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Wilmington, North Carolina (CNN) More than 1 million people on the southern East Coast of the United States faced mandatory evacuation orders as Hurricane Florence neared Category 5 strength Tuesday.

Florence was upgraded twice Monday, from Category 3 to 4, prompting evacuation orders in coastal areas of North Carolina, South Carolina and Virginia. One year after historic storms that ravaged the American Gulf Coast and Puerto Rico last year, officials warned not to underestimate the threat the storm poses. Residents boarded up their homes, formed long lines at gas stations and emptied the shelves of hardware stores and supermarkets.

Residents stock up on supplies at a Harris Teeter grocer in the Isle of Palms, South Carolina.

The size of hurricane-force winds doubled over 12 hours from 30 to 60 miles across, the National Hurricane Center said Monday afternoon. The wind field is expected to keep growing, which will increase the storm surge and inland wind threats, the service said in a statement.

"None of the guidance suggest that Florence has peaked in intensity, and this is supported by a continuation of a low-shear environment, and even warmer waters over he next 36 hours. Thus, the intensity forecast is raised from the previous one, bringing Florence close to Category 5 strength tomorrow," NHS said.

More counties placed under mandatory evacuations

National Guard soldiers are being mobilized in the affected states. President Donald Trump declared an emergency in North Carolina, allowing it to access federal funds.

Six more North Carolina counties were placed under mandatory evacuation Monday night, hours after the first order in Hatteras Island. Certain areas of Brunswick, Currituck, Dare, Hyde, New Hanover, and Onslow counties are affected, and more orders are expected Tuesday.

In South Carolina, mandatory evacuation orders take effect Tuesday at noon in eight counties along the state's 187-mile coastline. Starting then, all roads on I-26 and Route 501 will be directed away from the coast, South Carolina Gov. Henry McMaster said.

"This is a real hurricane we have coming," McMaster said Monday. "We don't want to risk one South Carolina life."

Mandatory evacuations start in coastal South Carolina Tuesday at noon.

State government offices, including schools and medical facilities, will be closed in 26 counties, McMaster said. He estimated that about one million people will be affected by the order, including residents and visitors.

In Virginia, mandatory evacuations begin 8 a.m. Tuesday for about 245,000 residents in a portion of Hampton Roads and the Eastern Shore area, Gov. Ralph Northam said Monday. "Everyone in Virginia needs to prepare," he said. "This is a serious storm and it's going to affect the entire state of Virginia."

Residents prepare to flee and hunker down

Residents of coastal areas boarded up homes and packed their bags as they braced for Florence's wrath. Lines formed at gas stations as people heeded warnings to gas up in case they're ordered to leave.

Crystal Kirwan didn't think she had much choice as she got her family ready to leave their home Monday. She and her husband will make the 4.5-hour drive with their children to family in Dover, Delaware, as soon as he is cleared from military duty. "Probably not too much better, but most likely safer than here," she said.

Grocery stores began selling out of water, milk and bread, and supply stores were struggling to stock batteries, plywood and generators as early as Sunday.

At a Home Depot in Wilmington, North Carolina, Billy Nivens told CNN they ran out of water, plywood, generators, flashlights, propane, batteries and five-gallon gas cans by Monday afternoon.

Since noon Sunday, a Lowe's in Lumberton, North Carolina, has been sold out of generators, store manager Mackie Singletary said.

In Charleston, South Carolina, the Coburg Dairy Cow landmark was removed for safekeeping from its perch atop a sign on Highway 17 on Monday, according to CNN affiliate WCIV.

Storm growing quickly

Florence became a hurricane Sunday with sustained winds of at least 74 mph. By Monday morning, the hurricane center classified Florence as a "major" Category 3 hurricane, with sustained winds of at least 111 mph. It was picking up speed over the Atlantic's warm waters, causing concerns about landfall and flooding from heavy rains afterward, possibly late Thursday or Friday.

An enhanced satellite image shows Hurricane Florence west of Isaac and Helene.

About an hour after the Category 3 upgrade, the storm was reclassified once again, now as Category 4. That means sustained winds of at least 130 mph and expectations of catastrophic damage, the hurricane center says.

States of emergency were declared in the Carolinas, Virginia and Maryland. In Maryland, where some coastal areas are still recovering from weather Gov. Larry Hogan said flooding could be catastrophic if it hits the state.

"We are hoping for the best and preparing for the worst," Hogan said.

Gov. Roy Cooper urged residents to prepare for lengthy power outages after the hurricane arrives and learn evacuation routes they may need to take.

"North Carolina is taking Hurricane Florence seriously, and you should, too," Cooper said. "Action today can avoid losses due to Florence."

This is peak season

The hurricane season in the Atlantic will hit its peak Monday.

Preparations for Florence come as the Atlantic hurricane season hits its peak. Monday is the climatological peak date of hurricane season, the height of the eight-week period when the most powerful storms usually form, CNN meteorologist Pedram Javaheri said.

Two other storms also are churning in the Atlantic. Hurricanes Helene and Isaac are not expected to hit the US mainland.

The Atlantic hurricane season officially began June 1, but cooler water and higher wind shear -- winds moving at different speeds and directions -- early in the season are less than ideal for tropical systems to gain and maintain strength.

Storms increase in frequency and intensity by mid-August and into September as temperatures in the Atlantic climb to their highest levels, Javaheri said.