MIAMI (Reuters)—The U.S. East Coast is mopping up after Hurricane Irene's weekend battering that killed about 40 people and authorities and residents are looking out anxiously over the Atlantic and asking: Is another one coming?

Tropical Storm Katia was jogging west at a brisk 18 mph and the U.S. National Hurricane Center said it is expected to become a hurricane by late Wednesday or early Thursday.

But beyond predicting that Katia will be a major hurricane northeast of the Caribbean's northern Leeward Islands by Sunday, the Miami-based center says it is not possible now to predict its path with certainty, or say whether it will threaten the U.S. East Coast.

“It's still well out to sea. A lot of things can happen...We don't show it affecting any land areas for five days. Beyond that is merely speculation,” Richard Pasch, senior hurricane specialist at the hurricane center, told Reuters.

Nevertheless, he recommended that the U.S. East Coast and the Caribbean “keep an eye” on Katia.

Some private forecasters were citing long-range models beyond five days, some of which show Katia swinging over Bermuda toward Canada and avoiding the U.S. coast. But Mr. Pasch cautioned that such long-range predictions were unreliable and contain errors of “hundreds of miles” in the envisaged track.

“You can look at what the long range forecasts did with Irene, taking it across Miami, which of course didn't happen,” he said, stressing that even the hurricane center's five-day forecast “cone” had an average margin of error of about 250 miles.

Mr. Pasch said Katia's likely track in about a week's time will depend on shifting weather patterns over the Atlantic and the U.S. coast—troughs and ridges that will steer the gyrating storm in one direction or another.

“The long-term fate of Katia is unknown,” hurricane expert Jeff Masters of private forecaster Weather Underground Inc. wrote in his blog.

He cited a historical probability chart drawn up by Robert Hart of Florida State University indicating that tropical storms in Katia's current position had a 19% chance of hitting North Carolina, a 16% chance of hitting Canada, an 11% chance of hitting Florida and a 47% chance of never hitting land.

Irene, the first hurricane to form in the busy storm-filled 2011 Atlantic season, was a Category 1 storm on the five-step Saffir-Simpson scale when it swept from North Carolina to New York Saturday and Sunday. It was the first hurricane to hit the U.S. mainland since Ike pounded Texas in 2008.

Although it subsequently slackened to a tropical storm, its large width, at times more than 500 miles across, swirled rain and wind over a huge area, causing the worst flooding in decades in New Jersey and Vermont.

The hurricane center sees Katia becoming a major Category 3 hurricane by Sunday morning on its track over warm ocean waters, which act as boosters for a hurricane's power.

“Bear in mind, it's the end of August, the beginning of September. It's the peak of the hurricane season,” Mr. Pasch said.

He said Katia was in the main development region—the broad Atlantic bowling alley down which rotating storms roll off the coast of western Africa.

Forecasters have predicted a very active 2011 Atlantic season with between eight and 10 hurricanes, above the long-term June-to-November average of six to seven hurricanes.

Mr. Pasch said some computer models showed the possibility of storm development in the next several days in the Gulf of Mexico, where major U.S. oil and gas facilities are located. But he cautioned this too was speculative because of the uncertainty of forecasts looking ahead beyond three days.