Government forecasters on Thursday upped the odds of a major weather mess, now saying there’s a 90 percent chance that the East will get steady gale-force winds, heavy rain, flooding and maybe snow starting Sunday and stretching past Halloween on Wednesday.

Meteorologists say it is likely to cause $1 billion in damages.

The storm is a combination of Hurricane Sandy, now in the Caribbean, an early winter storm in the West, and a blast of arctic air from the North. They’re predicted to collide and park over the country’s most populous coastal corridor and reach as far inland as Ohio.

The hurricane part of the storm is likely to come ashore somewhere in New Jersey on Tuesday morning, said National Oceanic and Atmospheric Administration forecaster Jim Cisco. But this is a storm that will affect a far wider area, so people all along the East have to be wary, Cisco said.

Coastal areas from Florida to Maine will feel some effects, mostly from the hurricane part, he said, and the other parts of the storm will reach inland from North Carolina northward.

Once the hurricane part of the storm hits, “it will get broader. It won’t be as intense, but its effects will be spread over a very large area,” the National Hurricane Center’s chief hurricane specialist, James Franklin, said Thursday.

One of the more messy aspects of the expected storm is that it just won’t leave. The worst of it should peak early Tuesday, but it will stretch into midweek, forecasters say. Weather may start clearing in the mid-Atlantic the day after Halloween and Nov. 2 in the Northeast, Cisco said.

“It’s almost a weeklong, five-day, six-day event,” Cisco said Thursday from NOAA’s northern storm forecast center in College Park, Md. “It’s going to be a widespread serious storm.”

With every hour, meteorologists are getting more confident that this storm is going to be bad and they’re able to focus their forecasts more.

The New York area could see around 5 inches of rain during the storm, while there could be snow southwest of where it comes inland, Cisco said. That could mean snow in eastern Ohio, southwestern Pennsylvania, western Virginia, and the Shenandoah Mountains, he said.

Both private and federal meteorologists are calling this a storm that will likely go down in the history books.

“We don’t have many modern precedents for what the models are suggesting,” Cisco said.

It is likely to hit during a full moon when tides are near their highest, increasing coastal flooding potential, NOAA forecasts warn. And with some trees still leafy and the potential for snow, power outages could last to Election Day, some meteorologists fear.

Some have compared it to the so-called Perfect Storm that struck off the coast of New England in 1991, but Cisco said that one didn’t hit as populated an area and is not comparable to what the East Coast may be facing. Nor is it like last year’s Halloween storm, which was merely an early snowstorm in the Northeast.

“The Perfect Storm only did $200 million of damage and I’m thinking a billion,” said Jeff Masters, meteorology director of the private service Weather Underground. “Yeah, it will be worse.”

But this is several days in advance, when weather forecasts are usually far less accurate. The National Hurricane Center only predicts five days in advance, and each long-range forecast moves Sandy’s track closer to the coast early next week. The latest has the storm just off central New Jersey’s shore at 8 a.m. on Tuesday.

As forecasts became more focused Thursday, the chance of the storm bypassing much of the coast and coming ashore in Maine faded, Cisco said.

The hurricane center’s Franklin called it “a big mess for an awful lot of people in the early part of next week.”