Agree with all that, but Sandy had no back side when it hit land. Was having trouble wrapping up its west end which was being stripped away and taken north by the cold front. Then when it sped up just before landfall, it could not pull anything from the west side at all. There was no moisture to speak of on its SE side when landing. My guess is when the cold low pressure joined up with Sandy, it pulled Sandy in and sped up its forward motion basically ripping the Hurricane/Tropical Storm apart. So the joining of the storms actually made Sandy less powerful, which was the opposite of what they predicted. Forecasters forgot that cold air is more dense then warm air.
Having stated that, it was obvious that north and north east of landfall was going to experience record flooding. That was always a given, if it took the path that it took. Speeding up its forward motion, which was not predicted, actually lessened the severity of the flooding. IE - The storm had less time to apply its pressure and wind forces to water motion.
That was dry air wrapping in. The cold front was still out in central PA.