DCA was really unimpressive during last years' 3-5"ers. I think folks will have trouble coming in and out of DCA on Monday and Tuesday... with the way plane delays and cancellations roll over it could be tricky. I got my family to push our flight out of DCA from Tuesday morning -> Sunday morning instead, as much as it pained the weenie in me.
The good news about DCA (or IAD, if that's your destination) is that the metro hooks right up to the airports. It operates close to normal in up to 4-6", and they'll be clearing it throughout the day. If you land, you can get to downtown proper and hopefully close to your hotel.