the skies clear, we should have a better idea of the extent of avalanches.

Seasonal temperatures have returned, the snowpack has refrozen, and wet snow is no longer a problem. Between 6” and one foot of new snow fell in the past 36 hrs. Above 8000’ in the central Bitterroot, there is up to 24” of new snow. This has bonded surprisingly well to the old snow and rain crusts. We did not find a storm slab problem yesterday, but pockets of reactive storm slab may exist. Until you are certain of the storm snow stability, it is best to avoid slopes over 35º and convexities.

The snowpack has dried out above 6500’, and crusts are mostly too buried to feel. Below 6500’ yesterday, the snowpack was still moist to the ground, with a breakable crust. Cold overnight temperatures have likely refrozen this. Areas where you find a supportable crust are unlikely to slide. If you are still breaking through to moist snow, stick to slopes under 30º

Low elevations below 5000’ have mostly melted out. The snowpack is thin, with an abundance of hazards and anchors. The exception may be the Lolo pass area, which had much deeper low-elevation snow before the rain.

Despite all the water and warmth, the persistent problems have not gone away. It’s an assumption that rain and warmth will fix persistent problems by freezing up the snowpack or that the problem layers will become too bridged by stiff layers and crusts to trigger. These are common ways that people try to “outsmart a problem layer,” potentially with disastrous results.  Persistent layers remain a problem at upper elevations and demand respect. While low there is a low likelihood of triggering a slide on these layers, the consequences are very consequential.

Please let us know what you see out there by submitting an observation, especially evidence of avalanches. 

Ski and ride safe

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