Flash flood risk increases in New York City as powerful storms threaten millions in Northeast

By Meteorologist Chris Dolce, CNN

(CNN) — Drenching storms sweeping through the East Coast on Thursday could unleash significant flash flooding, damaging wind gusts and a tornado or two.

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The flooding rainfall risk level was increased from a Level 2 to a Level 3 of 4 in New York City, parts of New Jersey and eastern Pennsylvania, including Philadelphia, on Thursday afternoon. This is where “numerous instances of flash flooding are likely,” the Weather Prediction Center said.

All told, more than 25 million people are in flood watches along the Interstate 95 corridor in the Northeast; they stretch as far north as southwest Connecticut to as far south as northern Delaware.

A few showers and storms have already popped up in these areas, but the heaviest rain is expected from late afternoon through the evening hours. Evening commuters could encounter serious disruptions.

The storms are being generated by a cold front that already caused flash flooding in central Ohio early Thursday morning. Multiple water rescues and road closures were reported near Columbus, Ohio, according to CNN affiliate WBNS.

The Northeast storms won’t last long, but some will wring out very intense rain falling at rates of 1 to 3 inches per hour.

Heavy bursts of rainfall like this are notorious for causing flash floods, especially in urban locations in the East. New York City’s aging sewer infrastructure can only handle around 1.75 inches of rain per hour before it’s overwhelmed and flash flooding can occur

Extreme rainfall rates are becoming more common as planet-warming pollution pushes temperatures higher because warmer air holds more moisture. Weather systems can then wring that moisture out like a water-laden sponge, yielding heavy and often highly localized downpours.

Most locations will see at least 1 to 2 inches of rain, but areas hit by multiple storms or the most intense rainfall rates could receive up to 3 to 5 inches of rain in just a couple of hours.

Some storms could intensify

Some of the storms could intensify enough to bring damaging winds and hail in many of the same areas at risk for flooding.

Damaging wind gusts will be the most widespread threat, especially from the southern half of New Jersey to Philadelphia, Baltimore and Washington, DC. The most intense bursts of wind could down trees and knock out power.

This same general area also has the biggest risk of isolated tornadoes.

Much of the flooding rain and severe storm threats should end around midnight as the cold front pushes off the East Coast.

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