JONESBORO, Ark. (KAIT) - Summer started June 21, but temperatures have been mostly below average. A prolonged heat wave is set to arrive Monday, June 29 and last through at least Thursday, July 2. Heat index values are set to soar over 100°, with many days seeing peaks between 105°-110°. According to the National Weather Service, heat is the top killer among all weather related fatalities.