The Weather Channel posted an interesting map earlier this month showing how the date of the peak heat in the summer has changed over time. Parts of the US are peaking later in the summer, but in most of the Southeast, the average date of the highest temperature is actually coming several days earlier now than in 1951-1980.
So far it is not clear what is causing the difference, but it may be related to changes in the temperature of the Gulf of Mexico or to changes in cloudiness over time. You can read more about it at https://weather.com/news/climate/news/peak-summer-temperature-changes?cm_ven=T_WX_BD_82516_1.
