The U.S. National Hurricane Center said Maria had maximum sustained winds of 175 kph Monday morning.
"The storm compounds already existing problems on the island", said Deepak Lamba-Nieves, research director at the Center for a New Economy in San Juan.
"We are planning for the unexpected, we are planning for the worst", said Chris Austin, head of a United Kingdom military task force set up to deal with Irma, as the British Virgin Islands readied for a new onslaught.
Maria's strength diminished slightly to a Category 4 storm after making landfall, but meteorologists increased it back to a Category 5 on Tuesday morning.
After passing the Dominican Republic Maria is forecast to turn north toward the Bahamas, missing both Cuba and Florida.
Hurricane warnings have also been issued for the USA and British Virgin Islands, Puerto Rico, St Kitts, Nevis and Montserrat. Those islands are likely to see hurricane conditions within the next 36 hours. A Tropical Storm Warning is in place for the Outer Banks.
Dominica has faced a lot of damage and devastation in the wake of Hurricane Maria.
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Mapp warned that hurricane-force winds were expected to howl across St. Croix for eight hours, accompanied by up to a foot and a half (46 cm) of rain that would be followed by almost a week of additional showers.
The National Hurricane Center has a page updating every few hours with the latest watches and warnings for Maria.
Dominica is bracing for what could be a direct hit from the storm.
"This storm promises to be catastrophic for our island", he said.
Harvey was followed by Irma, the most intense storm in the Atlantic.
It hit the island of Dominica with 160 miles per hour maximum sustained winds. Certain islands hit hard by Irma, like St. Thomas and St. John, appear to be in Maria's path. "You will not survive". As many as 20 inches of rain are possible across the region, and storm surge could reach as high as 9 feet.
The eye of the potentially catastrophic category 5 hurricane and its intense inner core is nearing Dominica. There have been 13 named storms so far - the average by September 18 is 7.6. As a major hurricane, Maria will push those measures even further ahead. One of the worst hit islands, Barbuda, had to completely evacuate for the first time ever.