Storm Hanna, off Florida, keeps experts guessing

MIAMI (Reuters) - While powerful Hurricane Gustav bore down on the U.S. Gulf Coast on Sunday, Tropical Storm Hanna swirled east of Florida, embedded in a complicated climatic environment that made it impossible to forecast its destination and likely strength.

The eighth tropical storm of the 2008 Atlantic hurricane season could just as easily end up over Cuba, bring heavy rainfall to citrus country in central Florida or drift northward toward South Carolina. It was not possible to say if the storm might eventually end up in the U.S. oil patch in the Gulf of Mexico, hurricane experts said.

"Unfortunately there is still considerable uncertainty with the forecast," said Jamie Rhome, a hurricane specialist at the U.S. National Hurricane Center in Miami. "It's impossible to say that this system is going to do this or that."

The cyclone was tangled up in a middle to upper level low that was making it difficult for Hanna to develop, and was likely to slow down in two days when it came across conditions of weak steering current that could make it meander.

Another trough would then swoop over the tropical storm, bringing with it considerable uncertainty as to the likely wind shear as Hanna drifted near the Bahamas. Wind shear -- the difference in wind speed at different levels of the atmosphere -- can tear storms apart.

"At the end of the forecast track the wind shear could let up a bit," Rhome said.

None of the computer models used to predict storm tracks actually took Hanna into the southeastern United States at this point, Rhome said.

Some oil analysts reported on Friday that one of the myriad computer models available to forecasters had indicated that Hanna could eventually make landfall on the U.S. Gulf Coast near New Orleans where Hurricane Gustav was expected to come ashore on Monday as a dangerous storm.

Those reports triggered concerns in energy markets of a potential one-two punch by Gustav and Hanna on some of the 4,000 Gulf of Mexico offshore platforms that provide a quarter of U.S. crude oil and 15 percent of its natural gas.

Hurricanes Katrina and Rita destroyed more than 100 oil rigs in 2005 when they roared through, causing oil prices to soar to then record highs. Katrina went on to swamp New Orleans, kill 1,500 people on the U.S. Gulf Coast and cause $80 billion in damages.

Rhome said it was folly to highlight a single computer model, especially so far out. "It's a mistake, and often a grave one, to focus on a single model," he said.

The accuracy of hurricane forecasting has come a long way since the days when entire fleets of Spanish galleons sank in unexpected storms as they carried South American gold and treasure back to Europe.

But even with the start of "hurricane hunter" flights in 1944 and the advent of satellite imagery in the 1960s, long-range forecasts are prone to enormous margins of error.

The National Hurricane Center estimates the average error in its track forecasts is near 260 miles by day four and 345 miles by day five. The hurricane center does not project a storm's track beyond day five.

Intensity forecasts are even more difficult. The hurricane center calculates that the error in its forecasts for a storm's top sustained winds averages 23 miles per hour (37 km per hour) per day.

The last official forecast for Hanna takes it in five days to minimal Category 1 hurricane strength with 80-mile-per-hour (130 km per hour) winds by next Friday.

It might then be somewhere off central Florida. But its potential position at that point also encompasses the southern Bahamas, eastern Cuba, south Florida and South Carolina.

(Editing by Tom Brown)