ATLANTA — A hurricane and a tropical storm continue to pose a threat in the Caribbean as they approach the Gulf Coast, with Louisiana potentially in the path of both of them in what could be an unusual occurrence, forecasters said.
Hurricane conditions, life-threatening storm surge and rain are expected along portions of the Gulf Coast from Hurricane Marco, which strengthened from a tropical storm on Sunday, the National Hurricane Center said.
Marco is predicted to make landfall as a Category 1 hurricane, and forecasters said Laura was projected to arrive as a Category 2, with a possibility of it growing stronger. Forecasters expect rain, rather than wind, to pose the most significant threat.
Meteorologists said that there was potential for storm surges along the banks of the Mississippi River in the range of three to six feet from Marco, and that Laura could bring a surge of seven to 10 feet along the coast.
Both storms are forecast to drench a portion of the state with five to 10 inches of rain, and they warned of the potential of the two storms overlapping in some places, unleashing one to two feet of rainfall.
“We have a one-two punch that’s going to hit the state of Louisiana,” Benjamin Schott, the meteorologist in charge of the National Weather Service in New Orleans, said on Sunday.
Laura is forecast to become a hurricane late Tuesday, officials said.
The last time a hurricane and a tropical storm were both in the Gulf of Mexico was in 1959, Joel Cline, a tropical program coordinator for the National Weather Service, said on Saturday.
There are no known cases of two hurricanes in the Gulf at the same time, according to the National Weather Service and the National Oceanic and Atmospheric Administration. The closest occurrence of this was on Sept. 4, 1933, when a hurricane was over South Florida and another was over the western Gulf of Mexico.
Dennis Feltgen, a meteorologist with the National Hurricane Center, dispelled rumors on social media that the storms would collide, forming a single monster storm. “They cannot merge,” he said. “They actually repel each other because of the rotations.”
The last time a hurricane was in the Gulf was in July when Hurricane Hanna hit Texas, Mr. Feltgen said.
Storm surge and hurricane warnings are in effect for some cities in states from Louisiana to Mississippi.
Some areas of Louisiana have issued mandatory evacuations, including portions of Plaquemines Parish, Lafourche Parish and Jefferson Parish. Orleans Parish was asking for voluntary evacuations, the television station 4WWL reported.
Many in Louisiana have grown accustomed to powerful storms blowing in from the Gulf of Mexico, punishing the coast with heavy rains, winds and surges. But two storms in rapid succession, the second coming within 48 hours of the first, have spurred a heightened sense of alarm.
Gov. John Bel Edwards warned residents on Sunday that the dual storms, both of which are expected to make landfall at hurricane strength, “pose a challenge that, quite frankly, we’ve not seen before.”
“We don’t know exactly what to expect,” Mr. Edwards said during a briefing from a state operations center with a countdown clock for the two storms flashing on a screen above him.
The routine admonishment by officials for residents to prepare took on added urgency as the authorities said the looming second storm would probably hinder search and rescue efforts for the first one. Over the weekend, the concern intensified as forecasts showed that Louisiana was positioned to face a direct hit from both storms.
“Wherever you are at dark tonight is where you need to be prepared to ride out these storms,” Mr. Edwards said, noting that residents could have to shelter in place for as long as 72 hours. “This is a situation where you really need to be prepared.”
In Iberia Parish, on the central Louisiana coast, the local authorities beseeched residents living in low-lying areas prone to flooding and near bayous, lakes and drainage structures to get to higher ground.
“Those residents that wish to evacuate should begin that process immediately,” the parish president, M. Larry Richard, said in a statement that included warnings of wind and rain, storm surges and floods. “Residents who choose to remain behind may find themselves without power and unable to leave the area due to possible floodwaters and storm debris.”
Tropical storm conditions were expected from Laura through Monday across portions of the Dominican Republic, Haiti, Turks and Caicos, the southeastern Bahamas and Cuba.
Beginning on Sunday night and into Monday, tropical storm conditions are possible over the central Bahamas, Andros Island and in the Florida Keys from Tropical Storm Laura.
At least four people died during the storm, The Associated Press reported. Haitian civil protection officials said they also received reports that a 10-year-old girl died when a tree fell on a home.
On Sunday, nearly 33,000 customers remained without electricity, Gov. Wanda Vázquez Garced of Puerto Rico said at a news conference on Sunday.