Hurricane Helene unleashed a “catastrophic storm surge and life-threatening winds” as it battered Florida‘s Big Bend region before making its way to Georgia.

More than four million people across five southeastern U.S. states were without power Friday morning, according to tracking website poweroutage.us.

Helene made landfall in northwestern Florida as a Category 4 storm Thursday night, before it weakened to a tropical storm over Georgia, where a flash flood emergency was in effect for metropolitan Atlanta.

At least 21 storm-related deaths have been reported in four U.S. states, according to The Associated Press.

Areas within the Big Bend region of Florida near Keaton Beach, Steinhatchee, and Horseshoe Beach had water levels reach more than 15 feet above ground level, according to a preliminary post-landfall modelling.

Hurricane Helene

In this aerial view, vehicles drive through a flooded street as Hurricane Helene churns offshore on Thursday in St. Pete Beach, Florida.

Photo by Joe Raedle/Getty Images
Hurricane Helene

A truck drives through a flooded street around the Sunset Park neighborhood as Hurricane Helene makes its way toward the Florida panhandle on Thursday in Tampa, Fla.

Jefferee Woo/Tampa Bay Times via AP
Hurricane Helene

Eric Midili, 45, examines and walks around a flooded street around the Sunset Park neighbourhood as Hurricane Helene makes its way toward the Florida panhandle on Thursday in Tampa, Fla.

Jefferee Woo/Tampa Bay Times via AP

Helena had already flooded Mexico’s Yucatan Peninsula and western Cuba earlier in the week.

The National Hurricane Center said Helene was producing “historic, catastrophic and life-threatening flash and urban flooding, including numerous significant landslides” across portions of the Southern Appalachians, which will continue into the evening.

“Widespread significant river flooding is ongoing, some of which will be major to record breaking,” the Center said in an update at 11 a.m. ET. 

The Center is also warning residents in portions of the southeast U.S. of the “possibility of long-duration power outages.”

Flash flood emergencies were also in effect for much of upstate South Carolina and western North Carolina.

Hurricane Helene

A sign closing down a road is posted around the Sunset Park neighbourhood as Hurricane Helene makes its way toward the Florida panhandle on Thursday in Tampa, Fla.

Jefferee Woo/Tampa Bay Times via AP
Hurricane Helene

Melvin Juarbe, right, attempts to assist an unidentified driver whose car stalled in floodwaters from Hurricane Helene on Thursday in Madeira Beach, Fla. The men tried to pull the car to dry land with their pickup truck but opted to call AAA after several failed attempts.

Max Chesnes/Tampa Bay Times via AP
Hurricane Helene

Guests at the Magic Kingdom break out ponchos at Cinderella Castle as bands of weather from Hurricane Helene move through Walt Disney World in Bay Lake, Fla., on Thursday. All four of Disney’s Florida theme parks remained open Thursday as the storm passed to the west in the Gulf of Mexico.

Joe Burbank/Orlando Sentinel via AP
Hurricane Helene

The St. Pete Pier is pictured among high winds and waves as Hurricane Helene makes its way toward the Florida panhandle, passing west of Tampa Bay, on Thursday in St. Petersburg, Fla.

Martha Asencio-Rhine/Tampa Bay Times via AP
Hurricane Helene

Flooded streets after the Hurricane Helene are seen in Madeira Beach, Fla., on Thursday.

Max Chesnes/Tampa Bay Times via AP
Hurricane Helene

Charles Starling, a lineman with Team Fishel, is pelted with rain as he walks by a row of electrical line trucks stage in a field in The Villages, Fla., on Thursday, in preparation for damage from Hurricane Helene.

Stephen M. Dowell/Orlando Sentinel via AP
Hurricane Helene

A man crosses a storm surge flooded area on the coast of Gulfport, Fla., as Hurricane Helene passed through the Gulf of Mexico to the West on Thursday.

Photo by Thomas Simonetti for The Washington Post via Getty Images

Meanwhile, the Canadian government is urging its citizens who are in the storm-affected areas to “exercise caution.”

“The storm brought excessive rainfall and violent winds,” the Canadian government’s travel advisory states.

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It also warned about potential for flash flooding and landslides that could disrupt essential services, like transportation, power distribution, water and food supply, telecommunications networks, emergency services and medical care.

Hurricane Helene

Residents fill sandbags at Helen Howarth Park ahead of the possible arrival of Hurricane Helene on Wednesday in Pinellas Park, Fla.

Photo by Joe Raedle/Getty Images
Hurricane Helene

Sharonda and Victor Davis, of Tallahassee, sit with their children Victoria background left, and Amaya, background right, inside a hurricane evacuation shelter at Fairview Middle School, ahead of Hurricane Helene in Leon County, Fla., on Thursday.

AP Photo/Gerald Herbert
Hurricane Helene

The Gulf of Mexico’s water pushes up against the beach as Hurricane Helene churns offshore on Thursday in St. Pete Beach, Fla.

Photo by Joe Raedle/Getty Images
Hurricane Helene

Vehicles move slowly around trees that have fallen after Hurricane Helene on Friday in Valdosta, Ga.

AP Photo/Mike Stewart
Hurricane Helene

Ronda Bell looks on after an Oak tree landed on her 100-year-old home after Hurricane Helene moved through on Friday in Valdosta, Ga.

AP Photo/Mike Stewart
Hurricane Helene

A damaged 100-year-old home is seen after an Oak tree landed on it after Hurricane Helene moved through the area on Friday in Valdosta, Ga.

AP Photo/Mike Stewart
Hurricane Helene

Jesus Hernandez guides his granddaughter Angelina via a container through a street flooded in the passing of Hurricane Helene, in Batabano, Mayabeque province, Cuba, on Thursday.

AP Photo/Ramon Espinosa
Hurricane Helene

A man in a kayak uses a makeshift paddle as he travels through a street flooded in the passing of Hurricane Helene, in Batabano, Mayabeque province, Cuba, on Thursday.

AP Photo/Ramon Espinosa
Hurricane Helene

A woman piggybacks on the back of a man as he wades through a street flooded in the passing of Hurricane Helene, in Batabano, Mayabeque province, Cuba, on Thursday.

AP Photo/Ramon Espinosa
Hurricane Helene

Residents wade through a street flooded in the passing of Hurricane Helene, in Batabano, Mayabeque province, Cuba, on Thursday.

AP Photo/Ramon Espinosa