Aug. 28 (Bloomberg) -- Tropical Storm Ernesto, which pelted Cuba with rain and winds this morning, is likely to reach southern Florida as a powerful tropical storm tomorrow and then strengthen again to a hurricane later this week.

Ernesto's storm system was buffeted by the mountainous regions of southeastern Cuba earlier today, and its maximum sustained winds dropped to about 40 miles per hour from 50 mph, the National Hurricane Center said. Once over water again, Ernesto will gain power and have winds of as much as 95 mph when it reaches southeastern Florida on the morning of Aug. 30.

Florida's southern counties began opening emergency operations centers, and the storm forced NASA to scrub its planned launch of the space shuttle. Governor Jeb Bush, who yesterday declared a state of emergency and ordered all visitors to leave the Keys, urged residents in areas that may be affected to have enough supplies for three days.

``Take this storm very seriously,'' Bush, a Republican, said in a televised briefing with reporters. ``A hurricane's a hurricane, and it has devastation that we've already seen. All you have to do is rewind back to last year and know what a Category 1 can do in terms of devastation to the community.''

Ernesto's center was over eastern Cuba as of 8 p.m. New York time, moving west-northwest at about 11 mph (19 kph), the hurricane center said.

``Ernesto is trying to come off the Cuban coast, where it was beaten up by the mountains; the storm is reorganizing itself,'' said Tom Downs, a meteorologist at Weather 2000 Inc. in New York. ``We think it will be a powerful tropical storm or a weak hurricane by the time it reaches Florida after sunset tomorrow.''

On to Miami

Ernesto, the fifth named storm of the June-to-November Atlantic basin hurricane season, formed just before the Aug. 29 anniversary of Hurricane Katrina's landfall on the Gulf Coast. That storm became the worst U.S. natural disaster in history, causing more than $81 billion in damage and killing more than 1,800 people.

The storm is forecast to bring as much as 20 inches of rain to parts of Cuba, the Dominican Republic and Haiti, which may cause life-threatening floods and mudslides, and as much as 4 inches of rain to the southern Bahamas.

Ernesto is then predicted to move northward toward Miami on Florida's coast as a hurricane, before heading out to the Atlantic coast off Georgia early on Aug. 31. The storm is expected to gather strength and could then head up the Eastern seaboard along the Carolinas, Downs said.

`Sigh of Relief'

Ernesto's projected path pushed natural gas prices down in New York today, as energy traders don't expect Ernesto to interrupt production in the Gulf of Mexico. Gasoline and heating oil futures fell for the first time in three days.

``For the moment the market is expressing a sigh of relief,'' said Michael Fitzpatrick, vice president for energy risk management at Fimat USA in New York. ``It seems at this point that the direction of the storm is going to be away from the Gulf oil and gas infrastructure.''

Tropical-storm warnings are in effect for the Cuban provinces of Guantanamo, Santiago De Cuba, Granma, Holguin, Las Tunas and Camaguey, as well as for Ragged Island and Great Exuma in the central Bahamas. A warning means winds as strong as 73 mph are possible in the next 24 hours.

A hurricane watch is in effect for coastal southern Florida south of Interstate 75, the so-called Alligator Alley, from south of New Smyrna Beach on the east coast to south of Chokoloskee on the west. The watch means hurricane-force winds are possible within the next 36 hours.

In Line for Fuel

Watches are also in effect for Andros Island, the Biminis and Grand Bahama Island in the Bahamas, and all of the Florida Keys, and watches may be needed for other parts of Florida later today, the hurricane center said.

The National Aeronautics and Space Administration postponed tomorrow's scheduled launch of the space shuttle Atlantis because of Ernesto, which is forecast to pass near the Kennedy Space Center in Cape Canaveral on Aug. 30.

Broward County, Florida, which includes Deerfield Beach and Fort Lauderdale, and Monroe County, where the Keys are located, opened their emergency operations centers this morning. Officials in Broward ordered mobile home residents to evacuate starting at 6 a.m. tomorrow and recommended that people living in low-lying areas and barrier islands also leave.

Lines were forming for gasoline throughout southern Florida as residents prepared to evacuate.

Tankers backed up at Port Everglades, which serves as the main point of entry for fuel for many of the state's gas stations, because of the increased demand. Shortages aren't expected, though, because ships are waiting in port and offshore to unload supply, said Broward County Emergency Management Agency spokeswoman Margaret Stapleton.

To contact the reporter on this story: Chris Dolmetsch in New York at cdolmetsch@bloomberg.net; Samantha Zee in Los Angeles at szee@bloomberg.net.

Last Updated: August 28, 2006 21:47 EDT