A flood advisory is in effect for Nassau County and western Suffolk County until midnight as a "large area of moderate to heavy rain" is poised to move in, the National Weather Service said.
Rainfall rates of between an inch to 1 1/4 inches were possible, the service said.
By about 10 p.m. Wednesday, a little more than an inch of rain had fallen at Long Island MacArthur Airport in Ronkonkoma, the weather service said.
Nassau County saw more rainfall, with 3.63 inches recorded in Uniondale by late evening, said Lauren Nash, a meteorologist with the weather service in Upton. Other high rainfall counts were recorded in Plainview, with about 3.21 inches.
The heaviest rain was expected to move in from the southwest and last through early Thursday morning, said Ashley Sears, another National Weather Service meteorologist.
Total rainfall is forecast to be 2 1/2 to 3 inches, possibly higher in isolated areas, she said.
"We're going to get periods of heavy, heavy rain and then we'll see times when it will be lighter," said Pete Wichrowski, also a meteorologist in Upton.
As of late evening, weather was causing arrival delays averaging 1 hour, 35 minutes at LaGuardia Airport, the Federal Aviation Administration said on its website. The FAA advised travelers to check with their airlines.
The weather service has issued a coastal flood advisory from 8 p.m. Wednesday to 3 a.m. Thursday for western Long Island Sound, the Great South Bay and Atlantic coast, with widespread minor flooding expected around the times of high tide. Minor beach erosion could also occur along the ocean, the weather service said.
A flood watch remains in effect for Suffolk County from 8 p.m. Wednesday until noon Thursday.
A flood watch is also in effect for Nassau until noon Thursday.
Wichrowski said clouds and intermittent rain could linger into the afternoon Thursday, depending on the area.
Clearing "should start by Thursday night," and the weekend is likely to be in the mid-60s with sunny skies.
Friday calls for a 20 percent chance of rain, likely after noon, with mostly sunny skies.
A high of 72 had been predicted for Thursday at MacArthur Airport, which hasn't seen 70 since Oct. 17, 2013. However that's been moderated to the mid- to upper 60s, with western areas of the Island still expected to reach 70, Sears said.
Since 1984, Long Island's weather records for the airport in Ronkonkoma have been kept by the weather service. From 1949 to 1983, the records were maintained by the Brookhaven National Laboratory in Upton.
Based on data since 1984, April 9 is the average date for temperatures at the Ronkonkoma airport to crack 70, said Joe Pollina, also a meteorologist in Upton. This year would be the third time that was delayed until May, the first being 1984, when the area had to wait until May 20 for 70 degrees to arrive.
With Candice Ruud