A heat advisory will take effect Thursday morning in Paterson and surrounding areas, with heat index values expected to reach 104 degrees, the National Weather Service said. The advisory covers Passaic, Bergen, Hudson, Essex and Union counties from 11 a.m. Thursday until 8 p.m. Friday. High heat and humidity could cause heat-related illnesses, particularly among seniors, people with chronic health problems and residents without air conditioning. Officials urged residents to remain in air-conditioned spaces, check on vulnerable relatives and neighbors and take frequent breaks when working outdoors. Thunderstorms Friday afternoon and evening could also produce locally heavy rain, isolated flash flooding and strong winds.