Bryan Norcross' Official Blog : The Sandy Paradox | Weather Underground:
The strong evidence we have that a significant, maybe historic, storm is going to hit the east coast is that EVERY reliable computer forecast model now says it's going to happen. The only way we can forecast the weather four or five days days from now is with the aid of these super-complex computer programs run on supercomputers. The two best, the European and the U.S. GFS (Global Forecast System) run by NOAA, are now in reasonable agreement that there IS going to be an extraordinarily unusual confluence of events that results in a massive storm.
The upper-air steering pattern that is part of the puzzle is not all that unheard of. It happens when the atmosphere gets blocked over the Atlantic and the flow over the U.S. doubles back on itself. Sometimes big winter storms are involved.
The freak part is that a hurricane happens to be in the right place in the world to get sucked into this doubled-back channel of air and pulled inland from the coast.
And the double-freak part is that the upper level wind, instead of weakening the storm and simply absorbing the moisture - which would be annoying enough - is merging with the tropical system to create a monstrous hybrid vortex. A combination of a hurricane and a nor'easter.
At least that's what the models are saying. And since all of the independent models are saying something similar, we have to believe them and be ready.