(CNN) -- Crystal Kirwan didn't see she had much choice as she got her family ready to leave their home Monday on the coast of North Carolina, with Category 4 Hurricane Florence threatening the state.
"This one is hard," she said, having been through hurricane damage before. "We live paycheck to paycheck, being a military family, and it's four days until payday."
She, her husband and children will make the 4½-hour drive to family in Dover, Delaware, as soon as he is cleared from duty.
"Probably not too much better, but most likely safer than here," she said.
Similar scenes were repeated up and down the US East Coast, from Virginia to Savannah, Georgia.
The National Hurricane Center upgraded Florence not once but twice Monday, saying the storm will continue to strengthen as it targets the Carolinas later this week. It could be the most devastating to hit the region in three decades. Swells up to 15 feet could batter the coast, CNN meteorologist Chad Myers said. And heavy rains could cause flooding inland for days after landfall.
"North Carolina is taking Hurricane Florence seriously, and you should, too," North Carolina Gov. Roy Cooper said, urging residents to prepare for lengthy power outages after the hurricane comes.
The first mandatory evacuation order was issued at noon, and Cooper expects more later Monday and Tuesday.
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.
"This storm gets stronger and stronger" on its way to a head-on impact with a zone that ranges some 80 miles, Myers said.
Storm growing quickly
On Sunday, the hurricane center reported that Florence had reached sustained winds of at least 74 mph to be classified a hurricane.
Some tourists and residents were still outside enjoying pleasant weather as forecasters said computer models narrowed the grim target with clearer consensus.
By Monday morning, the hurricane center classified Florence as a "major" hurricane, Category 3, meaning it was packing sustained winds of at least 111 mph. It was picking up speed over the warm waters of the Atlantic, causing forecasters to worry not only about landfall -- but about flooding brought by heavy rains afterward, possibly late Thursday or Friday.
Track the storm and compare different forecast models
A supervisor at the North Carolina emergency management office was welcoming workers back and telling them to prepare for a long week.
About an hour after the Category 3 upgrade came another -- to Category 4.
That means sustained winds of at least 130 mph and expectations of catastrophic damage, the hurricane center says. Well-built homes can lose walls and roofs. Trees can be snapped or uprooted. Electricity and power can go out for days.
Governors in the Carolinas and Virginia already had declared emergencies, and some sought federal aid as well.
At noon Monday, a mandatory evacuation order was issued for North Carolina's Hatteras Island. Dare County Emergency Management officials said they'll extend the order to all residents and visitors in other areas beginning at 7 a.m. Tuesday.
Fast facts: 2018 Atlantic hurricane season
Florence was about 575 miles south-southeast of Bermuda as of noon ET Monday, or about 1,230 miles from Cape Fear, North Carolina. The storm was moving west at 13 mph, the hurricane center said.
North Carolina's governor 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.
Preparations for Florence come as the Atlantic hurricane season hits its peak. Two other storms also are churning in the Atlantic.
Hurricanes Helene and Isaac are not expected to hit the US mainland.
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.
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.