In New York City, video shared on social media showed muddy brown water spewing like a geyser in front of a subway station’s turnstiles.
Multiple subway lines were briefly suspended or running with delays, while above ground, massive traffic jams paralysed several of the city’s main thoroughfares.
JFK, LaGuardia and Newark airports temporarily suspended departures on Monday night, forcing the cancellation of dozens of flights.
In Lancaster, Pennsylvania, severe flash flooding prompted a disaster declaration, while emergency responders plucked people from flooded basements and conducted 16 water rescues.
“Intense rainfall dropped over 7 inches of rain in less than five hours,” the fire department in the county’s Mount Joy Borough posted on Facebook.
Staten Island recorded between 10 to 15cm of rain on Monday night, according to the New York borough’s emergency notification system.
Zohran Mamdani, the Democrat running for mayor of New York, wrote on social media that the rapid flooding emphasised the need for climate-proofing the city.
“We must upgrade our infrastructure for this new climate reality,” he wrote.
The NWS said the storm would be concentrated “across the southern Mid-Atlantic/Appalachians on Tuesday, before shifting northward” on Wednesday.
The latest bad weather follows historic Fourth of July flooding that devastated parts of central Texas, killing at least 131 people, including three dozen children, and leaving more than 100 people still missing.
