The five day forecast has all that you need to know. When you say that upper 80's is a break from the heat, you know that it is going to get really hot. Monday through Thursday will be very very hot. Temperatures should be in the mid to upper 90's on Monday and Tuesday. Now it looks like the hottest day will be Wednesday when it looks like 100 will be a common number for the day's high temperature. It should be absolutely brutal with heat indices up over 110 degrees.
More on this dangerous heat wave later.