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.

Dangerous flash floods stranded motorists and left roadways under water. Some outdoor activities were canceled as communities scurried to keep pace with the rising waters.

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

Dozens of flash flood warnings are in effect for millions across several states and Washington, D.C., as slow-moving summer storms impact the East Coast Monday evening. Flooding has been reported near Newark,

Several Massachusetts communities south of Boston received as much as over 5 inches of rain, with flooding closing several streets and parts of Interstate 93.

Heavy rainfall caused areas of flooding on I-93 and other major roadways in Massachusetts on Thursday morning.

Mayor Lynn Crawford said hearts are broken over the lives lost and stomachs are in knots as residents begin to take stock of the damage.

Back-to-back flooding disasters in recent years — in Texas, New Mexico and Kentucky, among many others — have showed that preparing for flash flooding is a new necessity as the planet warms.