WASHINGTON — Hurricane Sandy seems straight out of the latest 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 stretch about 940 miles — greater than the distance from New York to Atlanta — said Jeff Masters, co-founder of the website WeatherUnderground.com and a former flight meteorologist "hurricane hunter" with the National Oceanic and Atmospheric Administration.

Sandy is the largest such storm to make landfall on the Atlantic seaboard since the 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 Center in Miami. Although its path is "not 100 percent unprecedented," it is unusual to have a storm of such size and strength coming inland so late in the year, Knabb said during a telephone news conference 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.

The true culprits are a couple of other atmospheric conditions — a high-pressure system centered west of Greenland and a huge wave of cold air moving eastward across the United States.

If not for those two enormous bundles of air, which pushed the hurricane toward land and beefed up its intensity, Sandy likely would have stayed offshore, causing relatively minimal damage as it wobbled past the mid-Atlantic region and finally died far out to sea, meteorologists say.

That shift toward the northwest has put Sandy on course to mesh with a cold front that began on the West Coast last week and has marched across the country, creating what Knabb and others call an unprecedented weather threat. The force of the eastward-moving front has slowed Sandy, Garrett said, and is changing the hurricane into a nor'easter.

That wave of cold air moving eastward across the United States "actually captured Sandy," also drawing it toward land, said meteorologist Wes Junker, a contributor to The Washington Post.

Sandy continues to merge with what was once two cold-weather systems already dumping snow in West Virginia, forming what the hurricane center calls post-tropical and others call Frankenstorm or Perfect Storm 2. Whatever name it visits as, it isn't leaving the Eastern U.S. anytime soon.

The storm began to look less and less like a hurricane over the colder mid-Atlantic, said meteorologist Brian McNoldy, another Post contributor. He said it lost the characteristics of a tropical storm, such as its warm core and its symmetrical cloud field, eye wall and eye.

The National Hurricane Center officially pronounced the storm "post-tropical" Monday evening, as the center of Sandy perched 20 miles south of Atlantic City, knocking at the coast's door. The change is part of a transition into a more diffuse storm that is bigger and sloppier, even as its force weakened.

Its massive girth will extend as far west as Chicago, where the National Weather Service already has issued high wind warnings and a lakeshore flood warning for Tuesday and Wednesday. Water may pile up on the south shore of Lake Michigan, Louis Uccellini, director of environmental prediction for the National Oceanic and Atmospheric Administration, said Monday.

One reason Sandy may have stayed tropical so long was the unusually warm waters of the Gulf Stream, a river of warm water that flows from the Caribbean up into the North Atlantic. It was 5 to 9 degrees warmer than normal for this time of year. And as a tropical system, Sandy fed on those warm waters and kept traveling north, Masters said. That could account for the last-minute boost in speed, too, that Sandy had as it neared shore, accelerating to 28 mph.

But once Sandy speeded up and reached land, it ran into a blocking ridge of air centered over Greenland. That won't let the hybrid storm go too far too fast. It's sort of like accelerating on a road to get stuck in a traffic jam. The weakened storm may still be over Maine on Saturday, Masters said.

Sandy could also produce record-setting storm surges. The swirling movement of its winds creates a big bulge of water that typically moves to the storm's center.

There, the low pressure in the atmosphere acts like a straw and pulls the water levels even higher, Masters said. If a storm is farther out to sea, that bulge would eventually sink. But because Sandy has moved to shallower inland waters, that bulge has nowhere to go but toward shore, he said.

The slowness of the storm means it could sit over the East Coast through several high-tide cycles.

Scientists are predicting 10- to 11-foot storm surges, including high-tide conditions, in such places as New York and New Jersey. Monday night's full moon could add another foot to the surge, Knabb said.

And many inlets and bays in the mid-Atlantic are funnel-shaped, which means water can surge in easily, but getting out is a challenge.

Knabb said he didn't expect water levels to return to normal until Wednesday.

The storm lost its status as a hurricane because it no longer has a warm core center nor the convection — the upward air movement in the eye — that traditional hurricanes have, but it is still as dangerous as it was when it was considered a hurricane, according to National Hurricane Center spokesman Dennis Feltgen.

It tipped into the post-tropical category because it has become "devoid of thunderstorms near the center," said Ed Rappaport, deputy director of the NHC.

That should mean a storm that is larger in physical dimensions affecting more people, but with weaker peak winds, meteorologists say.

That's because of the difference in barometric pressure between the hybrid storm and the calmer weather to the west, Uccellini said.

The transition from tropical to wintry won't affect the massive and life-threatening storm surge expected along the eastern coast, Masters said.

But Sandy hasn't been easy to label, said Chris Landsea, the hurricane center's science officer.

Meteorologists had expected Sandy to lose its tropical characteristics before Monday afternoon, but it approached the shoreline with the name hurricane attached even if some parts of didn't act that way.

Compiled from The Associated Press, The Washington Post and Tribune Washington bureau