The National Hurricane Center says Dorian has become a hurricane as it nears the US Virgin Islands.
The storm is expected to pass over or near Puerto Rico, with landslides, widespread flooding and power outages possible. It's expected to be the first major test of emergency preparedness since Hurricane Maria devastated the island in 2017.
Puerto Rico's governor urged those living in flood-prone areas to move into one of the island's 360 shelters, and most airlines have canceled all flights in and out of San Juan as of Wednesday.
President Donald Trump declared an emergency for Puerto Rico Tuesday night and ordered federal assistance for local authorities.
Forecasters say the hurricane could strengthen further as it approaches the U.S. mainland.
A National Hurricane Center meteorologist says Dorian could hit the southeast coast as a Category 3 hurricane and could land anywhere from South Florida to South Carolina on Sunday or Monday.
In Florida, storm preps are underway.
Polk County residents are already filling sandbags in preparation for the storm.
"Our house, we have a lot of standing water right now near our garage," said Bo Farkas of Lakeland, Florida. "We just moved, so we have a lot of stuff in our garage so might be stacking it up there. Other than that, maybe the front door."