Via: http://blogs.wsj.com/metropolis/2012/10/30/weather-journal-what-made-sandys-flooding-so-bad/
Over its lifespan, Sandy set several important records: just before landfall it was the most intense hurricane (as measured by its minimum central pressure of 940mb) ever recorded north of North Carolina (even including those that never made landfall), its potential wave/storm surge destructiveness (peaking at 5.8 on a 6.0 scale) was ranked by NOAA to be higher than any other hurricane in the Atlantic or Gulf of Mexico since at least 1969, and it was also at one point the largest hurricane ever recorded (since 1851) as measured by the diameter of its gale force winds (945mi). Though National Weather Service forecasts for Sandy appeared to have been very good, Sandy’s impacts — at least relating to storm surge — came in at the top end of anticipated ranges. It’s quite likely that Sandy’s mix of ingredients maximized damage for New York City in a way that no other hurricane has anywhere in the United States for at least the last 150 years.