Florence exploded into a potentially catastrophic Category 4 hurricane Monday as it closed in on North and South Carolina, carrying winds up to 140 mph (220 kph) and water that could wreak havoc over a wide stretch of the East Coast later this week.

NEW: Florence is now a category 4 hurricane. Data from a NOAA Hurricane Hunter indicate that Florence has continued to rapidly strengthen and has maximum sustained winds near 130 mph (195 km/h) and a minimum central pressure of 946 mb (27.93 inches) https://t.co/tW4KeGdBFb pic.twitter.com/wfLt6fJPl2 — National Hurricane Center (@NHC_Atlantic) September 10, 2018

Earlier this morning, @Astro_Ricky Arnold shared this view, saying "Hurricane #Florence this morning as seen from @Space_Station." Our @NASAEarth-observing satellites also continue to monitor this storm as it has developed an eye + strengthened: https://t.co/OQmhIyvaYQ pic.twitter.com/Npp0Zwb0cf — NASA (@NASA) September 10, 2018

View from inside the eye of category 4 #HurricaneFlorence today onboard the NOAA P-3 #NOAA42. (Video credit: Heather Holbach) pic.twitter.com/eEYOI2PBnh — HRD/AOML/NOAA (@HRD_AOML_NOAA) September 10, 2018

Communities along a stretch of coastline prepared to evacuate the storm, which forecasters expect to be close to Category 5 strength by Tuesday. South Carolina governor Henry McMaster ordered the state’s entire coastline to be evacuated starting at noon Tuesday and predicted that one million people would flee.

JUST IN: South Carolina Gov. Henry McMaster orders mandatory evacuation of all people in evacuation zones ahead of Hurricane Florence. Order goes into effect at noon on Tuesday. https://t.co/021hNnhqS8 pic.twitter.com/AKbTHNffcS — ABC News (@ABC) September 10, 2018

The storm’s first effects were already apparent on barrier islands as dangerous rip currents hit beaches and seawater flowed over a state highway. For many, the challenge could be finding a safe refuge: If Florence slows to a crawl just off the coast, it could bring torrential rains to the Appalachian mountains and as far away as West Virginia, causing flash floods, mudslides, and other dangerous conditions.

The storm’s potential path also includes half a dozen nuclear power plants, pits holding coal-ash and other industrial waste, and numerous hog farms that store animal waste in massive open-air lagoons. Airlines, including American and Southwest, have started letting passengers change travel plans that take them into the hurricane’s possible path.

National Hurricane Center Director Ken Graham warned that Florence was forecast to linger over the Carolinas once it reaches shore. People living well inland should prepare to lose power and endure flooding and other hazards, he warned. “It’s not just the coast,” Graham said. “When you stall a system like this and it moves real slow, some of that rainfall can extend well away from the center.”

#Florence nosing closer to 26.0 N tonight. We are drawing closer to watches and warnings going up in the coming hours… and then it starts feeling real for residents in the threat zone. Hurricane prep is taxing on multiple levels. pic.twitter.com/UW56lATDKb — Ryan Phillips – NBC6 Meteorologist (@RyanNBC6) September 11, 2018

Our models specifically used for forecasting hurricanes have great agreement in #Florence making landfall near New Bern, NC Thursday night. pic.twitter.com/Ty6apTgdQq — Ed Vallee | Vallee Wx Consulting (@EdValleeWx) September 11, 2018

A warm ocean is the fuel that powers hurricanes, and Florence will be moving over waters where temperatures are peaking near 85 degrees (30 Celsius), according to hurricane specialist Eric Blake. Further, with little wind shear to pull the storm apart, Florence’s hurricane wind field was expected to expand over the coming days, increasing its storm surge and inland wind threats.

By 5:00 p.m. Monday, Florence was centered about 1,170 miles (1,880 kilometers) east-southeast of Cape Fear, North Carolina, and moving west at 13 mph (20 kph). Its center will move between Bermuda and the Bahamas on Tuesday and Wednesday and approach the coast of South Carolina or North Carolina on Thursday, the National Hurricane Center said.

