The closest USHCN station to Washington DC is at Lincoln, Virginia. Between 1900 and 2010, there were eighty-eight March days over 80F. The vast majority of those days were below 350 ppm CO2.
Washington has seen three March days over 90F. Those occurred in 1907 and 1910.
Jeff Masters has been trying to link the warm weather in Washington this March to atmospheric CO2, but it is quite clear that the number of warm March days has decreased as atmospheric CO2 has increased.