It seems like the NAM/HRRR are hitting upon three different rounds. Effectively, they are taking the "complex" near Pittsburgh and washing that out, but developing some early storms mostly south of DC. The 2nd round is after sundown, and that appears to be our best shot for some thunder. Then the front comes through in the early am hours.
The NAM is more keen on the first round, while the HRRR generally likes the 2nd round better (nice cell coming down the Potomac on the 14z).