* NHC sees it being major hurricane by Sunday in Atlantic
* Northeast of Leeward Islands by then, but after that?
* Second system watched for possible Gulf of Mexico threat (Updates with U.S. oil companies monitoring second system)
By Pascal Fletcher
MIAMI, Aug 30 The U.S. East Coast is mopping up after Hurricane Irene's weekend battering that killed around 40 people and authorities and residents are looking out anxiously over the Atlantic and asking: Is another one coming?
Tropical Storm Katia is jogging west at a brisk 18 miles per hour (30 km per hour) and the U.S. National Hurricane Center says it is expected to become a hurricane by late Wednesday or early Thursday.
But beyond predicting 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," NHC senior hurricane specialist Richard Pasch told Reuters.
Nevertheless, he recommended that the U.S. East Coast and the Caribbean should "keep an eye" on Katia.
At 11 a.m. EDT (1500 GMT), Katia was about 630 miles (1,010 km) west southwest of the southernmost Cape Verde Islands, still far out to the east in the Atlantic.
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 Pasch cautioned such long-range predictions were unreliable and contained 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 NHC's five-day forecast "cone" had an average margin of error of about 250 miles (400 km).
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.
EYES ALSO ON SECOND WEATHER SYSTEM
"The long term fate of Katia is unknown," hurricane expert Jeff Masters of private forecaster Weather Underground 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 percent chance of hitting North Carolina, a 16 percent chance of hitting Canada, an 11 percent chance of hitting Florida, and a 47 percent chance of never hitting land.
Meanwhile, U.S. oil companies were monitoring another weather system in the northwest Caribbean Sea that some forecasters say may develop this week into a tropical storm in the northwest Gulf of Mexico, home to the U.S. offshore oil patch.
The hurricane center gave the system, which was expected to move west across the Yucatan peninsula, a 10 percent chance of becoming a tropical cyclone in the next 48 hours, but added it could develop further when it reaches the western Gulf.
U.S.-regulated Gulf of Mexico areas account for roughly 30 percent of U.S. oil production and 12 percent of natural gas output, the U.S. Bureau of Ocean Energy Management says.
Irene was the first hurricane to form in the busy 2011 Atlantic storm season. It was a Category 1 hurricane on the five-step Saffir-Simpson scale when it swept from North Carolina to New York on Saturday and Sunday. It was the first hurricane to hit the U.S. mainland since Ike pounded Texas in 2008.
But although it subsequently slackened to a tropical storm, its large width, at times more than 500 miles (800 km) across, swirled rains and winds over a huge area, causing the worst flooding in decades in New Jersey and Vermont.
The NHC 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," 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 West 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. (Additional reporting by Erwin Seba and Kristen Hays in Houston; editing by Mohammad Zargham)