As the summer heat begins to ease across the country, fall-like weather arrived in the Midwest and Northeast early this week.
Cold, dry air is expected to push into the Northern Plains and upper Midwest Saturday night, move through Chicago, Detroit and other parts of the Midwest tonight and Monday morning, and reach the Northeast and Mid-Atlantic by Tuesday morning.
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Temperatures will drop into the 50s overnight, bottoming out in the 40s in the interior Northeast and upper Midwest.
This cold air will drift in during the day, limiting afternoon warm-ups and keeping highs in the mid-70s across the region.
Chicago will stay in the 70s today, but will drop into the low 50s Monday night as cold air moves in.
New York City follows suit, dropping into the 60s Monday night before barely returning to the 70s on Tuesday. Boston and Providence will be even cooler, with overnight temperatures in the 50s and staying below 70 degrees into Wednesday morning.
Cooling is accompanied by a sharp drop in humidity. Especially along the East Coast, cold, dry air flows from the south into the mid-Atlantic and makes landfall east of the Appalachians.
Mountains help trap cold air in place through a process called cold air damming. The cold air is too dense to flow back west over the mountain, which acts like a dam on a river, so it accumulates on the east side of the slope.
The southern and central United States will be trapped in the heat that grips both regions for weeks, with highs still expected to reach 90 to 100 degrees. The first real taste of fall weather in the Southeast could come later in the week, possibly into next week.
But as the Northern Hemisphere heads into the dark half of the year, cold spells like this one will become more common.
Major cities such as Chicago, Minneapolis, New York, Washington DC, and Boston have all already lost more than two hours of daylight since the summer solstice, the longest day of the year. They are currently reducing their sunlight at nearly the fastest rate of the year, by more than 17 minutes per week.
Longer nighttime hours allow the atmosphere to cool for longer each day, making it easier for cold air masses to spread across North America.
