By AccuWeather senior meteorologist
Updated Jul. 28, 2021 7:51 AM EDT
AccuWeather meteorologists warn that atmospheric conditions may allow for a fast-moving high-wind event that could roar from Wisconsin to Ohio at midweek and impact the metro areas of Chicago and Milwaukee Wednesday evening.
The repetitive pattern of severe weather forecast for portions of the north-central United States this week includes the risk of a long-lived complex of severe thunderstorms, called a derecho, that can cause damage over a span of at least 250 miles. This system could pack wind gusts of up to 100 mph, causing life-threatening and damaging consequences to a swath of the country that is home to more than 10 million.
This image shows the threat level of severe weather for Wednesday and Wednesday night as forecast by the National Weather Service's Storm Prediction Center (SPC)
Farmers and residents in the Central states are still struggling to recover from a derecho that produced a path of destruction from Nebraska to Michigan and Indiana on Aug. 10, 2020. Crop and property damage, as well as business losses from the 2020 derecho totaled in the billions of dollars.
The risk for a derecho amid the approaching severe weather outbreak will cover an area farther to the north than the one that hit last year.
The rounds of severe thunderstorms are being set up by a northward bulge in the jet stream that represents a zone of high pressure at multiple levels in the atmosphere. This pattern produces hot air and a lack of thunderstorms within its dome, but on the edges of the high-pressure area, thunderstorms tend to erupt and can become severe.
Throughout Tuesday afternoon and evening, severe storms erupted across parts of the Northeastern states and farther west over parts of the northern Plains and the Upper Midwest. Storms in Wisconsin reportedly brought baseball-sized hail to North Bend in Trempealeau County, and strong winds raked across New York to Massachusetts and New Hampshire.
Wednesday evening into Wednesday night is when conditions will pose the biggest threat of a derecho, AccuWeather forecasters say.
AccuWeather is projecting the first big storms will erupt during the day Wednesday near the Minneapolis area. Assisting this burst in thunderstorm development will be a ripple in the jet stream. These first storms may not move at breakneck speed and have the potential to bring very large hail, isolated strong wind gusts and flash flooding.
As the storms grow skyward and approach the fast-moving air aloft, they are forecast to roll southeastward with increasing forward speed during the evening and nighttime hours on Wednesday.
Along the way, major cities, such as Duluth and Rochester, Minnesota; Milwaukee, Eau Claire, LaCrosse and Madison, Wisconsin; Dubuque, Iowa; Chicago and Rockford, Illinois; South Bend and Fort Wayne, Indiana; Detroit, Kalamazoo and Grand Rapids, Michigan; and Toledo, Ohio, will be at risk for dangerous, damaging and disruptive weather. As the storms approach and move through the metro areas, airline delays and ground stops are likely. The timing for the worst thunderstorms in the Milwaukee and Chicago areas are after dark Wednesday evening. The nastiest storms may not reach areas in southern Michigan, northern Indiana and northwestern Ohio until the late night or early morning hours on Thursday.
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Storms from late Wednesday through early Thursday morning could be moving along at 40-60 mph at times and result in rapidly changing weather conditions for those outdoors and on the highways. The sudden, powerful storms can knock over trucks and mobile homes and blind motorists.
Locations in central and southern Wisconsin, northeastern Iowa, northern Illinois, part of southern Michigan, northern Indiana and northwestern Ohio will be at risk for thunderstorms with winds gusting to hurricane force (74 mph or greater). At this force, regional power outages are likely due to falling trees. Property damage can be significant. Vast fields of corn and other crops can be flattened.
Rod Pierce stands in a cornfield damaged in the derecho that occurred on Aug. 10, 2020, near Woodward, Iowa. The fast-moving complex of severe storms hit several Midwestern states, but was especially devastating in Iowa as it cut west to east through the state's midsection with winds of up to 140 mph. (AP Photo/Charlie Neibergall)
AccuWeather is projecting local StormMax™ wind gusts to reach near 100 mph with the event Wednesday night. As storms approach, people are urged to move indoors and away from windows as winds that pick up loose objects can cause the glass to shatter. Tall trees can be a hazard for parked vehicles beneath as there is a significant risk of large limbs crashing down.
Seiche may develop on lakes Michigan, Erie
The sudden rush of wind associated with the fast-moving complex of storms has the potential to cause water levels on Lake Michigan and perhaps Lake Erie to fluctuate significantly over a few hours. This phenomenon, in which large bodies of water start to rock back and forth, is known as a seiche and sometimes referred to as a meteotsunami.
Should the seiche develop, there can be water level rises of several inches to 10 feet or more along the lakeshores of Michigan and Erie that can lead to flooding, depending on the severity of the rush of wind. The rocking motion could last from hours to a day or more.
Severe storm risk to settle southeastward Thursday
During Thursday afternoon and Thursday night, the risk of violent, fast-moving complexes of storms will tend to diminish over the Upper Midwest, but storms near the Ohio Valley, central Appalachians and mid-Atlantic regions can be locally severe with flash flooding, hail, strong wind gusts and an isolated tornado.
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