The Long Island Power Authority restored power Thursday night to all but 200 customers who lost power after Wednesday's thunderstorms.
But shortly after 10 p.m., only scattered outages remained, LIPA's website showed.
The number of customers without power had fluctuated since the storm first touched Long Island, on the North Shore, about 1 p.m. Wednesday.
LIPA spokeswoman Elizabeth Flagler said the hard-hit communities suffered significant damage -- downed trees, poles and power lines -- adding that compounding the issue is that some of the damage is on private property and not in the power authority's right of way.
That means crews must get permission from homeowners before they can begin work.
"We will have extensive manpower in the area all day, with additional crews on overnight," Flagler said. "We're working to have a majority of customers back on line by this evening."
Flagler said LIPA "can't guarantee" all power will be restored Thursday.
Officials said that at one point Wednesday more than 22,000 customers were without power. That number was down to about 3,500 early Thursday, but then climbed and dropped again.
Flagler said the number of outages was expected to fluctuate, as emergency crews took homes offline to make permanent repairs to areas temporarily restored in the aftermath of the storms that hit Long Island on Wednesday.
Those storms downed trees and power lines, sparked fires, collapsed a historic gazebo in Glen Cove and injured two.
Rain remains a strong possibility again Thursday, the National Weather Service said, with high temperatures expected to be in the low 80s.
The eastward-moving thunderstorm first hit Long Island just before 1 p.m. Wednesday, touching down on Nassau's North Shore.
Soon after, a wind gust blew down the 1932 gazebo at Morgan Memorial Park in Glen Cove.
Three fishermen had taken shelter under the open-air gazebo when the roof caved in about 1:20 p.m., injuring one of them, Glen Cove parks and recreation director Darcy Belyea said. That victim was taken to North Shore University Hospital in Manhasset, she said.
Dozens of trees also came down, Belyea said, and one woman was trapped when a tree came down on her car.
She was taken to Glen Cove Hospital with neck and back pain, Belyea said.
As of late afternoon Wednesday, Nassau County Executive Edward Mangano said the damage was focused on the North Shore. Emergency operations were activated at about 2 p.m., with public works and emergency personnel called in to monitor the storm and clear debris from roads, Mangano said.
Fire departments in both counties were busy with storm-related problems, including several lighting strikes to buildings in Suffolk, police said.
Some flights at area airports were experiencing weather-related delays, according to the Federal Aviation Administration. Departures at Kennedy Airport were delayed about two hours, according to the FAA, while arrivals LaGuardia Airport were up to 45 minutes late and with delays increasing.
Early Thursday, the Port Authority was still reporting delays of up to two hours at those airports -- with a handful of flight cancellations, as well. Travelers are advised to continue checking with their airlines for flight updates.
Meanwhile, the New York State Department of Transportation reported that the westbound service road to the Long Island Expressway was temporarily closed on Thursday morning between exits 40 and 39 because of downed wires.
The road was closed about 5 a.m. and reopened by the afternoon.
The severe thunderstorms came on a day when a heat advisory was in effect for Nassau and western Suffolk from 10 a.m. to 6 p.m., officials said.
It also officially marked the third day of a heat wave, as the temperature at Long Island MacArthur Airport in Ronkonkoma hit 90 degrees at 10:32 a.m., rising to 94 by 11:37 a.m. The mercury had also hit 90 degrees on Monday and Tuesday.
Highs are expected to be in the mid-70s Friday, when, meteorologist Tim Morrin said, forecasters are looking at the possibility of "light but long-duration rainfall."
With Patricia Kitchen and Ellen Yan