From Accuweather:
The highest temperatures so far this year will combine with increasing humidity and intense summer sunshine to create dangerous, life-threatening conditions for roughly 170 million people, AccuWeather meteorologists warn. The heat wave will be the result of a heat dome shifting out of the West and parking over the central and eastern United States.
Summer officially arrived in the Northern Hemisphere Friday night. The heat wave will come into full bloom just after the summer solstice, during some of the longest days of the year and when the sun is most intense.