A dramatic pattern shift will bring the coldest air since spring to millions of people across the central and eastern United States this week, making areas that have previously struggled to shed sustained warmth feel more like November than October.

Since the season began last month, much of the country has found it difficult to experience extended fall cold spells. But the combination of an extended cold front and an impending blast of cold Canadian air will change that and temperatures could fall to 10 to 15 degrees below average by midweek.

The first aspect of this cooling – a widespread cold front – began over the weekend. This cold front swept in from the north-central U.S. and will cross the eastern half of the U.S. by late Monday, ushering in a true fall chill for some along the way.

The coldest temperatures will remain in the Plains and Midwest on Monday before even cooler air from Canada is pushed much further south.

High temperatures from the Midwest and Northeast to the South will be seasonally cool on Monday, but temperatures will drop on Tuesday. Tuesday could be the coldest day of the week in Chicago, with the high temperature struggling to reach 50°. Chicago hasn't seen a high above 50 degrees since April.

Many areas outside the Midwest will see temperatures drop again on Wednesday. Wednesday is expected to be the coolest day since spring for millions, and in many places it will feel more like late November than October.

“True fall weather is forecast through the start of the weekend,” the National Weather Service in Atlanta said Monday.

The high temperature in Washington, D.C. on Wednesday could struggle to reach the mid-50s — 10 to 15 degrees lower than normal for mid-October.

Atlanta could struggle to break out of the low 60s on Wednesday and stay in the 60s through Friday. The city hasn't recorded high temperatures below 70 degrees since May, and highs typically hover around 75 degrees this time of year.

Wednesday will also be the coolest day of the week for much of the Northeast, with highs in the 40s and 50s. Because daytime temperatures are so cool, there is a chance that temperatures could drop below freezing overnight.

“Frosty and freezing conditions are likely for several nights and in many areas this week,” the National Weather Service in State College, Pennsylvania, warned Monday.

Frost warnings are in effect for parts of Pennsylvania until Tuesday morning. Additional advisories may be issued in the coming days for other parts of the Northeast and cooler areas of the Mid-Atlantic.

Frost can cause plants not exposed to cold to die or can end the growing season of some regional staple crops. These conditions are also dangerous for people who do not have access to adequate heating.

Temperatures around freezing or near freezing are possible every night Tuesday through Thursday in western North Carolina, which was devastated by Helene late last month.

By the end of this week, conditions will weaken back to more typical October conditions across much of the central US and Northeast. Areas farther south, such as the southern Appalachians and the Southeast, will see cold temperatures persist through at least Friday.

By Saturday, seasonal highs in the 70s and 80s will return to the Southeast, while the 60s and 70s will spread north into the Midwest and parts of the Northeast.

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