And if they meet Tuesday morning around New York or New Jersey, as forecasters predict, they could create a big wet mess that settles over the nation's most heavily populated corridor and reaches as far inland as Ohio.
With experts expecting at least $1 billion in damage, the people who will have to clean it up aren't waiting.
Utilities are lining up out-of-state work crews and canceling employees' days off to deal with the power outages. From county disaster chiefs to the federal government, emergency officials are warning the public to be prepared. And President Barack Obama was briefed aboard Air Force One.
"It's looking like a very serious storm that could be historic," said Jeff Masters, meteorology director of the forecasting service Weather Underground. "Mother Nature is not saying `trick-or-treat.' It's just going to give tricks."
National Oceanic and Atmospheric Administration forecaster Jim Cisco, who coined the nickname Frankenstorm, said: "We don't have many modern precedents for what the models are suggesting."
Government forecasters said there is a 90 percent chance -- up from 60 percent two days earlier -- that the East will get pounded starting Sunday and stretching past Halloween on Wednesday. Things are expected to get messier once Sandy, a very late hurricane in what has been a remarkably quiet season, comes ashore, probably in New Jersey.
Coastal areas from Florida to Maine will feel some effects, but the storm is expected to vent the worst of its fury on New Jersey and the New York City area, which could see around 5 inches of rain and gale-force winds close to 40 mph. Eastern Ohio, southwestern Pennsylvania, western Virginia and the Shenandoah Mountains could get snow.
And the storm will take its time leaving. The weather may not start clearing in the mid-Atlantic until the day after Halloween and Nov. 2 in the upper Northeast, Cisco said.
"It's almost a weeklong, five-day, six-day event," he said from a NOAA forecast center in College Park, Md. "It's going to be a widespread, serious storm."
It is likely to hit during a full moon, when tides are near their highest, increasing the risk of coastal flooding. And because many trees still have their leaves, they are more likely to topple in the event of wind and snow, meaning there could be widespread power outages lasting to Election Day.
Eastern states that saw blackouts that lasted for days after last year's freak Halloween snowstorm and Hurricane Irene in late August 2011 are already pressuring power companies to be more ready this time.
Asked if he expected utilities to be more prepared, Massachusetts Gov. Deval Patrick responded: "They'd better be."
Jersey Central Power & Light, which was criticized for its response to Irene, notified employees to be ready for extended shifts. In Pennsylvania, PPL Corp. spokesman Michael Wood said, "We're in a much better place this year."
Some have compared the tempest to the so-called Perfect Storm that struck off the coast of New England in 1991, but that one didn't hit as populated an area. Nor is this one like last year's Halloween storm, which was merely an early snowfall.
"The Perfect Storm only did $200 million of damage and I'm thinking a billion," Masters said. "Yeah, it will be worse."
In the Caribbean, Sandy killed at least two people in Haiti before the 18th named storm of the Atlantic hurricane season passed over Cuba, ripping the roofs off homes and damaging fragile coffee and tomato crops.
Norje Pupo, a 66-year-old retiree in Holguin, was helping his son clean up early Thursday after an enormous tree toppled over in his garden.
"The hurricane really hit us hard," he said. "As you can see, we were very affected. The houses are not poorly made here, but some may have been damaged."