Tropical Storm Fiona is expected to become a hurricane and could dump as much as 20 inches of rain on Puerto Rico beginning Saturday, the National Hurricane Center warned.

Driving the news: The meteorological agency issued a hurricane warning for the U.S. territory, where the impending flooding and high winds could imperil the island's power grid, which is still recovering from 2017's Hurricane Maria.

Catch up fast: Fiona has already dumped 17 inches of rain on the island of Guadeloupe and brought significant flooding to some eastern Caribbean islands.

• As of Saturday morning, the tropical storm was located about 130 miles southeast of St. Croix and was moving west. It’s on pace to pass near or over Puerto Rico on Sunday night.
• A Saturday afternoon updated from the NHC revealed no significant chances in the forecast for Puerto Rico, though the agency stressed the likelihood of "life threatening" flooding occurring as a result of the storm. The NHC also said the storm is expected to be a hurricane when it makes landfall sometime on Sunday.

Threat level: Forecasters are calling for 12 to 16 inches to fall in Puerto Rico with the potential for as many as 20 inches, particularly across eastern and southern swaths of the island.

• A hurricane watch is in effect for the southern coast of the Dominican Republic from Cabo Engano westward to Cabo Caucedo and its northern coast, from Cabo Engano westward to Puerto Plata. The island nation is forecast to get four to eight inches of rain, with up to a foot possible along its eastern coast.
• The U.S. Virgin Islands, also under a hurricane watch as of Saturday morning, are expected to receive four to six inches, with as much as 10 inches of rain possible.
• The deluge could produce flash and urban flooding, as well as mudslides along higher terrain, particularly in southern and eastern Puerto Rico and the eastern Dominican Republic. On the nearby island of Guadeloupe, 19.85 inches of rain has fallen since Fiona arrived.

Our thought bubble: From Axios’ Andrew Freedman: The National Hurricane Center raised the possibility of faster intensification prior to the storm’s arrival in Puerto Rico on Sunday, so it’s possible the storm could hit as a stronger Category 1 or even a Category 2 storm, depending on its development.

• Once the storm emerges over the southwestern Atlantic, it is likely to further intensify, and appears destined to curve out to sea, away from the East Coast. However, the ultimate track is still subject to considerable uncertainty, so the East Coast from Florida to the Carolinas in particular should keep a close watch.

Editor's note: This story has been updated with additional forecast information from the National Hurricane Center.