I’d compare DCA to BWI, SBY, and EZF though. The lows at DCA are often warmer than all three.
BWI is closer to DCA at 59 for the same reason, while SBY and EZF are both in the 56-57 range.
Like raindance says, the 60F line is probably down closer to richmond. It’ll take a while for it to creep up, even if locally it gets there first at the river and bay areas.