@jwilson
The summer creep into September is indeed a real change occurring. As I posted in the Extreme Run thread last fall, every September in the 2010's at BWI finished with above average temperatures except 2013, which was only -0.1 below. The new 1990-2020 norms, when released, will definitely increase the September temperature.
March has varied wildly over the last decade and it's difficult to draw any conclusions. For BWI, March should return to being snowier on average than December under the 1990-2020 norms, but this is just a return to how it's always been historically.