Fall temperatures finally in the forecast for Midwest and Northeast

A blast of fall-like weather is coming to the Midwest and Northeast early this week as summer's grip begins to loosen across the country.

A pool of cold, dry air pushed into the Northern Plains and Upper Midwest Saturday night and will sweep through Chicago, Detroit and the rest of the Midwest this evening through Monday morning, reaching the Northeast and Mid-Atlantic by Tuesday morning.

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Temperatures dip into the 50s overnight, bottoming out in the 40s across the interior Northeast and Upper Midwest.

That cold air will stick around during the day and limit the afternoon warmup, holding high temperatures to the mid-70s across the regions.

Chicago will stay in the 70s today, dropping to the upper 50s Monday night as the cold air pushes on.

New York City will follow, falling to 60 degrees overnight Monday and barely climbing back into the 70s on Tuesday. Boston and Providence will be even cooler, in 50s overnight and staying below 70 degrees through Wednesday morning.

The cooldown is accompanied by a sharp drop in humidity — especially along the East Coast as cool, dry air pushes south into the Mid-Atlantic and lands on the eastern side of the Appalachians.

The mountains will help lock the cold air in place through a process called cold air damming. Cold air is too dense to climb west back over the mountains, which act like a dam in a river, so it piles up against the eastern side of the slopes.

The South and central US will stay locked in the heat that's gripped both regions for weeks, with highs still reaching into the 90s and 100s. The Southeast likely won't get its first real taste of fall weather until later in the week, possibly stretching into next week.

But cold outbreaks like this one will become more common as the Northern Hemisphere tips into the darker half of the year.

Major cities including Chicago, Minneapolis, New York, Washington DC, and Boston have all already lost upwards of two hours of daylight since the summer solstice, the longest day of the year. They are now losing daylight at close to their fastest pace of the year, over 17 minutes per week.

The longer nighttime hours give the atmosphere more time each day to cool down, allowing cold air masses to expand across North America.