The hot and humid weather is expected to persist into Friday, with the peak of the Massachusetts “heat advisory” Thursday afternoon as heat index values approach 100 degrees and as Boston faces a “heat emergency” on the first day of school.

National Weather Service meteorologists were warning residents about the risk of heat exhaustion and heat illnesses amid this early September blast of brutally hot weather. The “heat advisory” remains in effect through Friday evening.

“Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors,” the National Weather Service wrote in its advisory. “Young children and pets should never be left unattended in vehicles under any circumstances.

“Take extra precautions if you work or spend time outside,” NWS added. “When possible reschedule strenuous activities to early morning or evening. Know the signs and symptoms of heat exhaustion and heat stroke. Wear lightweight and loose fitting clothing when possible.”

In Boston, Mayor Michelle Wu declared a “heat emergency” through Friday. Cooling centers will be open at 15 Boston Centers for Youth and Families (BCYF) community centers from 9 a.m. to 5 p.m.

Also, 64 splash pads will be open at parks and playgrounds throughout the city. Boston Public Library locations are also available for residents to seek relief from the heat.

With the school year for Boston Public Schools beginning on Thursday, BPS was encouraging students and their families to prepare for hot weather by staying well hydrated and dressing appropriately. Most BPS schools have air conditioning. The city was supplying water and fans to those schools that need it.

“The impacts of climate change are more palpable than ever, with extreme heat posing risk to our communities,” Wu said in a statement.

“Although extreme heat affects Bostonians of all ages, with the new school year starting, our Boston Public Schools staff will be following protocols to ensure our kids have an enjoyable, safe first week back at school,” the mayor added. “I’m grateful to our City employees who are working tirelessly to support residents, and ask residents to take precautions.”

BPS is following the MIAA guidelines for weather and will be checking in with coaches and BPS athletics regularly to ensure that students are safe, the city said.

During last summer’s heat wave, Boston EMS experienced a 15% to 20% rise in daily calls to 9-1-1.