A strong thunderstorm moved through part of Long Island Wednesday afternoon, dropping almost three-quarters of an inch of rain in the Smithtown to Stony Brook area, said Mike Layer, meteorologist with the National Weather Service, based in Upton.

That happened as a slow-moving frontal system traveled through the region, leaving a 30 percent chance of scattered showers and thunderstorms Wednesday into the overnight hours, he said.

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Chances will increase mid-to-late Thursday morning for the same, including the possibility of isolated thunderstorms that could produce heavy rain, leading to localized flash flooding, he said.

Area air traffic was affected, with weather-related arrival delays late Wednesday afternoon averaging about 21/2 hours at Kennedy Airport and just over 2 hours at LaGuardia, according to the Federal Aviation Administration website. Some departures from Kennedy were averaging just over two hours and at LaGuardia, just over 11/2 hours, the FAA said. With some departure delays, too, at Long Island MacArthur, the FAA advises travelers to check with their airlines.

A high rip current risk is in effect for Long Island ocean beaches through 9 p.m. Wednesday, the weather service said, with surf height of around 3 to 4 feet and a moderate lightning risk.