The storm dropped 6 to 20 inches of rain in Puerto Rico and the U.S. Virgin Islands, with up to 30 inches of rain falling in southern and southeastern Puerto Rico. The rain caused rivers to rise over their banks and triggered rock and mudslides, according to officials with the Federal Emergency Management Agency.
As of early Thursday morning, the storm system was carrying maximum sustained winds of 130 miles per hour as it continued to move over open water, just west of Bermuda. Fiona's wind field has expanded, meaning stronger winds extend farther from the center of the storm than they did before, according to the National Weather Service.
The storm is forecast to move parallel to the eastern United States, passing between the East Coast and Bermuda late Thursday into early Friday.
The National Weather Service said Fiona is not expected to directly impact the mainland U.S., but the East Coast could see an increased rip current on beaches for the rest of the week and into the weekend. The storm itself will pass by Bermuda, where a tropical storm warning and a hurricane watch both remain in effect, but it is not forecast to directly hit the British island territory.
Amid flooding and with many people still without power, a heat advisory was issued for the northern and western part of Puerto Rico on Wednesday, where the temperature and humidity will make it feel like it's 100 to 112 degrees.
FEMA officials said during a press conference Tuesday that at least four people have died in Puerto Rico due to Fiona. A public health emergency was declared in the U.S. territory, and on Wednesday, President Joe Biden approved a disaster declaration for Puerto Rico.
On Monday, officials reported that one person was killed as the then-Category 1 storm slammed the island. The Arecibo resident was attempting to fill his generator with gasoline while it was on, causing an ignition, officials said.
No one was reported missing as of Tuesday afternoon, according to Steve Goldstein, the National Weather Service's liaison to FEMA.
FEMA officials were still assessing the extent of the damage in Puerto Rico, saying it is too early to estimate the financial impact of the storm.
Fiona made a second landfall on Monday in the Dominican Republic near the town of Boca de Yuma, along the eastern side of the island, with sustained winds of 90 mph and even higher gusts.
Puerto Rico Gov. Pedro Pierluisi warned residents that more rain was expected on the island through Tuesday evening.
"We are going through a difficult moment but our people are strong and very generous," Pierluisi said at a press conference.
Four helicopters were in the air surveying damage from Fiona on Tuesday. The governor said it would take at least a week to determine the extent of the damage left by the storm.
In addition to the four deaths cited by FEMA, at least two other people died in a shelter due to natural causes, but those have not been labeled as storm-related, according to Pierluisi.
Restoring power in Puerto Rico
LUMA Energy said that only 376,000 out of 1.5 million clients have had power restored on the island as of Wednesday morning, with more expected in the coming days.
“We assure you that a large part of Puerto Rico will have electricity today and tomorrow,” Abner Gomez, spokesperson for LUMA Energy, said during a press conference Tuesday.
In an update Tuesday afternoon, FEMA said that 80% of customers still remain without power.
The governor said Monday the goal is for "a large number of LUMA customers" to have power "in a matter of days." However, LUMA said in a statement Sunday that "full power restoration could take several days."
Hospitals on the island are currently operating on generators, according to the governor.
Only 34% of households on the island have potable water after rivers grew and heavy rainfall impacted the system -- meaning more than 834,000 people are without drinking water, the governor said Monday.
More than 1,000 people have been rescued by authorities, including a woman rescued Sunday who was stuck in a tree for seven hours after trying to look at the damage, officials said.
Heavy rainfall causes flooding across Puerto Rico
Fiona strengthened to a hurricane from a tropical storm Sunday morning. The National Hurricane Center said Fiona made landfall in southwestern Puerto Rico on Sunday at 3:20 p.m. ET, dumping torrential rain on much of the island.
Some regions measured up to 25 inches of rain by 8 a.m. Monday.
A flash flood emergency was issued due to many rivers rising very quickly out of their banks. The Rio Grande de Arecido river rose 13 feet in one hour.
A bridge near Utuado, a town in the central mountainous region of the island, has collapsed, cutting off the communities of Salto Arriba and Guaonico, local newspaper El Vocero de Puerto Rico reported.
