An official heat wave was declared Saturday in Eastern Massachusetts as the area entered a third day of sweltering temperatures in the 90s. The scorching day was capped with lightning storms bringing hail as large as nickels in some northeastern communities, according to the National Weather Service. Cooling centers were opened across Boston Saturday as Mayor Martin J. Walsh declared a heat emergency Friday, effective for the next several days. A list of cooling centers is available on the City of Boston website. A hailstorm formed in Burlington late Saturday afternoon and swept east through Woburn and Melrose, and into Revere, moving out over the ocean around 6:15 p.m., said Rebecca Gould, a meteorologist with the National Weather Service in Taunton. The Weather Service issued a marine warning for Boston Harbor, Massachusetts Bay, and Ipswich Bay, and a second warning for the Merrimack River area up to Cape Elizabeth, Maine, advising boaters to seek harbor immediately due to thunderstorms with high winds and lightning. Meanwhile, a line of storms forming in New Hampshire was expected to affect much of northern Massachusetts, and perhaps more of the state as well, said Gould. The storms were expected tto be finished by midnight, said Gould. The mayor's heat advisory asks residents to use sunscreen; never leave children or pets alone in cars, even for short periods; drink lots of fluids; and because of dehydration concerns, call 911 if they observe a homeless person in distress. Some relief is expected Sunday, which should be "gorgeous," according to Alan Dunham, meteorologist with the Weather Service in Taunton. Sunday is expected to have temperatures in the upper 80s, perhaps cracking 90 in a few spots, but less humidity than Saturday. The start of the work week will likely be marked by a return to heat and humidity, as well as possible showers and thunderstorms, he said.