Emergency officials urged Florida residents to create emergency kits and be prepared for Hurricane Dorian as the storm pushed over open waters Thursday, unleashing heavy flooding along the eastern and southern coasts of Puerto Rico.

Jeff Byard, associate administrator with the Federal Emergency Management Agency, said the hurricane is expected to move inland and "create a lot of havoc with infrastructure, power and roads." He said the federal agency was already mobilizing disaster assistance teams and would move into areas hit by the storm.

The U.S. National Hurricane Centre (NHC) said the Category 2 storm is expected to further strengthen and hit the U.S. somewhere between the Florida Keys and southern Georgia as a powerful Category 4 on Monday.

"If it makes landfall as a Category 3 or 4 hurricane, that's a big deal," said University of Miami hurricane researcher Brian McNoldy. "A lot of people are going to be affected. A lot of insurance claims."

U.S. President Donald Trump cancelled his weekend trip to Poland and warned Florida residents to be prepared.

"All indications are it's going to hit very hard and it's going to be very big," Trump said in a video he tweeted Thursday evening, comparing Dorian to Hurricane Andrew, which devastated South Florida in 1992.

With the storm's track still unclear, no immediate mass evacuations were ordered.

At 11 p.m. ET, Dorian was 470 kilometres east northeast of the Bahamas with maximum sustained winds of 165 km/h. It was moving northwest at about 20 km/h. It was expected to continue on this path into Friday.

Here are the 11 PM AST/EDT August 29 Key Messages for Hurricane <a href="https://twitter.com/hashtag/Dorian?src=hash&ref_src=twsrc%5Etfw">#Dorian</a>. The risk of devastating winds, life-threatening storm surge, and heavy rains continues to increase across portions of the Bahamas and Florida. For more information, visit <a href="https://t.co/tW4KeFW0gB">https://t.co/tW4KeFW0gB</a>. <a href="https://t.co/Ft5jPlAbqX">pic.twitter.com/Ft5jPlAbqX</a> —@NHC_Atlantic

Forecast models show Dorian could have maximum sustained winds as high as 209 km/h as it approaches the Florida coast over the Labour Day weekend, putting it right at the threshold of Category 4 strength. Also imperilled were the Bahamas, with Dorian's forecast track running just to the north of Great Abaco and Grand Bahama islands.

Florida Gov. Ron DeSantis had declared a state of emergency for 26 counties on the east coast but extended it on Thursday to the whole state, and urged people to have a week's worth of supplies. Georgia Gov. Brian Kemp followed suit, declaring a state of emergency for 12 counties nearest the state's coastline.

Authorities cancelled some commercial flights, planned precautions at space rocket launch sites along the Florida coast, and prepared to give out sand to residents for sandbags ahead of the storm's arrival.

Across much of Florida's east coast, residents began flocking to grocery stores, stocking up in anticipation of the storm. There were lines at many gas stations in South Florida as people began filling gas cans and topping off their gas tanks. Some residents using community Facebook groups gave updates on new shipments of water to restock the nearly empty shelves at local grocery stores.

Josefine Larrauri, a retired translator, went to a Publix supermarket in Miami only to find empty shelves in the water section, and store employees were unsure of when new cases would arrive.

Larrauri fled to Orlando two years ago when Hurricane Irma was expected to hit South Florida as a Category 5 storm and ended up shifting west to land on the Lower Keys and then Marco Island. This time, she said the uncertainty made her nervous.

"I feel helpless because the whole coast is threatened," she said. "What's the use of going all the way to Georgia if it can land there?"

The weather forecast also forced the Rolling Stones to reschedule a concert that had been planned for Saturday in Miami Gardens. Publicist Amal Mokhtar said the band will instead play Friday night, and all tickets will be honoured for the new concert date, but there will be no opening act.

Kimberly Gaston, left, and Madelin Valdez fill sandbags in preparation for Hurricane Dorian on Aug. 29, 2019 in Casselberry, Fla. The National Hurricane Center is forecasting Dorian to strengthen into a Category 3 or greater storm as it heads toward Florida. (Gerardo Mora/Getty Images)

Puerto Rico largely spared

The storm was a Category 1 hurricane Wednesday when it swirled through the islands of the northeastern Caribbean, causing power outages and flooding in places.

The tail end of the storm left cars, homes and gravestones in the southeast coastal town of Humacao halfway submerged in water after officials say the banks of a river burst amid heavy rains late Wednesday afternoon. Meanwhile, flooding closed several roads in the southeast.

The National Weather Service in San Juan issued a flash flood warning for the area, as well as for the nearby islands of Vieques and Culebra.

But Puerto Rico seemed to be mostly spared, a huge relief on an island where blue tarps still cover some 30,000 homes nearly two years after Hurricane Maria. The island's 3.2 million inhabitants also depend on an unstable power grid that remains prone to outages since it was destroyed by Maria, a Category 4 storm.

Several hundred customers were without power across Puerto Rico, said Angel Figueroa, president of a union that represents power workers.

Residents fill their cars with gas after waiting in line ahead of the arrival of Hurricane Dorian in Kissimmee, Fla., on Thursday. (Gregg Newton/Reuters)

Police said an 80-year-old man in the northern town of Bayamon died Wednesday after he fell trying to climb up to his roof to clear it of debris ahead of the storm.

Before the storm, Trump sent a tweet assuring islanders that "FEMA and all others are ready, and will do a great job."

He then added a jab at Puerto Rican officials who have accused his administration of a slow and inadequate response to Hurricane Maria: "When they do, let them know it, and give them a big Thank You — Not like last time. That includes from the incompetent Mayor of San Juan!"

The mayor, Carmen Yulin Cruz, tweeted that Trump needs to "calm down get out of the way and make way for those of us who are actually doing the work on the ground," adding that maybe he "will understand this time around THIS IS NOT ABOUT HIM; THIS IS NOT ABOUT POLITICS; THIS IS ABOUT SAVING LIVES."