MIAMI — Emergency officials from Florida to the Carolinas were closely watching Irene Tuesday as the first hurricane to seriously threaten the U.S. in three years churned over energizing tropical waters. The storm has already cut a destructive path through the Caribbean.

Forecasters say the hurricane could grow to a monstrous Category 4 storm with winds of more than 131 mph before it's predicted to come ashore this weekend on the U.S. mainland. The U.S. National Hurricane Center in Miami expected Irene to reach Category 3 strength on Tuesday, said spokesman Dennis Feltgen.

Officials could begin issuing watches for parts of the U.S. mainland later in the day. Because the storm is so large, Florida could begin feeling some effects from the storm late Wednesday.

Current government models have the storm's outer bands sweeping Florida late this week before it takes aim at the Carolinas this weekend, though forecasters caution that predictions made days in advance can be off by hundreds of miles. Georgia is also likely to be affected.

The last hurricane to make landfall in the U.S. was Ike, which pounded Texas in 2008.

For now, the first Atlantic hurricane of the season had maximum sustained winds early Tuesday around 100 mph (160 kph) and was centered about 55 miles (90 kilometers) northeast of Puerto Plata in the Dominican Republic. The hurricane was moving west-northwest near 10 mph (17 kph).

"For residents in states that may be affected later this week, it's critical that you take this storm seriously," said Craig Fugate, administrator at the Federal Emergency Management Agency.

Emergency officials in North Carolina were checking "pre-landfall operations" to make sure equipment such as trucks, forklifts, generators and computers were working, said Ernie Seneca, spokesman for the state Department of Crime Control and Public Safety. Also, they were taking inventory of food and water supplies.

To the south in Miami, Julio Gonzalez was heeding the warnings and headed to a hardware store to pick up what he needed to protect his home.

"I'm gonna board up," he said Monday. "It's best to play it safe."

Others were stocking up on bottled water and plywood. And Hurricane Irene was trending on Twitter, with many users sharing updates on the storm's progress while others hoped it wouldn't come their way.

"We want to make sure Floridians are paying attention," said Bryan Koon, director of the Florida Division of Emergency Management, who met Monday with the governor. "We are at the height of the hurricane season right now. If it's not Hurricane Irene, it could be the follow-up storm that impacts us."

After several extremely active years, Florida has not been struck by a hurricane since Wilma raked across the state's south in October 2005. That storm was responsible for at least five deaths in the state and came two months after Hurricane Katrina devastated New Orleans.

Irene slashed directly across Puerto Rico, tearing up trees and knocking out power to more than a million people. It then headed out to sea, north of the Dominican Republic, where the powerful storm's outer bands were buffeting the north coast with dangerous sea surge and downpours. President Barack Obama declared an emergency for Puerto Rico, making it eligible for federal help.

Irene was forecast to pass over or near the Turks and Caicos Islands and the southeastern Bahamas by Tuesday night and be near the central Bahamas early Wednesday.

In the U.K. territory of the Turks and Caicos, a steady stream of customers bought plywood and nails at hardware stores, while others readied storm shutters and emergency kits at home.

"I can tell you I don't want this storm to come. It looks like it could get bad, so I've definitely got to get my boats out of the water," said Dedrick Handfield at the North Caicos hardware store where he works.

Many of the center's computer models had the storm veering northward away from Florida's east coast toward Georgia and the Carolinas. A hurricane center forecast map said the storm's center could come ashore in one of the states on Saturday or Sunday, but forecasters said much was still unclear.

"In terms of where it's going to go, there is still a pretty high level of uncertainty," said Wallace Hogsett, a National Hurricane Center meteorologist. "It's a very difficult forecast in terms of when it's going to turn northward."

In South Carolina, emergency agencies went on alert for what could be the first hurricane to hit there in seven years.

"This is potentially a very serious hurricane," longtime Charleston Mayor Joseph P. Riley Jr. said. He led Charleston's recovery from the massive destruction of Hurricane Hugo's 135 mph winds and waves back in 1989.

It's been more than a century since Georgia has taken a direct hit from a Category 3 storm or greater. That was in 1893 and the last hurricane to make landfall along the state's 100-mile coast was David, which caused only minor damage when it struck in 1979.

Across Florida, emergency management agencies were closely monitoring Irene's movements and track. They urged residents to make sure they have batteries, drinking water, food and other supplies available in case Irene takes aim at the state.

"We must prepare for the worst and hope for the best," said Joe Martinez, chairman of the Miami-Dade County Commission.

Gov. Scott met with state emergency management officials and the state meteorologist, poring over detailed charts involving windspeed and steering currents. Scott, a first-term Republican who has not experienced a hurricane as governor, asked questions such as how much advanced notice would be needed for evacuations of low-lying areas.

"Irene's going to be close," Amy Godsey, the state meteorologist, told Scott. "We're not out of the woods yet."

Scott replied, "I'm an optimist."