Tropical Storm Fiona formed into a Category 1 hurricane Sunday as it continues to approach landfall on Puerto Rico.

Fiona first formed as a tropical storm Thursday, and experts say the storm is likely to strengthen further over the next few days before it hits the island. President Joe Biden approved an emergency declaration for the island ahead of the storm, opening up federal resources to both prepare for and respond to damage.

More than 660,000 Puerto Ricans were without power as of Sunday afternoon, compared to just 82,000 earlier in the morning.

Fiona is the season's sixth named storm and third hurricane, and it has sustained winds of 80 mph and gusts of up to 100 mph, according to forecasters.

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The Puerto Rican government is warning of flash floods and mudslides as the storm heats up.

When first detected, Fiona was located about 580 miles east of the Leeward Islands and moving at 12 mph with maximum sustained winds of about 50 mph, according to the Associated Press.