The National Weather Service issues heat alerts mostly based on Heat Index values. The warm counter-part to Wind Chills, the Heat Index is a measure of what it feels like outside when you consider both temperature and humidity. The NWS will issue heat alerts when the Heat Index is expected to exceed 105-110F for at least 2 consecutive hours.
IMPORTANT: Since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15F. also, strong winds, particularly with very hot, dry air, can be extremely hazardous.