Mayor Michelle Wu declared a heat emergency for Boston from Wednesday through Friday, with the heat index predicted to fluctuate from 91 to 97 degrees with little overnight relief, the city announced.

Thursday will be the second day of expected 90-degree temperatures for Greater Boston, with high temperatures and humidity in the forecast through the end of the week.Most of Massachusetts is under a heat advisory that lasts through Thursday afternoon and Boston Mayor Michelle Wu declared a heat emergency for the city from Wednesday through Friday.

Cars were submerged, roadways flooded and nearly 2,000 flights canceled throughout the Northeast on Monday as flash flooding caused significant delays to regional transport -- and with more wet weather forecast through into Tuesday.

Heavy rain swept across parts of the U.S. Northeast on Monday night, inundating sections of New York and New Jersey with flash flooding that stranded vehicles in roadways, closed subway lines and led to the declaration of a state of emergency.