Bollettino valanghe

East Cabinet Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Thu
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

Today's advice is simple...avoid traveling in, near, or below avalanche terrain. Copious amounts of rain - possibly 3 to 4 inches - will fall onto an already stressed snowpack. Dangerous natural avalanches are likely and they will break wide across terrain features and run long distances to lower elevation slopes.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Our persistent slab problem is very complex today, with multiple buried weak layers in the snowpack ranging from 2-3 feet deep. Heavy rainfall that climbed to at least 6500-7000 feet Monday night, percolated rapidly through our dry December snowpack, and provoked a natural avalanche cycle of large avalanches and at least a few very large events (size D3).

    Cooler temps yesterday offered a short reprieve, but today we will once again be hammered by an atmospheric river with unprecedented moisture and very high freezing levels for the month of December. Despite being tested once already, these weak layers have not had adequate time to heal. This was evident in stability tests on Monday, as well as at a crown profile investigation yesterday afternoon. Slopes above 7000 feet have not had their exam yet, and will be especially worrisome for today’s rain on snow event.

    We recommend avoiding avalanche terrain all together during these remarkably warm and wet times. Very dangerous conditions exist on slopes steeper than 30 degrees.

    This large persistent slab avalanche released overnight on December 8th during a rain on snow event. The crown thickness was about 2 feet and the slope angle was a mere 32 degrees where the inital crack formed. We discovered this natural avalanche as well as several others decorating the Selkirk Mountains on Tuesday afternoon.

    Photo: Jackson George

  • Loose Wet

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

    As the snow line works its way up the mountain with rising temperatures and transitions to rain, liquid input to surface snow will create wet loose avalanches. Steep slopes, gullies, and cliffs are the most likely areas to encounter this problem. Seeing rollerballs that gain in size or sinking to your boot top in heavy, wet snow are red flags that this problem is materializing. Due to the amount of new snow at mid and lower elevations recently, wet loose avalanches could quickly gain in size, and run far distances down slope as they entrain more snow. Avoid traveling under any overhead hazard, and give yourself wide margins near cliff bands and chutes.

    Widespread runnel formation was eye-popping on Tuesday, as the snowpack recieved unprecedented moisture into upper elevations. Loose wet avalanche activity was omnipresent and some pinwheels worked to trigger small storm slabs on their slow roll down hill.

Avalanche Discussion

Rain on snow is starting to sound like a broken record. After a brief reprieve on Tuesday, the atmospheric river that dumped heavy snow and rain through Monday has moved back into our zones and will park itself over the Panhandle and NW Montana until it transitions out Friday. We are forecast to receive 1 to 4 inches of additional precipitation today into tomorrow. With temperatures climbing into the 40’s across our zones, the snowline will quickly rise to near, or exceeding, 8,000 feet today, wetting snow above 6,800 feet for the first time this week.

The conditions are tricky out there right now. A widespread rain crust up to around 6,800 feet overlays a complex early season snowpack. At mid to lower elevations today’s warm rain may help break down the crust, allowing liquid water to percolate into the snowpack. While the introduction of moisture in the snowpack will help erase distinct layers over time, in the short term the introduced water will lubricate and further destabilize persistent weak layers.

Yesterday, IPAC teams documented numerous persistent slab avalanches that had failed in the preceding days, including several large avalanches in the Selkirks. Results in pit tests demonstrated that the structure needed to sustain crack propagation still exists in the snowpack, and we can expect those conditions to be significantly more dangerous at elevations exceeding 6,500 feet where human triggered avalanches are very likely. The flood of warm liquid all the way up the mountain means wet loose avalanches are possible at all elevations today.

The forecast team did not issue a warning for the Silver Valley and Northern Bitterroot Zone this morning because we felt the conditions are sufficiently different from the northern zones to differentiate them. The overall shallower snowpack and lack of widespread continuity of persistent weak layers lower in the snowpack mean the avalanche problems are less pronounced, although still present in our mid elevation band. Considerable danger is challenging to navigate so if you venture out today don’t try to outsmart the problems. Conservative decisions today are the way to go.

Weird weather breeds weird avalanche behavior. These near historic atmospheric river events have created conditions in the snowpack that none of us have witnessed in recent memory. How exactly the snowpack reacts to an additional 4 inches of rain is an open question, so please treat this period of heightened avalanche danger with respect and caution.

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