Chicago kicked off the week with a combination of extreme heat and severe weather as a storm threat develops across the Midwest and Great Lakes.
The storms triggered a tornado warning in neighboring northwest Indiana Monday afternoon, with flash flood and severe thunderstorm threats moving in on the Windy City, according to the National Weather Service.
Severe storms target Midwest, Great Lakes
The Storm Prediction Center says storms could become strong to severe Monday, especially around the Great Lakes and areas near Lake Michigan. The biggest concerns are damaging wind gusts and large hail, though a few tornadoes are also possible.
Some storms may organize into larger clusters that could bring widespread damaging winds as they move through parts of Wisconsin, Michigan, Illinois, Indiana,
and Ohio.
"Severe storms capable of strong to severe wind gusts and large hail are expected across the Great Lakes and Midwest through tonight," the Storm Prediction Center wrote. "More sporadic occurrences of damaging wind are possible in the Mid-Atlantic, Tennessee Valley, central High Plains and southeast Arizona."
The NWS warns that winds could surpass 70 mph in Chicago, with hail up to the size of a golf ball through the afternoon and evening. Weather officials warn Chicago residents to "expect damage to roofs, siding and trees."
Eastern Illinois and northwestern Indiana may also see heavy rainfall and scattered instances of flash flooding, according to the center. The NWS issued flash flood warnings through late afternoon (4-5 p.m. CT) in northern and southeast Cook County, as well as in northeast DuPage County.
Why is the weather so extreme? Two unusual factors play a role
Meteorologists say a stubborn weather pattern called an omega block is helping keep the nation's weather stuck.
The pattern gets its name because the jet stream bends into a shape that looks like the Greek letter Ω (omega). When that happens, weather systems slow down instead of moving quickly from west to east.
Areas under the "ridge," or bulge, of the pattern tend to stay hot and dry for days, while storms ride along the edges of it. That's helping keep dangerous heat locked over parts of the central and southern United States while fueling rounds of thunderstorms in the Midwest and Great Lakes.
Another factor adding to the muggy conditions is something meteorologists call "corn sweat."
It sounds funny, but it's real. Corn plants release water vapor into the air as they grow, much like people sweat to cool off. During the peak of the growing season, millions of acres of corn across the Midwest can pump large amounts of moisture into the atmosphere.
That extra moisture doesn't make temperatures hotter, but it does make the air feel hotter by raising humidity. It can also provide more fuel for thunderstorms when other weather ingredients are in place.
The NWS says corn sweat is just one piece of the puzzle. Combined with the stagnant omega block, abundant Gulf moisture and an approaching cold front, it is helping create the hot, sticky and stormy weather affecting much of the country's midsection this week.
When will storm threat end in Chicago?
The NWS forecasts possible severe weather in Chicago any time between 10 a.m. and 9 p.m. CT, though any developing storms will likely occur between 1-7 p.m. CT.
Chicago storm radar
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This article originally appeared on USA TODAY: Chicago faces dangerous combo of heat, tornado risk and flash flooding
