Mary Marcado found herself torn between two seasons this weekend as an unusual early cold snap signaled the abrupt arrival of autumn in the Washington area. The native of Key West, Fla., visiting the District with her husband, Pedro, was crossing McPherson Square on Sunday with Pedro’s sweater over her shoulders and open-toe sandals on her feet, which were “freezing.”

“I was going to ask the concierge [of the Madison Hotel] if anyone had left any winter clothes behind,” she said. The couple planned to add a stop to the day’s sightseeing plans: the coat department at Filene’s Basement.

“I didn’t bring the clothes for this,” Marcado said. “We love your city, but we’re hating your weather.”

Across the region, tourists and residents scrambled to accommodate an abrupt shift from flip-flops to fleece as the jet stream dipped over the Mid-Atlantic and delivered a premature load of chilly air. Thermometers barely reached into the 50s Sunday, more than 20 degrees below the average high for the second day of October.

“It’s unusual,” said Washington Post meteorologist Jason Samenow. “This is more characteristic of temperatures you expect in late November.”

Building supervisors rushed to make sure boilers were ready for an early start to the heating season. Many homeowners who had been running air conditioning only days ago were surprised to wake up to find their furnaces running. Strosniders Hardware in Silver Spring reported a boomlet in sales of firewood and space heaters Sunday morning.

“We’ve sold more than a dozen” of the heaters, manager Jerry Bennett said. “People want a little heat right next to them.”

At the weekly Takoma Park farmers market, vendors said the change in weather brought a sudden interest in fall foods.

Patty Oakley-Audia of Audia Farms in Hampstead, Md., has had her pumpkins and winter squash out for weeks, but they had little attraction. “Last week, people walked by them and said, ‘I don’t want to think about that yet, I’ll have to eat that all winter,’ ” she said. “Now they’re buying it. They’re asking what goes well in soups.”

In an adjacent block, the Takoma Park Street Festival was a sea of dark sweaters and wool caps. When Julia Andrews felt the chill, she threw out her plans to serve popcorn and lemonade at the Andrews Tutoring booth. Instead, she handed out popcorn and hot cider.

And while dozens waited in lines for sizzling chicken skewers and steaming pad Thai, Julius Little sat undisturbed in his soft-serve ice cream wagon. After a good summer at street festivals around Washington, he recognizes it’s about time to put the truck away until next year.

“This is probably the end of the season,” Little said. “People don’t buy ice cream in the cold.”

While some lovers of autumn welcomed the return of sweater weather, others lamented the early portent of winter.

“I can barely feel my fingers,” said Ing Hamilton, 41, an artist from Ireland who is beginning a three-month visit to North America with her husband. With Canada on the itinerary, Hamilton had brought winter duds, all of which she was wearing months before she expected to: a down coat over a down vest, a pink wool cap and fingerless gloves.

“I was expecting to wear all my slippy dresses,” she said, “slippy” being Irish for light summer wear.

“We had shorts and T-shirts on two days ago,” said her husband, Andy. “I was being bitten by mosquitoes. At least they’re gone.”

For now. Temperatures in the region are expected to climb back into the mid-70s by the end of the week. A mosquito boom spurred by weeks of rain will not end completely until the first overnight freeze, which is not expected before next month.