In Puerto Rico, a U.S. territory with about 3.4 million inhabitants, Governor Ricardo Rossello urged residents to go to official shelters, saying, "It's time to act and look for a safe place if you live in flood-prone areas or in wooden or vulnerable structures."
Puerto Rico avoided a direct hit from Irma two weeks ago as that storm skirted north, although it still saw damage.
In the neighborhood of Miramar in Puerto Rico's capital, San Juan, Gerry Garraton said he was ready for the storm.
"I am prepared. I have water, I have cash, I boarded up my windows, I have gas. That's it," said Garraton, smoking a cigarette on the sidewalk near his home.
Garraton, 58, planned to be alone in his apartment in Miramar during the storm, and said he could stay there a few days, if needed.
Shelters in Puerto Rico have capacity for nearly 70,000 people, but Puerto Rico Housing Secretary Fernando Gil said he was concerned that only 299 people had taken refuge in official centers as of Tuesday morning.
Mary Luz, 43, and her daughter Summer Torres Varela, 23, who has epilepsy, went to the medical shelter at Puerto Rico Convention Center in Isla Grande, near old San Juan, on Tuesday. Torres Varela was vacationing in St. Thomas during Irma and experienced three seizures, Luz said.
Maria is the 13th named Atlantic storm of the year, the seventh hurricane so far this season and the fourth major hurricane - defined as Category 3 or higher - following Harvey, Irma and Jose, the NHC said. Those numbers are all above average for a typical season, which runs from June 1 to Nov. 30.