Although Reagan National Airport in Virginia is not part of the US Historical Climate Network (not used for calculating US or global averages), it is often used as the "official" temperature record for DC. It is by the Potomac which can keep temperatures warmer at night. That's not a problem because the river is a constant.
What is a problem is the increasing amount of gravel around the temperature measurement site. This violates the policy that the measurement sites must have natural landscaping (mowed grass in this case). The increased gravel reduces or eliminates radiational cooling on clear calm nights. Radiational cooling does not happen often at National due to the river and other factors, but when there is radiational cooling, National stands out as being a few degrees (F) warmer than nearby northern Virginia sites. Some of those can be viewed on the weatherbug network: http://web.live.weatherbug.com/Broadcaster/2/Home.aspx?zcode=z4798&list=1&lid=BSL although those stations are not quality controlled. Other stations at sea level and close to the water like Pawtuxent and Annapolis generally do not have a problem radiating lower than National Airport.