The storm, which has already been blamed for one death, was about 105 kilometers south-southeast of the city of Ponce with sustained winds of 110 kilometers per hour, just shy of the 119 kph threshold for hurricane strength, the NHC said.
Torrential rains and mudslides were also forecast for the Dominican Republic as the storm progresses northwestward.
"Additional strengthening is forecast, and Fiona is expected to become a hurricane before reaching the southern or southwestern coast of Puerto Rico later today," the NHC said in its latest update on the storm at 8:00 a.m. (1200 GMT).
U.S. President Joe Biden approved an emergency declaration for Puerto Rico on Sunday, a move that authorizes the Federal Emergency Management Agency (FEMA) to coordinate disaster relief operations and provide emergency protective measures.
Puerto Rico, a Caribbean island of 3.3 million people, could be hit by up to 25 inches (63.5 cm) of rain.
"These rains will produce life-threatening flash flooding and urban flooding across Puerto Rico and the eastern Dominican Republic, along with mudslides and landslides in areas of higher terrain," the NHC said.
Puerto Rico's grid remains fragile after Hurricane Maria in September 2017 caused the largest blackout in U.S. history. In that category 5 storm, 1.5 million customers lost electricity with 80% of power lines knocked out.
Authorities have opened about 80 shelters and closed beaches and casinos, and residents were urged to seek shelter.
The one death reported so far from Fiona was in the French Caribbean island of Guadeloupe, after heavy rains swept away his house in the Basse-Terre district, authorities said on Saturday.
(Reporting by Ivelisse Rivera in San Juan; additional reporting by Nathan Layne in Wilton, Connecticut; editing by Grant McCool)
By Ivelisse Rivera
