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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 into Friday evening.
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.
The threat of heavy, locally gusty thunderstorms will extend along the Interstate-95 corridor as well as part of the central and southern Appalachians and the lower Ohio and Tennessee valleys.
"Compared to recent days, the storms into Friday evening 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 evening and result in just as much rain in a shorter period of time," Pydynowski said.
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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.
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.
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