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.

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

A "considerable" flash flood warning was issued in Boston on Thursday morning as heavy rain pounded the city and areas to the south. Four to 6 inches of rain fell in the towns of Canton, Quincy and North Scituate. Rainfall rates reached up to 2 inches per hour.