Cooling Sites Opened During Heat Wave This Week Take precautions in hot weather TRENTON—The National Weather Service has issued an Excessive Heat Warning for Mercer County and the surrounding area that will remain in effect until 8 p.m. Tuesday, June 29, and a Heat Advisory that will be in effect from 8 p.m. Tuesday until 8 p.m. Wednesday. County Executive Brian M. Hughes reminds residents that many cooling sites will be open. Temperatures through Wednesday are expected to reach the middle to upper 90s with a heat index (a measure of the combination of heat and humidity) of up to 105 degrees on Tuesday and 102 degrees on Wednesday. Children, older adults, people with disabilities and pets are most at risk during excessive temperatures.