Light showers had already moved into some areas of Long Island Tuesday afternoon, a prelude to the 2 to 3 inches of rain expected overnight into Wednesday -- with isolated spots seeing a possible 4 inches, forecasters said.
Overall, the evening commute shouldn't be greatly affected by the rain and possible thunderstorms, which are not expected to be widespread until around 9 p.m., said Joe Pollina, National Weather Service meteorologist in Upton.
The best chance for the heaviest rain -- brief periods at a rate of 1 to 2 inches an hour -- is after 3 a.m. Wednesday, with the earliest morning commuters possibly affected, he said.
Residents should be aware of the possibility of flooding, with a flash flood watch in effect for the Island from midnight through Wednesday afternoon. Widespread minor to locally moderate coastal flooding is also expected for South Shore bays and western Long Island Sound.
Behind the interruption to a gorgeous stretch of recent summer weather are two low pressure systems, one moving eastward across the Great Lakes and the other tracking upward from the mid-Atlantic, the weather service said.
"By Wednesday night, we should be drying out, and Thursday and the end of the week, up through the weekend, look nice," said Michael Silva, a meteorologist with the Upton office.