Winter Weather Advisory

Long-duration cool spell to dominate Midwest as downpours frequent eastern areas

By Alex Sosnowski, AccuWeather senior meteorologist
July 21, 2018, 12:00:30 PM EDT

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. Showers and thunderstorms will be most numerous east of Chicago and St. Louis during the upcoming week.

AP MLB Allstar Game

Field crews take cover during a heavy and sustained rain ahead of the 89th MLB baseball All-Star Game, Tuesday, July 17, 2018, at Nationals Park, in Washington. (AP Photo/Alex Brandon)

Since conditions in the atmosphere will be more typical of the middle of spring, rather than the middle of summer, a few thunderstorms can become severe.

While these storms will not be an every-day occurrence, the overall weather pattern may periodically allow some of the rounds of storms to produce isolated tornadoes.

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.

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.

Static US Late July 1 pm

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."

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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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