Flash floods hit cities and towns south of Boston Thursday as historic rain brought traffic to a crawl and shut down part of I-93.

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.

Boston, Quincy, Brockton, and other South Shore communities are under a flood watch until 9 a.m., according to the National Weather Service.