Slow-moving summer storms capable of extreme rain are poised to drench the East Coast Monday, threatening millions from North Carolina to New England with even more dangerous flash flooding.

Places recently hit hard -- like Kerrville, Texas, and Burnet, Texas -- are inside this heightened potential for flash flooding, with any heavy rain and flash flooding that occurs over or adjacent to these areas likely to flow downstream via rivers that run through or nearby these devastated areas.

The National Weather Service warned that the Guadalupe River could surge to nearly 15 feet—5 feet above flood stage—by Sunday afternoon.