Cool end to July in Midwest to starkly contrast humid, stormy weather along US East Coast

By Alex Sosnowski, AccuWeather senior meteorologist
July 18, 2018, 11:17:29 AM EDT

Following a push of dry air during the middle part of this week, a humid and rather wet weather pattern is forecast to evolve over the eastern third of the nation during the latter part of July.

A large bubble of cooler and less humid air will settle southeastward from the upper Great Lakes to the mid-Atlantic and New England coasts through Thursday.

NE regional 7.18 AM

People who have been minding the high humidity will get a break as will fans and air conditioners across the Upper Midwest and interior Northeast.

However, as AccuWeather long-range meteorologists have been predicting over the past couple of weeks, a change in the jet stream pattern will also force some changes in the lower part of the atmosphere beginning late this week.

Static Saturday NE

The weather pattern change is forecast to last through the end of July and into early August.

A semi-permanent southward dip in the jet stream will allow waves of cooler air to flow from central Canada to parts of the Appalachians.

Where the jet stream bottoms out and bends northeastward, over much of the central and eastern Gulf Coast to much of the Atlantic seaboard, humid air will be around much of the time.

Static Late July Pattern

"While cool conditions will become fairly routine over much of the Upper Midwest, the pattern looks quite wet across the Southeast and mid-Atlantic states during much of the second half of July," according to AccuWeather Lead Long-Range Meteorologist Paul Pastelok.

The weather could become a bother for people on vacation, walking or biking to work or anyone with outdoor plans.

However, the pattern may also bring much-needed rain to parts of the region. For example, from June 28 through July 16, Washington, D.C., did not receive a mere 0.01 of an inch of rain. It was the driest first half of July on record. The prior record was 0.08 of an inch of rain during early July of 1900.

As the weather pattern evolves, showers and thunderstorms may become frequent and could occur on a daily basis or perhaps multiple times a day along the Interstate 10, 20, 81 and 95 corridors.

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"The pattern is likely to bring below-average temperatures with multiple days that have highs in the 70s to lower 80s F in Chicago, Minneapolis, Detroit and perhaps the central Appalachians as well," Pastelok said.

Most nights are likely to be cool enough to keep air conditioners off in all but the most urban areas.

Ninety-degree highs may become scarce over the interior South, along the mid-Atlantic coast and in much of New England during the pattern as the ground trends wetter and cloud cover increases. But, because of warm, muggy nights and high humidity, temperatures may still be within a couple degrees of average, and air conditioning may be needed on a daily basis.

"We also can see how a tropical depression develops near the northeastern Gulf Coast late this week and wanders northward along the Atlantic coast this weekend," Pastelok said.

Such a development may enhance locally drenching showers and thunderstorms to the point where torrential rain and flooding occur along with locally rough surf.

AccuWeather will continue to provide updates on this potential tropical system as well as tropical activity around the globe.

"Another area where frequent showers and thunderstorms may develop is over parts of the central Plains to the lower Mississippi and Tennessee valleys during the latter part of July," Pastelok said.

This zone will mark a boundary between cool air and low humidity over the Midwest and hot, stifling conditions over the southern Plains.

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