It’s not officially winter yet, but a polar vortex is bringing frigid conditions this week.

The polar vortex is a large upper-level, low-pressure area or circulation that typically resides above the Arctic Circle. When this storm is strong, it tends to keep the coldest air in the Northern Hemisphere locked up over the pole. However, when it weakens or stretches, frigid air can move south.

“The polar vortex has been in a weakened and stretched state since late November,” AccuWeather Lead Long-Range Meteorologist Paul Pastelok said. “We are seeing the result as Arctic air pushes southward across the central and eastern U.S. in recent days and may continue to do so for the next couple of weeks or so in waves

Temperatures across many areas of the Midwest could drop below zero Fahrenheit on Monday morning, roughly 15 to 25 degrees below the historical average.

“We believe that two to three more rounds of intense cold are possible from the Midwest to much of the East, spanning Dec. 10 to 19,” Pastelok said. “There is a chance for a frost or freeze late next week as far south as central Florida.”

Here is the latest forecast from the National Weather Service:

Tonight: Mostly cloudy, then gradually becoming mostly clear, with a low around 17. West wind 5 to 10 mph becoming north after midnight. Winds could gust as high as 25 mph.

Monday: Mostly sunny, with a high near 28. North wind 5 to 15 mph, with gusts as high as 30 mph.

Monday Night: Partly cloudy, with a low around 6. North wind around 5 mph becoming calm.

Tuesday: Increasing clouds, with a high near 31. Light and variable wind becoming south 5 to 10 mph in the afternoon.

Tuesday Night: A slight chance of snow after 1 a.m. Mostly cloudy, with a low around 22. Southwest wind around 5 mph. Chance of precipitation is 20%.

Wednesday: Rain and snow likely, becoming all rain after 1 p.m. Cloudy, with a high near 42. Chance of precipitation is 60%. New precipitation amounts of less than a tenth of an inch possible.

Wednesday Night: A chance of rain before 1 a.m. Mostly cloudy, with a low around 28. Chance of precipitation is 30%.

Thursday: Partly sunny, with a high near 38.

Thursday Night: A chance of snow after 1 a.m. Mostly cloudy, with a low around 21. Chance of precipitation is 30%.

Friday: A chance of snow. Partly sunny, with a high near 34. Chance of precipitation is 30%.

Friday Night: A chance of snow. Mostly cloudy, with a low around 16. Chance of precipitation is 30%.

Saturday: Partly sunny, with a high near 30.

Saturday Night: Mostly cloudy, with a low around 14.

Sunday: Partly sunny, with a high near 28.