Damaging storms, vivid lightning strike Chicago with new round forecast

Significant severe thunderstorms, some with hurricane-force winds, are expected across portions of the Midwest and Great Lakes region on Monday evening following a round of intense storms Sunday night.

Chicago — which had wind gusts around 60 mph Sunday night — is square in the center of Monday’s most serious storm risk. Thunderstorms, capable of producing wind gusts up to 85 mph and a few tornadoes, are expected to chug eastward toward Indianapolis late Monday night before diminishing.

The National Weather Service’s Storm Prediction Center placed the zone from Moline, Ill. to Chicago, in a Level 4 out of 5 risk for severe storms late Monday afternoon, a one level increase from earlier in the day. Milwaukee is in a surrounding Level 3 out of 5 risk.

A severe thunderstorm watch has been posted until 11 p.m. Central time for central and eastern Iowa, northwest Illinois, northeast Missouri and southwest Wisconsin for the potential for widespread damaging wind gusts, scattered large hail and “a tornado or two.” This watch will probably be expanded east to cover Milwaukee and Chicago.

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The storms in Chicago on Sunday night produced gusts of 59 mph at Midway Airport, 67 mph over Lake Michigan just a few miles offshore, and 70 mph at DuPage Airport. Numerous large tree limbs and a few trees went down from 48th and State streets to 46th Street and MLK Drive downtown. There was even a brief tornado confirmed by radar in the far southwest suburbs. Downtown Chicago was included in a tornado warning until 11 p.m.

Even after the storms left, lingering positive charge in the upper parts of the storm helped several lightning bolts leap upward off skyscrapers simultaneously; at 12:21 a.m., a trio of strikes hopped off the Hancock, Trump and Sears towers at once.

The timing of Monday’s storms

The storms predicted for late Monday could be even more intense than Sunday’s.

Thunderstorms should develop rapidly in central or eastern Iowa around 6 p.m., then quickly race east, reaching northwest Illinois by sunset. The first scattered thunderstorms to erupt may contain hail up to golf-ball size before consolidating into a squall line with damaging winds. Then they’re forecast to blast through Chicago between 9 and 11 p.m., and continue southeast. Indianapolis might see storms a little after midnight. Thunderstorms will begin to fade passing east of Indianapolis.

How the storms may come together

Low pressure will pass north of Lake Michigan on Monday. Ahead of it, exceptional warmth and moisture, with a steamy summertime air mass, is in place. A trailing cold front being dragged east by the low pressure system will act as a trigger, kicking that moisture and instability, or storm fuel, upward to spawn severe storms.

With a strong jet stream racing overhead, storms that grow tall enough will probably encounter the fierce high-altitude winds. Some should be able to propel those strong upper-level winds to the surface in the form of damaging gusts.

There’s even a chance the squall line becomes a derecho. Those powerful windstorms produce widespread 60 mph gusts, and a few gusts over 80 mph, that travel at least 400 miles.

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In addition, the atmosphere will contain low-level vorticity, or spin. That could foster short-lived tornadoes on the leading edge of the squall line. Any tornadoes would be brief, but they would develop quickly and move erratically. Regardless of whether a tornado warning is in effect, it’s a good idea to shelter as storms approach.

The National Weather Service will issue watches in the afternoon, connoting the likelihood of later severe weather. Once storms hit, warnings will be issued for areas in line for damaging hazards. If a storm is deemed to be “destructive,” and spawns gusts of at least 80 mph, wireless emergency alerts will automatically activate smartphones.

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