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Shyla MacAlister, 8, of Lolo, makes a running jump on to her sled Wednesday afternoon to get a fast run down the hill near Tower Street and Spurgin Road while on a winter outing with her family. Forecasters are predicting as much as a foot of new snow in the Missoula Valley by Sunday.

“Meteorologically beautiful” and “crippling” don’t often show up in the same sentence, but that’s the forecast for this weekend’s end-of-2017 snowstorm.

A wet channel of air blowing east from the Pacific Ocean should collide with an arctic air mass flowing south out of Canada over the next few days. As of Thursday morning, the forecast called for intensifying snowfall along the Montana-Idaho border, starting in the Kootenai-Cabinet region and Bitterroot-Sapphire mountains.

By Thursday afternoon, the storm should reach the West Glacier and Seeley-Swan areas. The Flathead-Mission valleys and Missoula-Bitterroot valleys should see their winter storm watches go into effect Thursday evening.

“As if the forecast hadn’t been complicated enough to this point, what happens late Friday afternoon into Saturday has potential to be one of the most impressive snow events in recent memory,” Missoula National Weather Service meteorologists wrote Wednesday morning.

“Models consistently paint the most robust plume of Pacific moisture (dubbed the Pineapple Express) moving across the area at this time. Meanwhile, an impressively deep and extremely cold arctic air mass will begin to wedge its way across the Continental Divide," the NWS said.

"What occurs next will be both meteorologically beautiful, but also likely quite crippling, for many areas as a full-on collision of these distinctly different air masses happens across parts of West Central and Northwest Montana (generally north of I-90).”

The much warmer coastal air could make valley snowfall come and go in pulses of light and moderate intensity, and might even change to freezing rain in the southern Bitterroot Valley on Friday.

Missoula’s high temperature may reach 32 on Saturday, making any snow that falls very heavy and wet. Nighttime temperatures Thursday and Friday should be in the mid- to low 20s.

The West Central Montana Avalanche Center considered the weather system an “increasing danger” factor for backcountry travelers on Wednesday. Wind slabs of snow deposited on the lee sides of ridgelines, as well as persistent slabs of heavy snow sitting on unstable layers below were the main risks.

Highway drivers could encounter near-whiteout conditions in the Flathead and Mission valleys on Friday as between 8 and 14 inches of snow could land there. The mountains expect 1 to 2 feet of snow, making travel conditions dangerous especially at Lookout, Marias, Lolo and Lost Trail passes.

“The snow should really start after dark Friday night in Missoula and Bitterroot valleys,” National Weather Service meteorologist Chris Gibson said Wednesday. “The heavy snow will be throughout the day Saturday, along with cold and blustery northeast winds. By Sunday, New Year’s Eve, it should be some snow flurries but mostly dry. It’s a prolonged event.”

Snowplow crews have already prepared for the storm. The Montana Department of Transportation clears about 25,000 miles of road, including almost 5,000 miles in Missoula County alone. It has used an average 168,859 cubic yards of sand and 8.4 million gallons of liquid deicer to keep them clear.

“Crews within the Missoula Division have approximately 70 plow trucks, including 4 tow plows with one additional unit being added in Clinton in early January,” MDT spokeswoman Lori Ryan noted in an email. “Other tow plows are located in Missoula, Lolo, Drummond and Look Out Pass.

"The lane miles plowed and materials used this season in the Missoula District are 169,117 miles of roadways either plowed, sanded or deiced in the Division so far this season, 11,757 cubic yards of sand/salt used Division wide this season and 630,109 gallons of salt brine used Division wide so far this season.”

Both Missoula’s city and county snowplows also have been put on alert for the year-end storm. Missoula City Street Superintendent Brian Hensel said he’s braced to get 15 inches of snow through Friday.

“After last winter, there’s not a lot that scares me,” Hensel said Wednesday. “We’ve made some changes to have more people working at night, so we can get the main routes done when there’s less traffic.”

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Natural Resources & Environment Reporter

Natural Resources Reporter for The Missoulian.