New Yorkers in basement apartments were told to be ready to leave and New Jersey’s governor declared a state of emergency after heavy rains in the northeast caused flash flooding, officials said.

Western Union County, New Jersey, was experiencing life-threatening flash flooding by storms that were nearly stationary Monday evening, the National Weather Service office for the New York City region said.

“Please stay indoors and avoid unnecessary travel,” New Jersey Gov. Phil Murphy said in announcing he was declaring a state of emergency due to the heavy rainfall. “Stay safe, New Jersey.”

All five boroughs of New York City were under a flash flood warning Monday night.

Emergency management officials warned residents of basement apartments to be ready to leave.

“If you live in a basement apartment, stay alert. Flash flooding can happen with little warning, including overnight,” New York City Emergency Management said on X. “Keep a phone, flashlight, and Go Bag nearby. Be ready to move to higher ground.”

Video on social media showed people wading out into floodwaters above their knees Monday in New Providence, New Jersey, around 10 miles west of Newark.

Early rainfall reports Monday evening in New York City included 1.67 inches in Staten Island and 1.47 inches in the Chelsea neighborhood of Manhattan, New York City Emergency Management said on X at around 7:30 p.m. — and it cautioned more rain was forecast Monday night.

LaGuardia International Airport and Newark Liberty International Airport both reported flight disruptions due to the weather, and John F. Kennedy Airport warned of possible problems. All three airports led in cancellations Monday night on tracking website FlightAware.

Ground stops due to thunderstorms were issued for LaGuardia and Newark airports Monday evening, the Federal Aviation ministration website showed, and long delays were reported at all three.

Standing water on the Cross Bronx Expressway caused the traffic artery to be closed in both directions at Macombs Road, emergency officials said.

There was also standing water on FDR Drive that was causing delays Monday night, as well as standing water on Harlem River Drive, the city warned.

New York City Transit said it was working on flooding at multiple subway stations in Queens. E, M and R trains were “severely disrupted,” the agency said. The Staten Island Railway suspended service in both directions in the southern part of the borough because of floods.

The National Weather Service warned earlier Monday that showers and thunderstorms could bring rainfall rates of up to 2 inches an hour, and up to 3 inches in total could fall in the New York City area.

“Much of this could fall in only 3 to 4 hours, causing isolated to scattered instances of flash flooding,” the weather service said.

At around 7:40 p.m., the weather service warned that an additional 1 to 2 inches, on top of the rain that already fell, could hit the city.

Jamaica, Flushing, Rockaway Beach, Great Neck, John F. Kennedy Airport and Bayside were among the New York City areas expected to see flooding, the weather service said in the flash flood warning issued Monday.