CHICAGO (AP) -- In addition to some snow and heavy rain, bitterly cold temperatures have begun moving into parts of the U.S. and will be staying put for at least part of this week.
Here are some questions and answers about the weather:
Q: WHAT'S THE FORECAST?
A: The Midwestern heartland will see the tail end of a storm that could leave as many as 6 inches (15 centimeters) of snow in Chicago by early Tuesday. After that, Arctic temperatures like those seen in North Dakota and Minnesota will rush in. Parts of those states were expecting wind chills of between 25-50 degrees below zero through Monday morning.
It'll be a similar story in New York, where rain showers will give way to cold air. By Thursday, New York City could see lows near 10 degrees Fahrenheit (-12.2 Celsius), according to Michael Musher with the National Weather Service's Weather Prediction Center.
Atlanta, in the normally warm Southeast, will see temperatures dip to about 15 degrees Fahrenheit (-9.4 Celsius) Monday and Tuesday.
In the West, a stream of Pacific moisture will drop as much as 6 inches of rain in the Seattle area and could mean substantial snowfall in the Cascades. But in San Francisco -- a region that desperately needs rain -- skies will be sunny.
Q: WHAT'S CAUSING THE TEMPERATURE DROP?
A: The jet stream is dipping, meaning cold air from Canada and other northern areas is plummeting into the eastern two-thirds of the United States.
Q: IS IT A POLAR VORTEX?
A: The phrase took on a life of its own last year, and it was blamed for everything from ice storms to the inability of football teams to score. But the National Weather Service is skittish about going anywhere near it this time around.
But the answer is yes and no. Yes, because as Musher noted, the cold air is coming from near the North Pole. But also no, because the low-pressure system isn't going to sink into the U.S. this year, just the temperatures that precede it.
Meteorologists say it's simply winter.