Motorists and people spending time outdoors will be at risk for heavy and locally gusty thunderstorms ahead of a press of cooler and less humid air in the Northeast and Midwest to end this workweek.
Some communities may be hit with an inch or two of rain in a couple of hours, which is enough to cause urban flooding. It may lead to small stream flooding where the ground is saturated from recent rainfall.
While the storms will be most numerous and strongest from the mid-afternoon to the early evening, there can be a torrential downpour with thunder and lightning at any time until a few hours after the leading edge of cooler air arrives.
Into Thursday night, the main concentration of showers and thunderstorms is likely to extend from western, northern and central New York state to the Lower Peninsula of Michigan, Pennsylvania, Ohio, West Virginia, northern Virginia, western and central Maryland, Indiana and Illinois.
The risk of drenching storms may ramp up over Kentucky and Tennessee during Thursday night, when many of the storms will be slow-moving and repetitive in nature.
During Friday afternoon and evening, the threat of heavy, locally gusty thunderstorms will extend along the Interstate-95 corridor as well as the central and southern Appalachians and the lower Ohio and Tennessee valleys.
"Compared to recent days, the storms on Friday will be more numerous in coastal areas and in the southern Appalachians," according to AccuWeather Senior Meteorologist Kristina Pydynowski.
"Even though there will be more forward speed to the storms on Friday, they can be heavier than Thursday and result in just as much rain in a shorter period of time," Pydynowski said.
Motorists should be on the alert for flooded roads and seek an alternative route should a significant amount of water gather. The water may be much deeper than it appears. By driving too fast in heavy rain, there is an increased risk to hydroplane.
Summer weather across the US in 2018
Summer weather across the US in 2018
NEWPORT BEACH, CA - AUGUST 17: A man and his nephew enjoy the warm water at Salt Creek Beach in Dana Point on Friday, August 17, 2018. (Photo by Paul Bersebach/Orange County Register via Getty Images)
NEW YORK, NY - AUGUST 17: People cool themselves during a warm day at Central Park on August 17, 2018 in New York City. Severe thunderstorms and even an isolated tornado could strike New York City on Friday. (Photo by Eduardo Munoz Alvarez/Getty Images)
WEEHAWKEN, NJ - AUGUST 17: A man takes a look of the haze over the New York skyline and One World Trade Center on August 17, 2018 in Weehawken, New Jersey. Severe thunderstorms and even an isolated tornado could strike New York City on Friday. (Photo by Eduardo Munoz Alvarez/Getty Images
NEWPORT BEACH, CA - AUGUST 10: A surfer loses it on a wave in Newport Beach on Friday, August 10, 2018. A swell from Hurricane John will bring larger waves to south-facing beaches through the weekend. (Photo by Paul Bersebach/Orange County Register via Getty Images)
A man sunbathes on Venice Beach in Los Angeles, California on August 7, 2018. Temperatures climbed to near 100 degrees as a week-long heat wave continues in Southern California. (Photo by Ronen Tivony/NurPhoto via Getty Images)
WELLFLEET, MA - JUNE 21: Mark Wilke and his wife Sharon enjoy the first day of summer at White Crest Beach in Wellfleet, MA on June 21, 2018. (Photo by Craig F. Walker/The Boston Globe via Getty Images)
ARLINGTON, VA - JUNE 20: A pedestrian passes near the Netherlands Carillon as the Washington Monument and United States Capitol are seen at sunrise on Wednesday June 20, 2018 in Arlington, VA. The summer solstice is Thursday. (Photo by Matt McClain/The Washington Post via Getty Images)
Pedestrians watch sailboats and windsurfers on the Charles River on the summer solstice in Boston, Massachusetts, U.S., June 21, 2018. REUTERS/Brian Snyder
A women keeps in the shade of her umbrella as she tries to beat the heat at Cardiff State beach in Encinitas, California, U.S. July 6, 2018. REUTERS/Mike Blake
A man cools off in fountain on the Rose Kennedy Greenway during a summer heat wave in Boston, Massachusetts, U.S., July 2, 2018. REUTERS/Brian Snyder
OLD ORCHARD BEACH, ME - JULY 4: Ryan Parsons, left, of Unity watches as his daughter, Lily, 10, rides the waves on her inflatable whale at Old Orchard Beach on a hot, beautiful Fourth of July. The Parson family was spending the day at the beach keeping cool and watching the fireworks. (Staff photo by Jill Brady/Portland Press Herald via Getty Images)
NEW JERSEY, USA - JUNE 30: People enjoy Manhattan skyline at the Hamilton Park during the hot weather in New Jersey, United States on June 30, 2018. (Photo by Atilgan Ozdil/Anadolu Agency/Getty Images)
NEW YORK, July 2, 2018 -- People cool themselves at a fountain at Washington Square Park in New York City, the United States, on July 2, 2018. The highest temperature reached 35 degrees Celsius in New York City on Monday as a result of a prolonged heat wave. (Xinhua/Wang Ying via Getty Images)
NEW YORK, USA - JULY 23: Rain clouds are seen over Lower Manhattan in New York, United States on July 23, 2018. (Photo by Atilgan Ozdil/Anadolu Agency/Getty Images)
NEW JERSEY, USA - JULY 27: People walk with their umbrellas during a rainy day at the Liberty State Park in New Jersey, United States on July 27, 2018. (Photo by Atlgan Ozdil/Anadolu Agency/Getty Images)
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Flash flooding occurred in Pittsburgh and State College, Pennsylvania, during Independence Day.
Campers settled along small streams may want to consider an alternative site above the flood plain.
"In addition to an elevated risk of flash flooding in this weather pattern, a small number of the storms may bring strong downburst winds that can knock down trees, send lawn furniture airborne and cause sporadic power outages," Pydynowski said.
A downburst, or a form of strong straight-line wind, is produced when downdrafts, heavy rain and gravity cause winds to rapidly descend and reach the ground during a thunderstorm. For a few minutes it may seem like a hurricane is occurring due to strong winds and blinding rain when a downburst is in progress.
Anyone spending time outdoors should seek shelter indoors at the first rumble of thunder or distant flash of lightning. Wait at least 30 minutes until the last rumble of thunder or flash of lightning before returning outdoors.
In the wake of the storms, a two- to four-day break from high humidity and searing heat is in store, centered on this weekend.
"Highs in the upper 80s to middle 90s F will be replace with highs in the upper 70s to the middle 80s in many locations," Pydynowski said.
The stretch of days will allow for more comfortable conditions for exercising, manual labor or enjoying the outdoors. It will still be warm enough for swimming. Fans and air conditioners may still be needed in urban areas.