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Cooler weather will spread across the Midwest and East early this week due to a developing pattern change that could bring frost and freeze conditions deep into the South.

W​hat we mean by a pattern change

The jet stream will become more amplified as the holiday arrives. As shown in the map below, it's expected to take a sharp southward plunge over the eastern half of the country, while being diverted north into western Canada.

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Temperature Impacts

T​emperatures will cool in the East through the early week as this flip-flop in the jet stream turns off the tap of warmer air in the central and eastern U.S. and Canada.

A pipeline of more persistently cooler air will be established in the East, South and Midwest through early week and as astronomical spring officially arrives (Tuesday, March 19).

N​otice we're saying "cool", not "cold". It appears temperatures will be up to 20 degrees cooler than usual for this time of year in the Southeast by Tuesday morning, but we don't anticipate record cold nor a potent cold outbreak. The lack of widespread snowpack and longer March days with the sun higher in the sky compared to, say, January, are two reasons for this rather muted "cool" outlook.

T​hat said, low temperatures will be cold enough to bring frost and freeze conditions all the way into the South early this week.

T​his will also mean a warm-up in parts of the West, especially areas that aren't in valleys susceptible to fog and low clouds. A few daily record highs are likely in the Northwest through Monday, including Seattle and Portland. Most of the West Coast will see a drier pattern, as well.

(Further beef up your forecast with our detailed, hour-by-hour breakdown for the next 8 days – only available on our Premium Pro experience.)

T​he recent pattern

B​y contrast, the jet stream arrangement since late February couldn't have been more different than the upcoming change.

T​he jet stream's southward plunge has, instead, been in the West. That pumped much warmer than average air into the eastern two-thirds of the U.S., as well as central and eastern Canada.

T​hat lead to a late February warm spell that smashed not only monthly, but also warm records for any winter day.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. His lifelong love of meteorology began with a close encounter with a tornado as a child in Wisconsin. He completed a Bachelor's degree in physics at the University of Wisconsin-Madison, then a Master's degree working with dual-polarization radar and lightning data at Colorado State University. Extreme and bizarre weather are his favorite topics. Reach out to him on X (formerly Twitter), Threads, Facebook and Bluesky.

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