Midnight Update: Preliminary data indicate that Washington bottomed out at 28.63"/969.5 mb, and Baltimore hit 28.49"/964.9 mb. This would break the Baltimore record by 1 mb.
The National Weather Service reports that the post-tropical phase of Hurricane Sandy could threaten the modern record for low pressure readings at Washington and Baltimore:
PRELIMINARY INFORMATION ON LOWEST PRESSURE RECORDED AT WASHINGTON DC AND BALTIMORE MD...BOTH MEASURED IN THE MARCH 1993 SUPER STORM... WASH DC... 28.54 INCHES HG 966.5 MB MEASURED AT 23Z ON 13 MARCH 1993 BALT MD... 28.52 INCHES HR 965.9 MB MEASURED AT 00Z ON 14 MARCH 1993 THESE VALUES WERE ARRIVED AT BY SEARCHING AN INTERNAL DATABASE OF OBSERVATIONS MAINTAINED UNOFFICIALLY HERE AT THE WEATHER OFFICE. THE DATABASE GOES BACK TO 1929.