A cold front backing in from the east helps generate a chance of weekend showers, including the possibility of some heavy thunderstorms, the National Weather Service said Friday.
The incoming front and "Atlantic high pressure expanding slightly westward" means the likelihood of precipitation for Long Island and parts of southern Connecticut, perhaps as early as Friday afternoon, the service's office in Upton said.
David Stark, a meteorologist at Upton, said the two systems will create a "corridor of increased moisture," especially Saturday morning into Saturday afternoon.
Stark said Friday is most likely to be dry, but there is a 20 percent to 30 percent chance of rain, beginning at about noon, for most of Long Island.
The chances increase to 50 percent after 3 p.m. and to 60 percent overnight and into Saturday morning, he said.
Thunderstorms were causing flight delays Friday afternoon at LaGuardia and Kennedy airports, according to the Federal Aviation Administration website. Travelers are advised to check with their airlines, the agency said.
"We could see isolated showers and thunderstorms but determining where and when are challenges in the summertime," Stark said of the Saturday and Sunday forecasts.
Waves of precipitation could ride that corridor, at times increasing and then again decreasing, he said. "Timing is difficult," Stark said.
The system could also generate patchy morning fog on Sunday, the service said.
Overall, daytime temperatures for the weekend will be in the mid to high 70s with high humidity.
Chance of rain Sunday drops to 40 percent to 50 percent for most areas, with conditions slowly clearing by Monday morning. There also is a slight chance of Monday morning fog, the service said.
"Best overall chances of rainfall would be Saturday night into Sunday morning," the service said, with Monday possibly turning out to be "completely dry."
A slight chance of rain remains for Monday, with conditions clearing significantly by Tuesday, which calls for clear, sunny skies and daytime temperatures in the 80s.