Forecasters cast a wary eye Tuesday on Tropical Storm Isaac, which was moving west in the Atlantic Ocean and poses a potential threat to Florida during next week's Republican National Convention in Tampa, which culminates in the nomination of Mitt Romney for president and Rep. Paul Ryan for vice president.

It's much too early to say with any certainty whether it will gain hurricane strength or head straight for Tampa, on Florida's west coast. But it's the type of weather that convention organizers knew was a possibility during the peak of hurricane season _ and they have backup plans in place in a worst-case scenario.

It's been 90 years since a major hurricane made a direct hit on Tampa. The last to strike Florida's west coast was Hurricane Charley, a Category 4 packing 150 mph (241 kph) winds. The Aug. 13, 2004, storm was small yet powerful _ and was initially forecast to strike the Tampa Bay area before it turned and slammed Port Charlotte, about 100 miles (160 kilometers) south.

National Hurricane Center computer models predicted Isaac would become a hurricane over the next few days, meaning maximum winds must be at least 74 mph (120 kph). Some models had the storm striking Florida, including the Tampa Bay area, after moving across Cuba or the Bahamas as early as Sunday morning.

Jeff Masters, director of meteorology at Weatherunderground.com, said long-range storm track predictions five days in advance are notoriously inaccurate, often off an average of 260 miles (418 kilometers). But Masters said the climate situation has improved chances that Florida could be in the system's sights during the Republican event that runs Monday through Thursday.

"It would take a perfect storm of a scenario where a bunch of factors all conspire together," Masters said. "But we definitely have to watch this one."

At 11 p.m. Tuesday, Isaac had maximum sustained winds near 40 mph (64 kph) but was expected to strengthen. The storm was about 390 miles (628 kilometers) east of Guadeloupe and was moving west near 18 mph (29 kph). A tropical storm warning had been issued for Puerto Rico and more than a dozen Caribbean islands. A hurricane hunter plane confirmed the storm had strengthened.

Republican and state officials have contingency plans in place if the storm makes its way to Tampa, including an evacuation in a worst-case scenario. About 70,000 delegates, party officials, journalists, protesters and others are expected for the convention.

A four-day mock hurricane drill was held in May featuring a pretend major storm striking the Tampa area during the second day of the convention. Under that scenario, planners canceled. A major hurricane is a Category 3 or above with winds at least 111 mph (170 kph) and devastating damage can occur.

Forecasters say that fortunately for Tampa, most Gulf storms emerge earlier or later in the hurricane season, which runs from June 1 to Nov. 30.

Florida, historically the nation's top target for tropical systems, has not been hit by a major hurricane since Wilma in 2005. The new storm's potential threat comes just as South Floridians are marking the 20th anniversary of Hurricane Andrew, a Category 5 monster that resulted in 26 direct deaths and caused some $26.5 billion in damage when it came ashore south of Miami on Aug. 24, 1992.

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