Most of the rain is forecast to fall in as little as 3-6 hours. A small chance of heavier rain and flooding could pose "an elevated threat to life and cause significant disruptions to transportation, flood underground infrastructure and basements and first floors of residences and businesses," with water rescues needed, the weather service said.

A flash flood warning was issued just before 7 p.m. for northwestern Essex and northeastern Middlesex counties. It was set to expire at 9 p.m.