Avalanche forecast

Seeley Lake

West Central Montana Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Highlights

The hazard posed by slabs of wind-drifted snow will increase today as a cold front with strong southwesterly wind impacts the forecast area. Changing conditions are likely as you climb into the upper elevations, where stronger winds will build thicker slabs large enough to bury and injure you. Watch for signs of wind slabs including cornices above the slope, hollow-sounding pillows of drifted snow, and shooting cracks. Avoid wind-loaded slopes greater than 35º. It may still be possible to trigger a sizeable persistent slab avalanche in the southern Bitterroot and on isolated pockets on shaded aspects in the Rattlesnake. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2.5

    Small wind slabs were stubborn on test slopes yesterday. Warmer temperature followed by colder may have locked existing wind slabs in place. However, plenty of snow remains for wind transport, and cooler temperatures, fresh snow, and more wind will make the potential for wind slabs to grow dangerous once again. Snow surfaces are currently wind affected across many slopes in the forecast area, creating a smooth surface for new wind slabs to build and slide on. 

    We could receive up to 12 inches (0.70 SWE) of fresh snow in the next 24 hours with strong southwesterly winds and frequent wind gusts over 40 mph. Expect rapid wind slab growth on north and easterly aspects. Large and dangerous wind slab avalanches are possible. Be alert to changing conditions as you climb into the upper elevations, where stronger winds will build thicker slabs large enough to bury and injure you. Signs of wind slab include cornices above the slope, hollow-sounding pillows of drifted snow, and shooting cracks. Avoid wind-loaded slopes greater than 35º.

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    2–3

    It may still be possible to trigger a sizeable persistent slab avalanche in the southern Bitterroot. A layer of faceted snow near the base of the snowpack lingers on isolated areas above 7,500 feet. Recent observations suggest that propagation is still possible on this layer (PSTend 50/100). We also noticed a significant avalanche naturally triggered by a cornice fall that failed on this layer during a warming event on December 21st. See Lost Trail Backcountry observation for more details. 

    Areas with a shallower snowpack, previously wind scouring, near ridgelines, and outcroppings may remain susceptible to basal facetting. Yesterday we were surprised by reactive basal facets under 40 inches (100 cm) of snow on a northeast aspect at 7,800 feet in the Rattlesnake (ECTP22 down 100cm). However, these reactive facets seem to be isolated to exposed ridges and shaded aspects. A propagation saw test (PST) done adjacent to this pit showed low propagation likelihood, and another column test done  200 feet downslope had no failure. Triggering a slide on this layer in this area is unlikely due to limited distribution, but a slight possibility exists. Prolonged colder temperatures may encourage the faceting process and is something to watch for in the coming weeks.

Avalanche Discussion

The arctic airmass has had a slower approach than initially anticipated. Models show colder temperatures arriving sometime Saturday night or Sunday morning. A southwesterly cold front with pacific moisture will bring more snow and wind until then. Expect increasingly cooler temperatures regardless of when the air mass arrives. 

Stormier weather returned last night. Freezing rain and light snowfall are likely this morning. Expect increasing southwest winds to 25 to 30 mph with gusts in excess of 40 mph at the highest elevations. Accumulations in the 6 to 12-inch range are possible with this system. We will have a brief pause in the precipitation Friday afternoon, and then another wave will bring increasing snowfall into the weekend.  The central Bitterroot is positioned to receive the most snow. 

Daytime temperatures have been warm over the last 24 hours. Rollerballs (pinwheels) and loose wet slides on road cuts or steep rock bands were abundant below 6,000 feet near Lolo and in the Rattlesnake yesterday. The snowpack has settled, particularly for lower elevations; however, a wintery snowpack remains above that, especially on shaded aspects.

Many thanks to the folks who have been submitting observations. They are extremely helpful for assessing conditions throughout the forecast area. Please continue to submit them here:

https://missoulaavalanche.org/submit-an-observation/

Ski and ride safe.

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