(CNN) - Hurricane Florence is still on track to hit the East Coast of the United States this week -- and two other big storms are brewing right behind it.
Florence, which was upgraded from a tropical storm to a hurricane on Sunday, was in the Atlantic Ocean, about 1,400 miles from East Coast. It was moving west close to 7 mph, about 720 miles from Bermuda, the National Hurricane Center said.
Helene was also upgraded to a hurricane on Sunday, the center said, but neither Helene nor Tropical Storm Isaac are expected to hit the mainland United States. Isaac, which was moving toward the lesser Antilles in the Caribbean Sea, could bring heavy rain and become a hurricane overnight, CNN meteorologist Tom Sater said.
"Maximum sustained winds are now 75 mph, and further strengthening is forecast over the coming days," said CNN senior meteorologist Dave Hennen. "Florence continues on a track to impact the US mainland by Thursday or Friday."
The hurricane center said, "All indications are that Florence will be an extremely dangerous Category 4 hurricane while it moves over the western Atlantic toward the southeastern United States."
Hennen said computer models agree Florence is on track to hit the Carolinas.
It would be the first Category 4 hurricane to do so since Hugo in 1989. And it would be the first major hurricane (Category 3 or higher) to hit the East Coast since Jeanne, which struck Florida in 2004.
To become a hurricane, a storm must reach sustained winds of 74 mph. The hurricane center says "catastrophic" Category 4 storms generate sustained winds of at least 130 mph.
Swells generated by Florence are already affecting Bermuda and are beginning to reach portions of the US East Coast, the hurricane center said.
"These swells are likely to cause life-threatening surf and rip current conditions," the center said.
Another "life-threatening impact" from Florence could be freshwater flooding from prolonged heavy rainfall inland, the center said. It is too soon to say where, when or how severe the rainfall might be.
Most computer models predict Florence will slow down as it moves inland, Hennen said, which could add to the heavy rains and potential floods.
Virginia and North Carolina and South Carolina already are on alert. Their governors declared states of emergency Friday and Saturday.
"We are preparing for the worst, and of course hoping for the best," South Carolina Gov. Henry McMaster said, adding his declaration would allow state agencies to deploy assets quickly to the coast.
McMaster said Sunday he has asked President Trump for a federal disaster declaration. That would make state and local agencies eligible for FEMA reimbursement of some costs.
North Carolina Gov. Roy Cooper waived certain transportation restrictions so that farmers could harvest and move crops more quickly.
Cooper also urged people to learn what evacuation routes to take, and put fuel in their vehicles in case they're ordered to leave.
"Action today can avoid losses due to Florence," he said.
Lines forming at grocery stores
Crowds of shoppers formed at supermarkets on Sunday, as people tried to stock up on supplies before the storm.
"Checkout lines @Costco in Charleston running all the way to the back of the store. Hurricane #florence for the win! #chswx," tweeted Michael Livingston. "Wait was about 20 min - long but fast-moving. Prep is usual: foodstocks, fuel, cash, batteries, clean-up of property for high winds. Might buy a new board game or two. :)"
Erin Byrd checked in online from Publix in Apex, North Carolina.
"Water supplies being depleted ... Bread and milk supplies still robust," she posted on Instagram.
"We don't panic, which is why we are amused that water was so depleted a week out. We still have water supply from last year here," she told CNN.
Alicia Buchanan posted on Instagram from Walmart in Rock Hill, South Carolina.
She just moved to the area two weeks ago from Northern Virginia and still doesn't have her furniture.
"So, I'm prepping with some bottled water, a couple puzzle books, and making sure all my electronics and back-up batteries are charged," she told CNN. "I plan to do most of my cooking on the grill."
Other storms lining up
The Navy said Saturday that it may be necessary to send ships in the Norfolk, Virginia, area out to sea because of the coming storm. The Navy put them on Sortie Condition Bravo, which means the onset of destructive weather is within 48 hours.
In a news release, the Navy said the ships can handle destructive weather better while at sea and that "having the ships underway also makes them ready and available to respond to any national tasking, including any needed disaster response efforts in the local area after the storm has passed."
Tropical Depression Nine became Tropical Storm Isaac on Saturday afternoon over the eastern tropical Atlantic, the National Hurricane Center said. It was about 1,500 miles from the easternmost Caribbean islands on Sunday and could become a hurricane later in the day.
The three systems -- Florence, Isaac and Helene -- come right before the Atlantic hurricane season hits its peak Monday. The eight weeks around that date often are prime time for the conditions that fuel powerful storms.
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