Rodney, is DCA mostly widening the gap because lows? The location is at a serious disadvantage for radiational cooling. We had an unusual amount of good radiational cooling nights in Jul-Aug. Sept usually has them every year.
The high temp differential in August and Sept was 3.68 and 4.5 respectively but the low temp differential was 7.98 and 8.27. I have no idea if this spread is historically normal.
DCA also went through several large expansions in the late 90's and early 2000's. More buildings and more planes certainly doesn't help with temps. No matter which way you shake it, the location is almost perfect to be one of the warmest locales in the area (river, UHI on all sides, elevation).
It would be interesting to compare some recent notoriously humid months in the area. Logic would say the gap would narrow.