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Late-week severe storms in Midwest to mark start of long-lasting cooldown

AccuWeather logo AccuWeather 7/19/2018 Alex Sosnowski

MINNEAPOLIS, MN - JUNE 11:  A severe thunderstorm rolls east towards downtown Minneapolis, lower right, seen from the south side of Lake Harriet on Sunday, June 11, 2017 in Minneapolis, Minnesota. (Photo by David Joles/Minneapolis Star Tribune/TNS via Getty Images) © Getty MINNEAPOLIS, MN - JUNE 11: A severe thunderstorm rolls east towards downtown Minneapolis, lower right, seen from the south side of Lake Harriet on Sunday, June 11, 2017 in Minneapolis, Minnesota. (Photo by David Joles/Minneapolis Star Tribune/TNS via Getty Images)

A change in the weather pattern will favor periodic bouts of wet and stormy conditions as well as unusually cool air at times for the Midwest for the duration of July.

Where downpours persist, the risk of flooding will increase and people with outdoor plans such as picnics, ballgames and construction projects may get frustrated.

In part of the Midwest, baseball fans should expect more rain delays and postponements than average, while motorists and airline passengers can expect travel disruptions.

However, lower temperatures over a large part of the region may help to keep energy demands much lower than average for the time of the year.

Overall, temperatures are likely to run 3-6 degrees Fahrenheit below average across the region with some exceptions. This will be quite a turnaround as temperatures were 3-6 degrees above average during the first half of July including Detroit, Chicago and Minneapolis.

During the second half of July, highs typically range from the low to middle 80s across the northern tier to the upper 80s to near 90 across the Ohio and middle Mississippi valleys.

Severe storms to proceed cooler pattern

In the short term, some storms may turn severe from central and eastern Iowa to central and eastern Missouri, as well as part of western Illinois into Thursday night.

These storms will carry the risk of damaging winds, flash flooding and perhaps a couple of isolated tornadoes.

At the end of the week, the greatest risk of severe storms is forecast to include parts of the Ohio and Tennessee valleys.

Storms with strong winds, flash flooding and damaging hail could hit the cities of Indianapolis and Evansville, Indiana; Louisville and Lexington, Kentucky; Cincinnati and Dayton, Ohio; and Nashville, Knoxville and Chattanooga, Tennessee; during Friday afternoon and evening.

The stormy weather will kick off the change to a major, lasting cooldown across the Midwest.

Why will this pattern flip take place?

The latter half of July into early August typically represents the hottest weather of the summer. However, a semi-permanent southward dip in the jet stream is forecast.

When this happens in the summer, not only can it turn cool, but quite stormy. This is because the air high above the ground turns much colder than near the surface.

At times, when and where there is enough moisture, clouds can tower high into the atmosphere and produce drenching downpours and severe thunderstorms. High humidity may make for muggy nights.

On other occasions, when and where the air is very dry, cloud cover may be limited. Under these conditions, the air can get surprisingly cool at night, but strong sunshine this time of the year will cancel out the coolness during the day.

The persistence of the cool and/or wet pattern forecast is somewhat unusual for the summer, according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok. "But, the pattern may not only continue through the remainder of July but also linger into the first part of August."

During the pattern over the next couple of weeks, areas that are most likely to experience high humidity, wet and stormy conditions most often will stretch from the middle Mississippi, Ohio and Tennessee valleys to the eastern Great Lakes region.

Farther northwest, from the western Great Lakes to the northern Plains, cool, dry air will be present more often, but there can still be occasional spikes in temperature and brief episodes of showers and severe thunderstorms.

Large area of wet weather may get bigger

"During the first part of August, there is a chance the zone of wet weather shifts or expands farther to the west over the Central states," Pastelok said.

In addition to cool and/or wet conditions in a large part of the Midwest, wet, humid and stormy conditions will frequent much of the East into early August.

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