The storm's center is expected to pass near, or just south of, Puerto Rico on Wednesday before moving near the Dominican Republic on Thursday, with sustained winds perhaps just below the 74 mph threshold for a hurricane, according to the National Hurricane Center.
Hurricane or not, Dorian will batter Puerto Rico with heavy rain and strong winds, and it won't take much of either to cause trouble for the island's brittle infrastructure, CNN meteorologist Chad Myers said.
"There's already so much damage on the ground from (Maria) that this isn't going to take a lot to make a significant amount of damage, especially flooding," Myers said.
"Some of these power lines are not held up by very much -- 70 mph would bring them back down," he said.
Puerto Rico Gov. Wanda Vázquez Garced on Monday declared a state of emergency for the island, and urged people to prepare for the storm.
"For citizens who do not yet have safe roofs, we will have shelters ready," Vázquez said on Twitter.
About 360 shelters are available across the island for a capacity of 48,500 people, the government's official Twitter account said Monday. And about 70 hospitals were prepared to handle emergencies, officials said.
Puerto Rico and eastern parts of the Dominican Republic are under a tropical storm warning and a hurricane watch, the hurricane center said.
A tropical storm warning means tropical storm conditions are expected within 36 hours, and a hurricane watch means that hurricane conditions are possible within 48 hours.
After passing Puerto Rico, Dorian is forecast to move near or over eastern Hispaniola on Thursday and move north, the hurricane center said.
By the end of the week, what's left of Dorian is expected to move toward the Bahamas and possibly into the southeastern parts of the United States. Forecasts show Dorian approaching the Florida peninsula Sunday as a tropical storm, but it's far too early to predict impacts there, CNN meteorologists said.