From DDOT: "The District Department of Transportation (DDOT) will close N Street SE between 11th and 12th Street SE on Thursday, February 24. Traffic that would have used N Street SE will be permanently routed to the intersection of 12th Street and M Street SE.[.]" There are stoplights now at 12th and M; I saw them on Saturday night, and thought they were pretty new (and indeed they are!).
This is all part of the 11th Street Bridges reconstruction; see my project page
for more details. This closure would seem to mainly effect Navy Yard employees who come northward across the bridges and have been able to turn left at the bottom of the ramp on N to head toward the Navy Yard's gates, and who will now have to go up to M, turn left, and then turn left on 11th.
In looking at the plans
, and parsing the press release wording, I think the "permanently" refers to drivers coming off the 11th Street Bridge inbound having to go to 12th and M. It looks like N will still be in existence, and in fact will be built through to 12th. I can't quite tell from the various graphics how traffic will flow on the new N. But basically, the way that N is used now (as a turn off the exit ramp) won't exist anymore.