Dulles is definitely starting to develop some sort of UHI as that area continues to grow.
The freeze warning was def a bit excessive for the more urban locations in and around DC - DCA only got down to 41 which was also my low in Tenleytown.
You hear the same thing in the NY area, with people way out in the burbs complaining that KNYC is too warm - it's the station for New York City, not your town.
I am about 5-6 miles away from DCA and my low was 37 (so 3F cooler than DCA - which is about right). DCA is meant to represent the urban area of DC + Arlington/Alexandria.
79/61 here - low of 66.7F - elevation helps little lol. We are close enough to more built up areas around Wisconsin Ave that the heat island persists here pretty strongly.