There’s been an abnormal amount of extreme rainfall across the United States in recent days. Here’s what’s driving it.

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.