A/Cs and fans are going to be working overtime in the next few days.

A heat wave is expected to start on Wednesday, with it likely feeling north of 90 degrees each day through Saturday.

Heat index values could soar above 100 degrees, even hitting 104 at the peak of the heat wave.

As a result, the National Weather Service has issued a “Heat Advisory” for Massachusetts from 11 a.m. on Thursday to 8 p.m. on Friday. The worst of the conditions are expected to be from noon to 6 p.m. on both Thursday and Friday.

Hot temps and high humidity may spark heat illnesses, the meteorologists warn.

“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 alert. “Young children and pets should never be left unattended in vehicles under any circumstances.”

Due to the hot weather in the forecast, Boston Mayor Michelle Wu has declared a heat emergency for the city for Thursday and Friday.

Cooling centers will be open at 15 BCYF community centers on Thursday and Friday, from 9 a.m. to 5 p.m. each day.

“Call @BOS311 with any questions about available, non-emergency City services,” Wu tweeted. “Learn more about heat safety at boston.gov/heat.”

The National Weather Service warns that it will feel downright oppressive.

“Take extra precautions if you work or spend time outside,” the National Weather Service wrote. “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.

“To reduce risk during outdoor work, the Occupational Safety and Health Administration recommends scheduling frequent rest breaks in shaded or air conditioned environments,” NWS added. “Anyone overcome by heat should be moved to a cool and shaded location. Heat stroke is an emergency! Call 9 1 1.”