Record storm surges are wallopping the Sound Shore and are slated to push up the Hudson River to Putnam and Orange counties around midnight, forecasters said.
A 5-foot surge breached banks in Irvington Monday afternoon, and some other river towns. An 11-foot surge is expected at about 11 p.m., when the next high tide also could trigger massive flooding.
"We might never see in our lifetime anything like this again," News12 meteorologist Brysen VanEck said.
A storm surge is caused when sustained hurricane-force winds blow down on the ocean surface, causing water to move upward, forecasters explained.
The impact is felt most acutely along the coast when the hurricane reaches shallow water. There, unlike at greater depths, the water can no longer move downward. It has nowhere to go but upward and inland.
"We're looking at an amazingly high storm surge in Long Island Sound, New York Harbor and the north shore of New Jersey," said David Valle, the hydrologist in charge of the Northeast River Forecast Service for the National Weather Service in Taunton, Ma.
That water eventually will move inland, pressing rivers to crest, he said.
Hurricane Katrina caused severe damage to Louisiana with a storm surge that measured 28 feet.
That's about a foot short of what the weather service defines as "major," but enough to flood the river towns and make roads impassable.
"The surge is going to move water up the river," said Valle. "This is going to impact the river in a big way."