Sunlight peers through stratocumulus clouds creating a dramatic scene over...

Sunlight peers through stratocumulus clouds creating a dramatic scene over the waters of Long Beach on the first day of fall. Credit: Newsday/J. Conrad Williams Jr.

The National Weather Service has issued a flash flood watch for Nassau County for Thursday evening and into Friday as potentially heavy rain moves into the region.

Showers and thunderstorms are expected then, just ahead of a "slow moving cold front," and there is a "slight risk of excessive rainfall" for Nassau and other parts of the tristate Thursday overnight and Friday morning.

There is also a "marginal risk" of severe thunderstorms across the same area during that time period, the weather service said in a Wednesday evening briefing.

Those storms could bring wind gusts of up to 58 mph and an "isolated" tornado can't be ruled out, the weather service said.

Wednesday night could also bring showers, but they become much more likely for Long Island overall after 2 p.m. Thursday, forecasters said.

Temperatures over the next seven days will start out warmer than usual, climbing to the high 70s during the days and slipping to the high 60s at night. Temperatures will then descend to the low 70s by Friday.

Islip’s mean temperature in September is a cooler 65.6, according to weather service records that date back to 1963.

Dangerous rip currents are anticipated through Thursday evening, the weather service warned.

Long Island can expect mostly sunny and mild days Saturday and Sunday. While a passing shower from an approaching low pressure system might sweep in late Saturday night, it should remain off to the north and west, the weather service said.

And with a high pressure system sitting just south of the area, Monday should also be fair with sunny skies and a high of 71 degrees.

Tuesday looks equally promising — but the weather service cautioned it has "much less confidence" about the forecast for early next week as its models' predictions diverge, and there is a slight chance of showers Monday night through Tuesday night.

With Joan Gralla

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