Flash floods hit several cities and towns south of Boston Thursday morning as historic rain brought traffic to a crawl and clogged streets and highways during rush hour.With rain coming down so fast,

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

Thursday will be a “storm sandwich” with pockets of heavy rain in the morning, a dry midday, and scattered afternoon showers and thunderstorms popping up across the region.