MIAMI, Aug. 24 (UPI) -- A tropical weather system with heavy rains and powerful winds has an 60 percent chance of becoming Tropical Storm Hermine in the next two days, potentially threatening millions of residents in South Florida by Sunday.
Though Tropical Storm Gaston looks more impressive as a storm, it's headed into open water and is no threat to land. Invest 99L northeast of the Caribbean, however, has the potential to be the first hurricane to strike Florida in a decade.
Currently a tropical wave composed of disorganized thunderstorms, 99L could be in South Florida late Sunday or early Monday, potentially growing into a hurricane over the Bahamas. The impact for Florida depends on what happens in the next 48 hours.
"Although upper-level winds are expected to be only marginally conducive for additional development, this system could become a tropical storm or tropical depression at any time during the next couple of days," the National Hurricane Center said in a special Tropical Weather Outlook Wednesday.
Meteorologists said they have already measured tropical storm force winds in 99L, near the northernmost Leeward Islands. At 500 feet, it has generated sustained winds of 50 to 55 mph.
The NHC said strong winds, heavy rains, possible flash floods and mudslides are expected in part of the Leeward Islands, Puerto Rico, Hispaniola and the southeastern and central Bahamas.
99L appears to have two fates, depending on how far north it tracks. If it passes north of Puerto Rico and Hispaniola, as is expected, it will be in much better shape over the Bahamas, where conditions are ripe for strengthening before it approaches South Florida. If the storm's center crosses over mountains on those large islands, it could weaken or dissipate.
No hurricane has made landfall in Florida since Hurricane Wilma a decade ago. Forecasters and disaster preparedness officials in the state have worried that residents have become complacent.
"This storm has the potential to be trouble," weather expert and former NOAA Hurricane Hunter Jeff Masters told The Palm Beach Post. "It's one of the more significant threats to Florida in the past four years."
This time of year is the peak for the Atlantic hurricane season, which runs from June to November.
Gaston was swirling about 1,000 miles west of the Cabo Verde Islands -- between South America and Africa in the mid-Atlantic -- early Wednesday with maximum sustained winds at 70 mph. Forecasts say the storm will strengthen and probably turn into a hurricane Wednesday night before it loses steam Thursday.
A variety of sophisticated equipment are helping forecasters keep an eye on both storms, including a remote piloted Global Hawk aircraft supplied by NASA.
99L was lashing islands in the northeast Caribbean Tuesday and Wednesday, including St. Maarten where Instagram user @kphawk99 shared video showing torrential rains and strong winds on what he said was the last day of his vacation.
"So much for our last day in St. Maarten lol. If this is an invest I'd hate to see a real hurricane," he wrote Wednesday.