The agency predicted rain would fall at a rate of 1 to 2 inches per hour, potentially overwhelming small creeks and streams, streets, highways and underpasses, as well as other poor drainage and low-lying areas.

Millions across the Northeast and Mid-Atlantic remain under flash flood warnings early Tuesday as slow-moving summer storms bring heavy showers to the East Coast, impacting transportation and leaving people stranded in vehicles on waterlogged roads.

Cars were submerged, roadways flooded and nearly 2,000 flights canceled throughout the Northeast on Monday as flash flooding caused significant delays to regional transport -- and with more wet weather forecast through into Tuesday.