New York faces new flood threat after Hurricane Ida's havoc

As much as 5cm of rain could fall in New York and its surrounding suburbs starting Sept 23. PHOTO: NYTIMES

NEW YORK (Bloomberg) - New York and the US Northeast are facing another flood threat just three weeks after the remnants of Hurricane Ida devastated the region, but the heavy rain expected through Friday (Sept 24) won't match that storm's deadly deluge.

As much as 5cm of rain could fall in New York and its surrounding suburbs starting Thursday afternoon, with the heaviest showers coming between 8pm and 2am, likely causing some floods, said Paul Walker, a meteorologist with commercial forecaster AccuWeather.

Flood warnings and watches stretch from eastern Maryland to Connecticut.

"We are going to do things differently from now on," New York City Mayor Bill de Blasio said on Thursday in a briefing.

"Officially this looks like a limited storm, but we've learned that projections are not always right. The National Weather Service forecast is for 1.5 inches (3.8cm) of rain but they are talking about 1 inch an hour, which could cause problems."

The remnants of Hurricane Ida killed more than 40 people in the Northeast, sent torrents of water through New York's subways and inundated many homes and businesses throughout the region.

Ida's blow to the region came after the storm tore a path of destruction across Louisiana and the US South.

More than 17cm of rain fell in Manhattan as Ida moved through, with more than 7cm falling in a single hour.

The front, which has already passed Washington, Baltimore and Philadelphia, will likely drop around 5cm of rain in New York, Walker said.

Though de Blasio cited reports that rain could fall at a rate of about 2.5cm per hour, that metric can be deceptive.

If half a centimetre of rain falls in 15 minutes, that's considered about 2.5cm per hour, according to Walker.

"Whenever we get that much rain in a short period of time, it can lead to a lot of flooding in urban areas because of the pavement," Walker said. "We will see some underpasses flood that typically flood when we get heavy rain."

Those roads that are prone to flooding are a major concern, de Blasio said.

"With basement apartments, this is much less than what we saw with Ida," though he said residents should be ready to move to higher ground if the storm appears to be more severe than forecast.

Join ST's Telegram channel and get the latest breaking news delivered to you.