An 830 sq. mi blast ring has a radius of 14.4 mi. (23.2 km). Position that over New York City and you’d have destruction reaching deep into Queens in the east and Staten Island in the South; west to Paterson and Montclair, NJ; and north to Yonkers and New Rochelle, NY. Manhattan, the Bronx and Brooklyn would be swallowed whole. Evacuation in advance of the blast would be a massive challenge, since the array of bridges and tunnels that connect the boroughs are natural choke points. The many months of notice the residents would have before the big day arrived would make things a bit easier, but fleeing from an asteroid is very different from fleeing from other kinds of disasters. People evacuating in advance of, say, a hurricane can usually just load up their cars and go, since even after a superstorm like Katrina, most of them will simply be turning around and coming home. After a Tunguska-like blast, most people would not have any home left at all.
As with a nuclear blast, the devastation would be greatest at the epicenter of the event and fade the farther away you moved, and while there would be no radiation to contend with, the immediate destruction would be pretty much the same. In a 6.5 mi. radius, all that would be left of most buildings would be the foundations, though some sturdier, reinforced structures like stocky old banks might survive. Out to 11 or so miles, multi-story buildings would be skeletonized—their curtain walls stripped away and only their frameworks left standing. Small, individual family homes would be destroyed completely. It would not be until about 20 miles away that most tall buildings would survive—windowless, to be sure—and some single-family dwellings would too. The economic damage—nationally and globally—would be incalculable.