Midwestern, eastern US to catch break from dog days of summer

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Ever since the Fourth of July weekend, rounds of severe thunderstorms have been persistent in parts of the Midwest and Northeast. On top of that, hot, sultry conditions have dominated in what could truly be called the dog days of summer. But, that's all beginning to change, AccuWeather forecasters say.

A refreshing change to cooler and less humid conditions was already underway over the north-central United States Thursday and the interior Northeast on Friday. The cool wave is forecast to expand through the Northeast into this weekend and much of the South next week.

The pattern change is not only forecast to cool conditions down, but it is also predicted to end the frequent bouts of severe thunderstorms that have been erupting. However, that doesn't mean the weather will be rain-free, nor entirely free from severe weather. A couple of pockets of severe weather are likely in the Northeast on Sunday and the pattern switch may eventually lead to a great deal of rain in the Southeast and parts of the Northeast.

The Midwest and Northeast have been taking turns on a nearly daily basis with severe thunderstorms since Independence Day. Since the holiday, there have been thousands of incidents of high winds and hundreds of incidents of storms with hail, according to the Storm Prediction Center.

The changing weather setup unfolding is being brought on by a southward dip in the jet stream in east-central Canada, and the jet stream will plunge farther south into the eastern half of the nation through next week, AccuWeather Lead Long-Range Meteorologist Paul Pastelok explained.

"The setup will bring below-average temperatures over a broad swath of the Eastern states in northern areas this weekend and expanding southward next week," Pastelok said.

Around the start of August, temperatures are typically just past the peak of their highest levels of the entire summer. The middle of July, right in the heart of the dog days of summer, is typically when highs reach their peak. Normal highs in late July and early August range from the upper 70s F over the northern Great Lakes and northern New England to the middle 80s around the Ohio River, near 90 around the Chesapeake Bay and the lower 90s over the interior Southeast and the middle to upper 90s over the southern Plains.

Following highs near 90 at midweek in Chicago and as late as Friday in Washington, D.C., highs in the 70s and lower 80s are forecast for much of the Midwest on Friday and this weekend for much of the Northeast with substantially lower humidity levels.

Nighttime low temperatures will range from the middle 40s in the mountains over the Northeast to the middle 50s around Detroit, the upper 50s in Boston, the lower 60s in Chicago and New York City and the middle 60s in St. Louis and Washington, D.C., this weekend. At this level, nighttime temperatures will dip to 10-15 degrees below average. Record lows may even be challenged in some interior locales of the Northeast early on Saturday morning.

"Where it manages to stay cloudy at night, temperatures may not drop off too much from daytime levels, but where it manages to clear at night, temperatures can dip to chilly levels for the middle of summer," Pastelok said.

An end to the sweltering air with highs climbing well into the 90s in much of the Southeast is in sight as well. Highs are forecast to be in the lower to middle 80s much of next week in cities such as Birmingham, Alabama; Jackson, Mississippi; Columbia, South Carolina; Charlotte, North Carolina; and Atlanta. While the temperature change of 10-15 degrees may seem subtle, it is significant for this part of the nation this time of the year.

"The expanding southward dip in the jet stream is expected to even bring an end to the streak of 90-plus days in Kansas and Oklahoma with highs in the lower to mid-80s in this part of the Plains starting this weekend and progressing southward early next week," added Pastelok. The string of 90-degree days has been ongoing for about a week straight in parts of the Plains, with highs throttling up to the upper 90s and triple digits for some since early this week.

The expanse of cooler, less humid air will greatly tone down the number of thunderstorms with torrential rain and high winds from the Midwest to the interior Northeast this weekend, but the pattern will not be entirely rain-free in this zone even into next week. And, the presence of cool air at mid-levels of the atmosphere can lead to an eruption of spotty afternoon showers and thundershowers.

As a storm system pivots southeastward, a batch of showers and thunderstorms is forecast to roll through parts of the Midwest on Saturday and then into the Northeast on Sunday.

Because of the amount of cool air aloft associated with this advancing system, heavy, gusty and perhaps even severe thunderstorms can occur. The magnitude of severe weather is likely to be less intense than that which occurred in both regions this past week, which featured many reports of high winds and about a dozen tornadoes.

Beyond this weekend, there is the potential for a wet pattern to return to part of the Eastern Seaboard.

As the core of cool air becomes re-established over the Midwest, as it was earlier this summer, a southerly flow of tropical moisture is forecast to develop in the Southeast states and at least in coastal areas of the Northeast.

The Bermuda high tends to hover over the western part of the Atlantic Ocean in the summer, but it will shift closer to the East Coast of the U.S. next week.

That change will result in a "log jam" of cool fronts stalling over the Southeast next week -- a change that can lead to multiple rounds of heavy and perhaps torrential rainfall, Pastelok said.

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At the very least, there are going to be some bad days at the beaches from the Gulf Coast to the southern Atlantic coast and even in the mid-Atlantic and New England at times.

However, AccuWeather meteorologists believe the pattern could bring more than just bad beach days. There is the potential for several inches to perhaps a foot of rain to fall in parts of Southeast states from Louisiana to Virginia and could result in flooding next week.

This image shows potential rainfall (inches) for the seven-day period ending on Friday morning, Aug. 6, 2021 as issued by the National Center for Environmental Protection (NCEP).

There is still some uncertainty in how far west the wet weather will reach. Should the jet stream dip back westward, the door could be opened for rounds of heavy rain to reach not only the Interstate-95 corridor of the Northeast, but also the central and northern Appalachians as well. The most likely time for rounds of heavy rain to occur in the swath from Philadelphia to New York City and Boston would be the middle to latter part of next week.

As wet as the summer has been for places like Boston, which has picked up more than three times the normal rainfall in July, and New York City, which has recorded more than two times the normal monthly rainfall, there are some pockets in western Virginia, West Virginia and northern New England that could stand a soaking or two. Some locales within this corridor are experiencing abnormally dry soil conditions, and others are in the grips of a drought, according to the United States Drought Monitor.

AccuWeather forecasters say people heading to the beaches for an extended vacation next week from the central Gulf Coast to Virginia and perhaps farther to the north should keep a close eye on the day-to-day forecast. But, as the saying goes, "a bad day at the beach is better than a good day at work."

Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier, Spectrum, FuboTV, Philo, and Verizon Fios.

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