The portion of the bridge that collapsed is on Highway 123, a branch of Highway 10, which serves as a link between both roads and is one of the accesses to the University of Puerto Rico at Utuado campus, according to El Vocero.
The bridge, installed by the National Guard following Hurricane Maria, cost about $3 million to construct, the newspaper reported.
The rain saturated areas in the southeastern part of Puerto Rico, along with the mountainous areas, where potential mudslides could cause the most damage.
Prior to landfall, Pierluisi said Puerto Rico was prepared as it could be, with enough resources and manpower in place to respond -- adding that the island learned its lessons from the devastating effects of Hurricane Maria in September 2017.
"We're much in a much better position than we were five years ago," he said.
Where Fiona heads next
The storm system is now over open water as it continues to head northward, northeast of Turks and Caicos and just west of Bermuda. Fiona is expected to track near Bermuda, but forecasts show the island will not see a direct hit. The storm system will continue to gradually strengthen in the coming days as it moves north and then northeast this week.
The Dominican Republic was expected to receive up to 10 inches of rain and some regions in Turks and Caicos expected to see 8 inches of rain.
Winds could be as strong as 140 mph as Fiona passes near Bermuda, bringing even higher gusts, heavy rain and storm surge. The latest model shows the worst of the storm will pass just west of Bermuda, but Fiona is powerful and large enough that at least tropical storm force winds will be felt on the island.
Tropical storm conditions are expected to begin Thursday morning, just after sunset, while hurricane conditions are possible late Thursday into early Friday, depending on how closely Fiona passes by the island.
The U.S. State Department has issued a travel advisory for Bermuda due to the approaching storm.
While it won't make landfall in the U.S., the hurricane will affect the entire East Coast with huge waves, rip currents and coastal flooding from Florida to Maine as it moves northward. Fiona is expected to head north all the way to Canada. It will begin to transition to a post-tropical storm as it heads north.
Fiona's wind field is expected to continue expanding as the storm gains latitude and eventually impacts coastal Canada as a post-tropical low. The storm is forecast to make landfall in Nova Scotia on Saturday morning, after losing its tropical characteristics. However, a widespread area of powerful, gusty winds will span across Nova Scotia, Newfoundland and New Brunswick on Saturday morning.
U.S. President Joe Biden approved an emergency declaration for Puerto Rico on Sunday, which allows federal agencies to coordinate all relief efforts.
Biden's decision has the "purpose of alleviating the hardship and suffering caused by the emergency on the local population, and to provide appropriate assistance for required emergency measures, authorized under Title V of the Stafford Act, to save lives and to protect property and public health and safety, and to lessen or avert the threat of a catastrophe in all 78 municipalities in the Commonwealth of Puerto Rico," the White House said in a statement.
FEMA Administrator Deanna Criswell arrived in Puerto Rico on Tuesday to coordinate the emergency response, the White House said. "Hundreds" of federal responders are already on the island, including members of the U.S. Army Corps of Engineers.
Fiona leaves 1 dead in Guadeloupe
While still a tropical storm, the system battered other Caribbean islands. One person died in the French territory of Guadeloupe, according to The Associated Press. More than 20 others were rescued amid heavy wind and rain according to the AP.
Fiona's center moved through the island of Guadeloupe on Friday night, bringing heavy rain and gusty winds across the Leeward Islands.
The island's emergency management office in Puerto Rico even had a blackout during its Saturday morning press conference. Pierluisi reiterated during that press briefing Saturday evening that the fear is that heavy rains will produce mudslides.
LUMA Energy's Crisis Management Manager Abner Gomez told reporters over the weekend the energy distributor was working to prevent a repeat of Hurricane Maria's aftermath.
"We are going to make sure [a widespread outage] will not happen because we have the crews," he said. "There will be damage. There will be outages and we will be ready to respond."
ABC News' Daniel Amarante, Rachel DeLima, Kenton Gewecke, Max Golembo, Daniel Peck and Molly Nagle contributed to this report.