Confluence of events made Sandy such a big storm

Sandy seems like a storm created for a Hollywood apocalyptic blockbuster. But a confluence of environmental and topographical characteristics helps explain its vast size, slow progress, storm surge and multiple methods of wreaking havoc on the coast and deep inland, scientists say.

Sandy began as a big storm when it came together in the Caribbean, said Katie Garrett, a meteorologist with the National Weather Service. It grew even larger as it moved north and into the mid-Atlantic, fed by unseasonably warm waters.

The tropical-storm winds that constitute Sandy at one stage stretched 1,500 kilometres, said Jeff Masters, co-founder of the website WeatherUnderground. com and a former flight meteorologist "hurricane hunter" with the National Oceanic and Atmospheric Administration (NOAA). Sandy is the largest such storm to make landfall on the Atlantic seaboard since the federal government began keeping records in 1988, he said.

Sandy's course, from the southeast to the northwest, is not typical, according to Rick Knabb, director of NOAA's National Hurricane Centre in Miami. Although its path is "not 100 per cent unprecedented", it is unusual to have a storm of such size and strength coming inland so late in the year, Knabb said on Monday.

Even when Atlantic storms come inland, they usually head out to sea. Some very powerful storms in recent years did not make landfall at all, kept away by onshore weather patterns. But Sandy was pushed ashore into the mid-Atlantic states and New England because a high-pressure system near Greenland blocked its progress eastward.