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.

Temperatures in Boston for the week peaked with a heat index in the triple digits on Wednesday – but residents may not see a real cool off until Friday.

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.