Long Island weather: Heavy rain, thunderstorms could bring 'serious' flooding into Wednesday

A flash flood watch remains in effect for Nassau County and western Suffolk through Wednesday night, as multiple rounds of heavy showers and thunderstorms in the forecast threaten to produce potentially serious flooding in some areas.

"This is going to be a serious situation," said Newsday meteorologist Addison Green, adding that rain coming down repeatedly "could become a problem more times than not."

The National Weather Service said the flood watch is set to end at 8 p.m. Wednesday.

The heaviest rainfall and rainfall rates will occur Tuesday night into early Wednesday, said weather service meteorologist Bryan Ramsey.

Long Island can anticipate 1 to 3 inches of rainfall. However, the area will see multiple bands of rain across the same areas "over and over again," allotting for more than 3 inches of rain. 

The multiple rounds of rain is what may turn flooding into flash flooding, Green said, especially in areas where the ground cannot absorb or run water off properly into drainage systems

"Now we have roads becoming more like rivers, and we've seen that time and time again in low-lying areas across Long Island and down across the coastal areas as well," Green said. 

Parts of southern Long Island and the East End, which is not under a flood watch, are expected to receive under 2 inches of rain. 

New York City and northwestern Nassau County are categorized under a moderate flash flooding risk, or a 3 out of 4 on the threat scale, and could experience numerous flooding events. Eastern Nassau through western Suffolk fall under a slight flash flooding marginal risk, where scattered flash floods are possible.

Ramsey said there is a possibility that the axis of the heaviest rainfall could shift east by Tuesday afternoon. If this were to happen, the service will extend the flood watch into the East End.

"There's still a little bit of variability on exactly where that heaviest rain will fall, but overall, the threat is pretty general across the area," Ramsey said. "So everyone should keep an eye out."

Green said the metro area is forecast to receive two heavy waves of rain with 2 inches per hour: one beginning Tuesday afternoon followed by another band Tuesday night into Wednesday. 

"That combination of urban areas and topography will lead into potential of seeing flooding become more of a concern than elsewhere, especially with those heavy rainfall rates," Green said. 

By Wednesday the system is forecast to travel northwest, bringing risks of scattered flash floods to Connecticut and the Hudson Valley.

Long Island will be under a marginal risk for isolated flash floods.

Commuters should stay updated on weather and road conditions and consider working from home on Wednesday, as moderate to heavy rain is possible between 6 to 8 a.m.

"If you have an option to work from home, I would work from home Wednesday," Green said.

If you do have to go into the city, Green urged commuters to take mass transit as traffic could easily build up from floodwaters and debris.

"The biggest advice to give is that from the National Weather Service: turn around, don't drown," Green said. "Does not matter what vehicle you're driving, whether it's a brand new car, it's a high-rise SUV. It's a large pickup truck. You should not attempt to drive through flooded roadways because you don't know how deep that water is that you're going to be going through."

Ramsey shared the same sentiment, as drivers don't know if something is below the surface of the water that could possibly puncture tires. 

"Every flood event we get there are people who will drive through the water think they can make it, and then they're sitting on top of their car waiting to be rescued," Ramsey said. 

While Long Island experienced severe thunderstorms last week, Tuesday night's thunderstorms are categorized as general and widespread. The storms may bring gusts of up to 40 mph, but they will not be nearly as strong or damaging as last week's storm, Ramsey said. 

The metropolitan area and New Jersey are likely to experience damaging gusts of between 60 and 70 mph. 

Before 7 a.m. on Tuesday, Kennedy Airport reported 254 cancellations and 50 delays, according to the Flight Aware website. LaGuardia Airport reported 17 delays and 213 cancellations. Those figures had eased up in the early afternoon, with JFK reporting 18 delays and 46 cancellations and LaGuardia noting 28 delays and 64 cancellations shortly after 1:30 p.m.

Gov. Kathy Hochul also urged New Yorkers to exercise caution ahead of the storm. If you are in an affected area, have an emergency plan in place, do not touch downed power lines and do not go through floodwaters, Hochul said.

After floodwaters have receded, wait until an area has been declared safe before entering. 

"New York State will be experiencing a prolonged rain event beginning Tuesday that will bring heavy rains and cause flash flooding — New Yorkers should stay vigilant, stay informed, and use caution," Hochul said in a news release on Monday night. "As state agencies are ready and mobilized to keep New Yorkers safe, it is important for everyone to do their part by monitoring your local forecast and when in doubt, turn around, don’t drown."

Check back for updates on this developing story.