Thunderstorms have moved back into parts of Long Island that were drenched Tuesday afternoon, with the nighttime system bringing up to 2 more inches of rain, the National Weather Service said.
The storm, moving northeast at 55 mph, was expected to sweep again over southern Nassau and Suffolk counties, nearing Islip by 8:30 p.m., said meteorologists in the service's Upton office.
A flash flood warning was issued for Nassau and will remain in effect until 10:15 p.m. A flood watch was in effect for western Suffolk until 9:45 p.m.
Frequent lightning from clouds to ground will also occur, just as it did during the afternoon storm, and the system was expected to pack wind gusts of up to 40 mph, the service said.
This follows an afternoon thunderstorm that carried wind gusts of 60 mph, which downed trees and caused flooding in isolated areas, meteorologists said.
Just before 7 p.m., more than 6,500 customers, more than half of them in Hempstead Town, remained without power, according to PSEG Long Island.
The weather also caused signal problems for the Long Island Rail Road.
There were delays of up to 15 minutes in both directions on the Long Beach branch. There were also delays of up to 45 minutes on the Port Jefferson branch, according to the LIRR.
The weather also delayed flights at LaGuardia and Kennedy airports, according to the Federal Aviation Administration's website.
The initial storm system had swept over parts of the Island, hitting Levittown and other communities in the southern part of Nassau just before 3:15 p.m., the service said.
It brought lightning from clouds to ground, which is typical of many warm tropical storms, said meteorologist Lauren Nash at the service's Upton office.
Wind gusts of 59 mph were recorded at the Jones Beach Coast Guard Station, the service said.
Just before 3:15 p.m., meteorologists in the Upton office said they had tracked the system as a "torrential rainfall" hit Levittown and other communities in southeast Nassau before the storm went across Ocean Parkway in Babylon Town just before 4 p.m.
By the time it hit Suffolk, the storm weakened, moving northeast at 30 to 40 mph before departing Long Island, the service said.
With a moisture-rich air mass that thunderstorms can tap into, the threat of heavy rain and severe wind gusts is the main concern, the service's Upton office said.
Cool air from the front, combined with warm ground temperatures -- highs in the low- to mid-80s, and high humidity -- increase the chance for heavy rain and strong winds, the service said.
Rain remains in the forecast through Wednesday night, when the chance of precipitation drops to 30 percent.
By Thursday, the cold front slowly moves off Long Island, leaving behind slightly cooler temperatures. Skies will remain partly cloudy starting Friday night and into Saturday, with a slight chance of rain again on Sunday.
With Gary Dymski