Nassau was in the path of a "strong thunderstorm" Thursday night that moved from the Kennedy Airport area before clearing out to Connecticut, the National Weather Service said.
The storm carried winds of up to 40 mph and heavy rain, up to an inch per hour, meteorologists said in a 9 p.m. special weather statement.
But half an hour later, only an isolated thunderstorm remained amid heavy rain over the county, with Suffolk still dry, meteorologist Lauren Nash said.
In the next few hours for both counties, the "widespread rain" will become even heavier, with only a "brief break" that will start toward the end of Friday's morning rush hour.
Ponding on roads and localized flooding should be expected, the service said.
A flash flood watch remains in effect until 6 p.m. Friday due to on-and-off showers and thunderstorms, the service said.
The pending storms caused flight delays in the New York area for much of Thursday, an average of two hours and 30 minutes for some arrivals at Kennedy Airport and almost two hours at LaGuardia Airport, according to the Federal Aviation Administration's website.
The pattern of scattered rain and storms may linger through the weekend and until as long as Wednesday, but the bulk of the rain, as much as 1.5 inches overall, could fall overnight Thursday into Friday, said Joe Pollina, a meteorologist with the Upton-based service.
Pollina said the sweeping, southeasterly moving cold front and a Bermuda High -- a large, subtropical air mass that rests over the western North Atlantic near the islands for which it is named -- are creating a boundary of rain that runs along the Atlantic Coastal states.
As those two fronts struggle for dominance, the rainfall boundary either moves inland or out to the ocean, he said.
As far as Nassau and Suffolk counties are concerned, the rainfall boundary likely will push mostly inland.
The stretch of precipitation will add to June's already heavy annual rainfall total of 8.03 inches (as of Thursday), Pollina said. But it's not expected to surpass the wettest June, in 2003, when 10.8 inches were recorded.
From 1949 to 1983, Long Island's weather records were maintained by the Brookhaven National Laboratory. Since 1984, the records have been kept at Long Island MacArthur Airport in Ronkonkoma by the National Weather Service.
With Ellen Yan