People throughout the Carolinas are already dealing with bumper-to-bumper traffic to get out of the way of #HurricaneFlorence https://t.co/bPljNvvhmo pic.twitter.com/am57dLRIR2 — Circa (@Circa) September 11, 2018

Video: heavy rain, storms along I26 near Columbia, SC reducing visibility for early evacuees. Eastbound I-26 between Columbia and Charleston will be closed tomorrow starting at 5 am to accommodate coastal evac @accuweather @breakingweather #HurricaneFlorence pic.twitter.com/Mb019mZLBe — Reed Timmer (@ReedTimmerAccu) September 11, 2018

Two other storms were spinning in the Atlantic. Hurricane Isaac was expected to lose strength as it reaches the Caribbean, and Helene, much farther out to sea, may veer northward into the open ocean as the 2018 hurricane season reaches its peak. In the Pacific, Hurricane Olivia triggered warnings for multiple Hawaiian islands as it blew west toward an arrival over the state as soon as late Tuesday or early Wednesday.

Preparations for Florence were intensifying up and down the densely populated coast. Since reliable record-keeping began more than 150 years ago, North Carolina has been hit by only one Category 4 hurricane: Hazel, with 130 mph winds, in 1954. The parking lot has been full for three days at the Ace Hardware store in coastal Calabash, North Carolina, where manager Tom Roberts said he sold 150 gas cans in two hours Monday, along with generators, plywood, rope, manual can openers, sand bags and a plethora of other items.

Several meteorologists said Florence could do what Hurricane Harvey did last year over Texas, dumping days of rain, although not quite as bad. “I think this is very Harvey-esque,” said University of Miami hurricane expert Brian McNoldy. “Normally, a landfalling tropical cyclone just keeps on going inland, gradually dissipating and raining itself out. But on rare occasions, the steering patterns can line up such that a storm slips into a dead zone between troughs and ridges.”

On North Carolina’s Outer Banks, Dawn Farrow Taylor, 50, was gathering photos and important documents and filling prescriptions Monday before heading inland. She grew up on the island chain, and says this will be only the second time she’s evacuated. “I don’t think many of us have ever been through a Category 4. And out here we’re so fragile. We’re just a strip of land — we’re a barrier island,” she said.

In the village of Buxton, Liz Browning Fox plans to ride the storm out in her house on top of a ridge. She believes her home, built in 2009, will be secure, but it’s hard to foresee all potential hazards. “You never know, there could be tree missiles coming from any direction,” she said. “There is no way to be completely safe.”

An evacuation order from Virginia Gov. Ralph Northam applies to about 245,000 people, including parts of the Hampton Roads area and Eastern Shore. North Carolina Gov. Roy Cooper said his state was “in the bullseye” of the storm and urged people to “get ready now.”

North Carolina Gov. Roy Cooper urges residents to "get ready now" for #HurricaneFlorence, as evacuations begin along the coast. A federal disaster declaration has been requested. pic.twitter.com/MZoHGqRGHK — Joyce Koh (@JoyceKohTV) September 10, 2018

“We here in North Carolina are bracing for a hard hit,” Cooper said at a press conference. “North Carolina is taking Hurricane Florence seriously, and you should too. Get ready now.”

“When weather forecasters tell us [the hurricane could be] life-threatening, we know that it is serious,” he added.

President Donald Trump spoke with McMaster, Cooper, and Virginia Governor Ralph Northam by phone to offer around-the-clock assistance from the federal government.

Just had calls with South Carolina Governor Henry McMaster, North Carolina Governor Roy Cooper, and Virginia Governor Ralph Northam regarding the incoming storm. Federal Government stands by, ready to assist 24/7. — Donald J. Trump (@realDonaldTrump) September 10, 2018

Was just briefed via phone by @DHSgov @SecNielsen and @FEMA @FEMA_Brock, along with @VP Mike Pence and Chief of Staff, John Kelly on incoming storm which is very dangerous. Heed the directions of your State and Local Officials – and know that WE are here for you. Be SAFE! pic.twitter.com/sN8D5NvrBa — Donald J. Trump (@realDonaldTrump) September 10, 2018

The president on Monday evening approved emergency declaration for South Carolina and North Carolina ahead of the storm.

WH/ABC: Pres. Trump approves South Carolina and North Carolina emergency declarations in anticipation of Hurricane Florence pic.twitter.com/VADzATrsOI — Joshua Dov Caplan (@joshdcaplan) September 11, 2018

The Associated Press contributed to this report.