DCA's climo is just awful as you get to Mid-March. The last time there was a 6"+ snowstorm in Mid-March or later was the '93 superstorm. Prior to that was 1942.
IAD has at least scored a couple of other times, in 1964 and 2014.
All of BWI's 6"+ events appear to be 1964 or prior, with the exception of '93.
But I'm sure we'll score in the warmest year on record.