The National Weather Service has issued a heat advisory for the state of Massachusetts and all of southern New England.The advisory will be in effect through 8 p.m. Tuesday for most of the Bay State and all of Rhode Island and Connecticut.The only areas exempt from the heat advisory are the islands of Martha's Vineyard and Nantucket.According to the National Weather Service, heat index values of 95-100 degrees are possible.Thunderstorms will move into the area by midday Tuesday before moving off the coast late afternoon. The storms could bring downpours and lightning, with a few storms having the potential to turn severe with damaging wind threat forecasters said. On Friday, Boston Mayor Marty Walsh declared a heat emergency for the city, which will last through Tuesday.Walsh said 21 community centers were open to the public as cooling centers from from 9 a.m. to 5 p.m. on Monday and Tuesday. Boston residents were advised to call the centers before visiting to confirm they are not at capacity.The city's two outdoor pools, the Clougherty Pool in Charlestown and the Mirabella Pool in the North End, were also open, but visitors had to register for a swim time slot."I want to remind everyone that COVID-19 is still a threat," Walsh said Friday. "Everyone needs to keep doing their part to avoid large crowds and wear your face covering, unless you're in the water." Spray pads are also open at Boston's playgrounds, but libraries and city-operated indoor pools remain closed due to the pandemic. The National Weather Service is advising people who live in areas under the excessive heat watch to drink plenty of fluids, stay in an air-conditioned room and out of the sun, and to check up on relatives and neighbors.Extra precautions should also be taken with young children and pets, according to the weather service.