Forecasters warned Friday that the impact of a so-called "Frankenstorm" generated by Hurricane Sandy could exceed that of Irene, which killed 47 people on the US eastern seaboard in 2011.

Sandy is moving northwards from the Bahamas parallel to the US coast, but weather models predict that on Tuesday it will collide with cold fronts from the north, super-charging the weakening storm while drawing it west onto land.

"Compared to Irene, we expect much broader surge impacts. Same thing with the wind," said James Franklin, branch chief of the Miami-based National Hurricane Center during a telephone news conference.

In August 2011, Irene battered the US mid-Atlantic states with heavy rains and winds, leaving entire towns underwater and claiming dozens of lives.

Areas in North Carolina, Virginia, New Jersey and Vermont were particularly hard hit. The cost of the damage was estimated at more than $10 billion.

Franklin said the difference between a typical tropical cyclone and what Sandy is likely to become once it merges with the wintry storm is that there will be "a very, very large area with a more uniform wind field."

"In Irene, large portions didn't have thunderstorms, so winds didn't come down. That's not going to happen here."

Louis Uccellini, director of the National Centers for Environmental Prediction, warned Sandy could also be more powerful than the so-called "Perfect Storm" in October 1991 that killed 13 people.

"To some extent this has some similarities surrounding the 'Perfect Storm,'" he said, adding that Sandy involves "a stronger tropical system up front."

And he added, "in the 'Perfect Storm,' the snow event stayed separate," while Sandy is predicted to merge with the cold weather systems.