Updated at 5:30 a.m. ET Wednesday
Even though Maria has weakened to a Category 4 storm, it remains a dangerous hurricane. Maria's maximum sustained winds are near 155 mph. The National Hurricane Center says the storm should keep that intensity until it makes landfall. Puerto Rico has long been spared from a direct hit by a hurricane.
Updated at 2:20 a.m. ET Wednesday
The National Hurricane Center says Maria is on track to reach the southeastern coast of Puerto Rico Wednesday morning. Forecasters say it will remain a Category 5 storm until landfall. According to the center's 2:00 a.m. ET advisory, the hurricane is about 85 miles southeast of San Juan, Puerto Rico, packing maximum sustained winds of 165 miles per hour.
Updated at 11:10 p.m. ET
Hurricane Maria is about to hit St. Croix and then move on to Puerto Rico as a "potentially catastrophic" Category 5 hurricane.
"The eye of Maria will move near or over St. Croix in the U.S. Virgin Islands within the next couple of hours, then cross Puerto Rico on Wednesday," according to the National Hurricane Center's 11 p.m. ET advisory, "bringing life-threatening wind, storm surge, and rainfall impacts to portions of those islands."
The hurricane's maximum sustained winds are near 175 mph. "Preparations to protect life and property should be rushed to completion," the notice says.'
On the island of Dominica, which was raked by the storm late Monday, the prime minister says that "initial reports are of widespread devastation. So far, we have lost all what money can buy and replace."
In a series of posts on Facebook, Roosevelt Skerrit, the prime minister of Dominica, described the devastation to his own home.
"We do not know what is happening outside. We dare not look out," he wrote Monday evening. "Certainly no sleep for anyone in Dominica. I believe my residence may have sustained some damage."
Minutes later, the prime minister posted, "Rough! Rough! Rough!"
Then he wrote, "My roof is gone. I am at the complete mercy of the hurricane. House is flooding."
Skerrit later said he had been rescued. But the fact that the island nation's prime minister could see the roof fly off his own home highlighted the risks for the people living in the storm's path.
"So, far the winds have swept away the roofs of almost every person I have spoken to or otherwise made contact with," he wrote. "The roof to my own official residence was among the first to go and this apparently triggered an avalanche of torn away roofs in the city and the countryside."
The National Hurricane Center says hurricane warnings are in effect for the British and U.S. Virgin Islands, Puerto Rico, parts of the Dominican Republic and Puerto Rico's Vieques and Culebra.
The storm's strength has fluctuated over the past day. That is likely to continue, forecasters say. However, the NHC adds, "Maria is forecast to remain an extremely dangerous Category 4 or 5 hurricane until it moves near or over the Virgin Islands and Puerto Rico."
As of the NHC's 11 p.m. advisory, Maria was located about 30 miles south-southeast of St. Croix, moving west-northwest at 10 miles per hour.
It says Puerto Rico and the U.S. Virgin Islands could see water levels rise 6 to 9 feet, "if the peak surge occurs at the time of high tide." The Leeward Islands and the British Virgin Islands could see water levels rise "by as much as 7 to 11 feet above normal tide levels."
That is expected to be accompanied by rainfall that "will cause life-threatening flash floods and mudslides."
Maria is following a different projected path than Hurricane Irma did through the same region, as NPR's Bill Chappell reported. That means "Puerto Rico and other islands that suffered only glancing blows from Irma could now be directly confronted with hurricane conditions."
Authorities in Puerto Rico are calling for "people in wooden or flimsy homes" to find shelter, The Associated Press reports.
"You have to evacuate. Otherwise, you're going to die," said Puerto Rico's public safety commissioner, Hector Pesquera, according to the news service. "I don't know how to make this any clearer."
Late Monday, President Trump approved emergency declarations for Puerto Rico and the U.S. Virgin Islands and authorized federal assistance and disaster relief efforts.
Puerto Rico's governor, Ricardo Rosselló, said 500 shelters were prepared to take families who evacuated. An interactive map released by the government shows areas that are prone to flooding as well as the locations of shelters across the island.
As NPR's Colin Dwyer has reported, the U.S. territory is "already reeling from billions in debt" and is in the middle of a debt-restructuring process. The economic situation could make relief efforts that much more difficult.
On islands already devastated by Irma, there are fears that a second hit could make a dangerous situation even worse by kicking up piles of debris. BuzzFeed U.K.'s Jim Waterson, reporting from the British Virgin Islands, tells NPR:
"There is so much stuff on the island. There's nowhere to put it, it's just piled up by the sides of roads. ... And if Maria came in and picked up shrapnel, essentially, picked up pieces of glass, picked up poles, this could be somehow more dangerous than the initial hit. Because at least then everything started off in one piece and was worn down. This time everything's already broken up."
The U.K. military has sent reinforcements to the islands. But Waterson says, "There's not much they can do right now. It's just a case of waiting for the storm to pass and picking up the pieces afterwards."