Hurricane Isaias has made landfall in the US, bringing winds of more than 100mph and the risk of floods, storm surges and tornadoes.
The category 2 storm reached land near Destin in Florida on Friday.
Dangerous conditions are expected throughout the western panhandle, southern Alabama and southern Mississippi, the National Hurricane Center (NHC) said.
More than 594,000 homes and businesses in Florida and Alabama were without power, according to poweroutage.com.
Winds of 105 mph have been recorded - with warnings of a tornado threat across parts of Alabama, Georgia, the Florida Panhandle and the Carolinas.
Roads in Destin were flooded, while in nearby Pensacola, police warned residents to stay off the streets because of debris across roadways.
Florida's National Guard deployed about 5,000 soldiers and airmen to help with the response, search and rescue teams were on high alert and some prisons evacuated inmates.
The state's Governor Ron DeSantis warned residents to expect power outages, fuel shortages and the closure of bridges and highways.
"The storm is here. Buckle down at this point. Don't be getting out on the roads," he said.
Several counties announced mandatory evacuations while Mobile, Alabama, imposed a curfew beginning Friday evening until Saturday.
Some panhandle counties temporarily suspended emergency response on Friday night due to dangerous conditions.
Flights out of several Panhandle airports, including all flights into and out of Destin-Fort Walton Beach Airport, were grounded due to the weather.
Donald Trump said the government were watching the hurricane's progress very closely and preparations were "under control".
As well as approving an emergency declaration for Alabama, he also scrapped his plans to attend Saturday's football game between the University of Georgia and the University of Alabama in Tuscaloosa.
Isaias was expected to miss the biggest oil hubs along the Gulf Coast, but fuel prices could rise if power outages stop refineries for processing crude, industry experts said.
As it moved inland, Isaias was later downgraded to Category 1 hurricane, which covers storms with winds of 74 to 95 mph.
The centre of the storm was about 50 miles east of Pensacola, Florida at about 10pm local time, and was moving around 19 miles an hour, the NHC said.
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Despite this, heavy rain is forecast as it moves inland, with 4 to 8 inches (10 to 20cm) or even more expected through the weekend from southern Alabama to southwestern Georgia.
Rain and potential flooding is forecasted for much of the southeast over the weekend, and then to parts of the Midwest and northeast into Monday.
In an indication of the storm's severity, known as the Waffle House Index, dozens of Waffle Houses in the Florida Panhandle were closed, according to the company's website.
The Southern breakfast chain is open 365 days a year, except for during extreme emergencies, making it an informal barometer for storm severity.
Meanwhile in the Pacific, Hurricane Simon became a Category 2 storm on Friday and is expected to strengthen further before making landfall in Mexico this weekend.
Tropical Storm Rachel is also expected to bring heavy winds and rain to northern Mexico and Southern California over the weekend.
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