Ivan strengthened early Thursday to 160 mph and forecasters said if the storm stays on course it could reach Jamaica by Friday, Cuba on Sunday and the Florida Keys early Monday. The storm would be the third to hit Florida in a month.
Ninety percent of the homes in Grenada were damaged; looting erupted and a prison was reportedly destroyed leaving criminals on the loose. The storm is the most powerful hurricane to hit the island in a decade.
The United States declared Grenada a disaster area, allowing the immediate release of $50,000 for emergency relief.
Meanwhile in Florida, Gov. Jeb Bush said "people are at their wits end" from the previous two hurricanes, but officials and residents have no choice but to prepare for a third.
Monroe County emergency officials asked tourists to leave the Keys at 9 a.m. Thursday, while mobile home residents were urged to begin evacuating earlier in the day, and other residents were told they should prepare to leave Friday.
Officials are being careful to begin evacuation procedures early since only one highway links the low-lying islands with mainland Florida said Irene Toner, Monroe County's emergency manager.
Although authorities are calling on people to leave the Keys, many worry those in the possible path of the storm may not take the opportunity to seek higher ground. Officials estimated that only 15 percent of Keys residents left in 2001 for Hurricane Michelle, a 135 mph storm that narrowly missed a direct hit on the chain.
Residents may not heed the evacuation warning, but tourists appear to be taking the tumultuous weather seriously. Some businesses have seen a steep drop in the number of tourists since Charley and Frances. Charlie Bauer, general manager of Hog's Breath Saloon in downtown Key West, said he has had his worst six weeks of business since 1988.
Bauer said he normally rides out hurricanes in the Keys, but not this time: "I won't even hang around for a Category 3."
Should Ivan strike Florida, it would be the first time since 1950 that two major hurricanes -- ones with top sustained winds of at least 111 mph -- hit Florida so close together.
Charley struck southwest Florida on Aug. 13 with winds of 145 mph, causing an estimated $6.8 billion in damage and 27 deaths. Frances hit the state's eastern coast early Sunday with 105 mph winds, leaving $2 billion to $4 billion in insured damage and at least 16 dead.
On Wednesday, President Bush signed a $2 billion disaster relief package for victims of Frances and Charley, just before he toured the state's Atlantic coast by helicopter.