Even snow-hardened Montanans shivered on Saturday as a rare September blizzard began to dump snow on part of the state. After all, the first day of fall was Monday.

The tops of trees at Glacier National Park were just starting to turn yellow. Bowhunters had begun to stalk deer and elk. Temperatures were in the 70s earlier this week.

But on Saturday, the snow began falling, and the storm threatened to bring up to 48 inches of snow to Montana’s mountains, and at least two feet to a wide swath of the state. In some cities, longstanding September snowfall records could be doubled.

Snow piled high on cars and trucks in northwest Montana, and gusts up to 60 miles per hour knocked over trees, downed power lines and pushed snow up against the doors of houses.

“For any time in September, it’s on the crazy side,” Francis Kredensor, a meteorologist at the National Weather Service office in Great Falls, said on Friday. “This has the potential to be an exceptional early-season storm.”

On Saturday afternoon, more than a foot of snow had already fallen in several areas around the mountains in Glacier National Park. A weather station near Marias Pass, where a highway cuts through the mountain range, recorded 17 inches as of just 2 p.m. Mr. Kredensor said the snow would fall even more heavily on Saturday evening.

A trooper with the Montana Highway Patrol posted a picture on Twitter of an overturned car, warning of slick roads. The trooper, Amanda Villa said everyone who had been in the car was all right.

Fears of the unusual snowstorm led rangers at Glacier National Park to close some roads, and the storm may cause power failures for thousands of Montanans. Wet snow weighs down the leaves that remain on trees, and powerful winds on Saturday and Sunday, with the strong gusts, could bring down trees and power lines. A strong storm in early October 2017 left about 10,000 without power — and this year’s blizzard is expected to be worse.

The National Weather Service has issued a winter storm warning for more than a dozen counties in the northwest part of the state and a winter storm watch for at least 15 more. The storm is expected to calm down by Monday night.

Cut Bank, a small city just 25 miles south of Alberta, Canada, has never received more than 15.5 inches over a three-day period in September. It is forecast to get up to 30 inches from this weekend’s storm. Great Falls, the state’s third-largest city, is predicted to get up to a foot of snow, nearing the city’s record for a three-day storm, 13.2 inches, which was set in 1934.

Montana is one of America’s snowiest states. The largest snowflake ever seen is rumored to have fallen in the state in January 1887, although many have doubted a rancher’s claim that it was more than a foot wide. Winter weather can stretch into summer.

But a September snowstorm measured in feet has the potential to alter the routines of residents like Megan and Scott Lester, who run Whitefish Stage Organic Farms in Kalispell, in the northwest corner of the state. At this time of year, the couple usually opens up its farm to children and parents who come to pick pumpkins, munch on apple cider churros and get lost in a grizzly bear-shaped straw bale maze.

Instead, the Lesters were battening down the hatches on Friday.

“With this weather, everything’s got to be closed,” Mr. Lester said on Friday. “Some of the straw-bale walls will blow down and make the maze impassable. People will have snow on everything. Anywhere you want to sit will be wet.”

The Lesters closed the farm on Saturday for the first time ever during a fall season, which runs from mid-September to the end of October.

“To lose a whole weekend to a major storm is a huge bummer, obviously,” Mr. Lester said. “There are not going to be crowds of people coming out to get snowed on.”

Mr. Kredensor, the meteorologist, said the storm was the result of a “perfect mix” of cold and wet. Early cold snaps and moisture from the Pacific Ocean are nothing new, but the combination of single-digit temperatures and substantial moisture has made the storm unusually intense.

And while his forecasting colleagues may be looking forward to traversing the Montana plains in snowshoes or skis, Mr. Kredensor said he was not yet ready for that.

“If this was December, I’d be O.K. with it, or even late November,” he said. “But late September? It’s a little too early to get excited about it. I was excited for some fall.”

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