It's December, which means it's "meteorological winter" for the Northern Hemisphere. And while there are still a few weeks until the winter solstice — which marks the official beginning of "astronomical winter" on Dec. 21 — the weather across much of North America is already leaving little doubt that winter has effectively arrived.

Wintry weather wreaked havoc in swaths of the U.S. over the Thanksgiving holiday, unleashing heavy snow and powerful winds that snarled road traffic, disrupted air travel and knocked out electricity for thousands of people.

A dangerous winter storm swept east across the country during the weekend after Thanksgiving, and more severe weather is forecast along both the east and west coasts in coming days.

The weather reportedly contributed to a deadly plane crash Saturday in South Dakota, where nine people were killed when a single-engine aircraft went down. A day later, a passenger jet slipped off the runway while it was landing at Buffalo Niagara International Airport. There were also widespread flight delays as travelers tried to return home after Thanksgiving, with some 7,500 flights delayed and more than 900 canceled as of Monday, according to CNN.

Road travel was also affected in many places, including Interstate 68 in Garrett County, Maryland, where a 36-vehicle pileup was linked to heavy fog and snow. About 50 million people in the U.S. began this week under some kind of winter weather advisory, including some places that have been inundated with snow. Nearly 2 feet (0.6 meters) of snow fell on Duluth, Minnesota, within 48 hours, AccuWeather reports, while parts of South Dakota at the base of the Rocky Mountains received 30 inches (0.8 meters) of snow with drifts measuring 3 to 5 feet (0.9 to 1.5 meters).

The snow and ice was often exacerbated by strong winds, which gusted as strong as a category 1 hurricane Saturday in Nederland, Colorado, reaching speeds up to 94 mph (151 kph). Winds also gusted at 59 mph (95 kph) in Nebraska, according to AccuWeather, which notes icicles froze sideways during high winds in the city of Kimbell.

More on the way

snow shoveling in Colorado on Nov. 26, 2019
A trio digs out of the Green Mountain neighborhood ahead of Thanksgiving in Lakewood, Colorado. (Photo: Joe Mahoney/Getty Images)

After sweeping across the Central U.S. last week, the severe weather battered the Eastern U.S. on Monday, dumping several inches of snow and causing additional delays to air traffic. Parts of the Northeast may have up to a foot of snow by Wednesday, according to the National Weather Service. Forecasters warned of "disastrous" travel conditions in some major metro areas.

This round of severe weather should dwindle as the first week of December wears on, but more trouble may still be brewing farther west. By midweek, for instance, an atmospheric river will carry bring rain and high-country snow from the Pacific Ocean across Southern California and into the interior Southwest. While this precipitation may offer relief to areas plagued by wildfires in recent weeks, it could also lead to dangerous flooding in some places, AccuWeather warns.

A "train" of West Coast storms is expected to continue into late this week, potentially expanding into the Pacific Northwest by this weekend, according to AccuWeather. While temperatures are expected to be below average this month for the Eastern U.S., the latter weeks of December may warm up slightly for much of the rest of the country, the Weather Channel reports.

snowman in San Bernardino National Forest, California
A woman and a boy build a snowman in California's San Bernardino National Forest on Nov. 29. (Photo: Apu Gomes/AFP/Getty Images)

This marks a busy beginning to meteorological winter, which spans the three coldest months of the year in the Northern Hemisphere. It begins on Dec. 1 and runs through the end of February, differentiating it from astronomical winter, which begins with the winter solstice on Dec. 21. Determined by Earth's axial tilt and the sun's alignment over the planet's equator, rather than actual weather conditions on the surface, astronomical winter continues until the spring equinox on March 19.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.