What’s making Dallas-Fort Worth so cold?

All of a sudden, North Texas is facing temperatures the likes of which haven’t reached it in decades, but the weather patterns pushing the frigid air started back in early January.

The low Monday could drop to 1 degree, and wind chill values could plunge below zero. The last time Dallas-Fort Worth dropped to 1 degree was Dec. 23, 1989, when the low reached -1 degree.

By Tuesday morning, lows could even dip below zero across North Texas, the National Weather Service said.

There’s a few reasons for this week’s remarkable cold.

“We’re having a favorable pattern for this arctic air to come down,” said Allison Prater, a Weather Service meteorologist. “There’s a flow over the Great Lakes that’s just been kind of sitting there and keeps pushing down that cold arctic air.”

“Polar vortex” is a popular term that often gets tossed around during winter storms, but Prater said it can be misleading.

The polar vortex is not something that magically appears during the winter. It’s around all year long.

“It always exists near the poles,” Prater said. “But it weakens in the summer and strengthens in the winter.”

The vortex is an area of low pressure that hangs around the north and south poles thousands of feet in the atmosphere. Shifts in its position or its strength can affect weather across North America, Europe and Asia.

According to The Washington Post, this week’s winter storm and extreme cold can be traced to a disruption of a weaker-than-usual polar vortex in early January, which allowed extreme cold in the Northern Hemisphere to start moving farther south than usual.