ATLANTA — Tens of millions of Americans were warned on Monday to brace themselves for the threat of flooding, power outages and downed trees as Hurricane Isaias intensified while moving up the Atlantic Coast.
On the coast of the Carolinas, residents were preparing for the Category 1 storm’s arrival on Monday night, boarding up their windows and stocking up on generators, flashlights and gas cans. Farther inland, in North Carolina and Maryland, officials said that flooding would be one of the storm’s most perilous risks.
The storm is expected to soak much of the East Coast in the coming days, prompting state officials to caution residents that they must prepare for heavy rainfall and powerful winds while remaining vigilant against the coronavirus.
“I know that North Carolinians have had to dig deep in recent months to tap into our strength and resilience during the pandemic, and that hasn’t been easy,” Gov. Roy Cooper of North Carolina said. “But with this storm on the way, we have to dig a little deeper. Let’s keep each other safe from the wind and water as well as from the virus.”
Isaias, which is written as Isaías in Spanish and pronounced ees-ah-EE-ahs, strengthened back into a hurricane on Monday night before making landfall near Ocean Isle Beach, N.C., at about 11:10 p.m. Eastern. A hurricane warning was issued from the South Santee River in South Carolina to Surf City, N.C., a region that includes Myrtle Beach, S.C.
The eastern Carolinas and Virginia may get three to six inches of rain, with isolated areas receiving up to eight inches. Significant flash floods and urban flooding can be expected through the middle of the week, and widespread minor to moderate river flooding is possible. Tropical-force winds and heavy rain were also expected to hit Maryland.
The Middle Atlantic States, southeastern New York and New England can expect a few inches of rain. Tropical storm warnings and watches were in effect Monday night all the way up the Eastern Seaboard, including in Martha’s Vineyard, Mass., and Stonington, Maine.
Heavy rainfall in northeastern New Jersey, New York City and the lower Hudson Valley was expected to begin late Monday night, building into heavier downpours by Tuesday afternoon and evening, said Matthew Wunsch, a meteorologist with the National Weather Service. Emergency management officials in New York City said the storm might bring three to six inches of rain in some areas.
Gov. Philip D. Murphy of New Jersey asked people on Monday to stay inside during the storm, clarifying that it should not be an invitation to huddle together with friends and relatives for the kind of gatherings that have been behind a recent rise in coronavirus cases there.
“I’m not a fan of hurricane parties,” Mr. Murphy said, referring to the events that became something of a tradition in Florida during minor storms. “If it’s a hurricane party, you’re inside. It just doesn’t make sense, folks. It doesn’t end well. And we know that.”
Officials in Florida expressed relief over the weekend, saying that Isaias failed to deliver the punch they had feared after it first became a Category 1 hurricane in the Caribbean. It brought rain and wind, but not enough to create significant damage.
The Florida coast was hit by the storm’s outer flank as it shifted north, and it largely spared Georgia. But Isaias veered inland, positioning the Carolinas to face the brunt of the storm.
North Carolina declared a statewide emergency, as did coastal communities in South Carolina. In Maryland, Gov. Larry Hogan said that coronavirus testing sites were closing on Tuesday, and Mayor Jack Young of Baltimore encouraged residents to move their cars to higher ground.
In the Chesapeake Bay, wind gusts may reach more than 65 miles per hour, and around 50 to 60 m.p.h. southeast of Washington, said Jeremy Geiger, a meteorologist with the Weather Service.
Winds above 60 m.p.h. are strong enough to knock down trees, so residents could experience power outages. Flooding will be the biggest risk, with southern Maryland expected to receive three to seven inches of rain within an eight-hour period. The most rain was expected around the Interstate 95 corridor.
“People don’t realize it, but in the Mid-Atlantic and a lot of areas, flooding actually causes the most loss of life and damage,” Mr. Geiger said. “So be aware of where you live, and what’s going on.”
Along the shore in Myrtle Beach, the operators of the SkyWheel, which sends gondolas 20 stories into the air, have watched Isaias since it was brewing near western Africa. On Monday, they were locking down anything the wind could turn into a projectile.
“We’re always familiar with how storms can change at the drop of a dime,” said Rachel Beckerman, a manager for the attraction. “We take every storm as if it were a Category 4 coming for Myrtle Beach.”
On the coast of the Carolinas, storms blowing in from the Atlantic are a fact of life, sometimes arriving daily during the summer. Tropical storms and hurricanes can be capricious and destructive, but many residents know how to prepare.
“They’ve been through this before,” said Susan A. Freeman, the executive director of the Brunswick County Chamber of Commerce, which includes a strand of North Carolina beach communities directly in the storm’s path.
Rick Rojas reported from Atlanta, and Lucy Tompkins from Bozeman, Mont. Patricia Mazzei contributed reporting from Miami, and Michael Gold and Mihir Zaveri from New York.