Hurricane Nate is now expected to be a Category 2 hurricane when it makes landfall on the US central Gulf Coast in the coming hours.
The National Hurricane Centre in Miami said Nate's top sustained winds have recently risen to 90mph, and the core is now about 180 miles south-south-east of the mouth of the Mississippi River.
Nate is accelerating to 26mph and headed north-north-west on a course expected to take it onto the central Gulf Coast on Saturday night.
Forecasters said the hurricane-force winds extend out up to 35 miles, mainly to the east of the eye of the storm.
In addition to hurricane warnings and tropical storm warnings already in place along a wide stretch of Gulf Coast, a new tropical storm warning has been issued in the Florida Panhandle from east of the Okaloosa-Walton County line to Indian Pass, Florida.
Louisiana and Mississippi officials declared states of emergency, and Louisiana ordered some people to evacuate coastal areas and barrier islands ahead of Nate's expected landfall.
The National Hurricane Centre in Miami projected that Nate would brush by the south-east tip of Louisiana, near the mouth of the Mississippi River, around 7pm on Saturday (1am BST).
Louisiana Governor John Bel Edwards said he spoke with President Donald Trump on Saturday morning.
"He assured me that LA would have all the assistance we need as we prepare for #Nate," the governor posted on Twitter.
Mr Edwards thanked Mr Trump for approving Louisiana's pre-disaster emergency declaration for 17 south Louisiana parishes.
Nate has already killed at least 21 people in Central America.
A hurricane warning is in effect from Grand Isle, Louisiana, to the Alabama-Florida border and also for metropolitan New Orleans and nearby Lake Pontchartrain.
Tropical storm warnings extended west of Grand Isle to Morgan City, Louisiana, and around Lake Maurepas and east of the Alabama-Florida border to the Okaloosa-Walton County line in the Florida Panhandle.
In Louisiana, Mr Edwards mobilised 1,300 National Guard troops, some headed to New Orleans to monitor the fragile pumping system there.
With forecasts projecting landfall on the central Gulf Coast as a Category 1 hurricane, Mr Edwards urged residents to ready for rainfall, storm surge and severe winds - and to be where they intend to hunker down by the time it gets dark on Saturday.
Mr Edwards said forecasts for the fast-moving storm indicate the greatest threats are winds and storm surge.
The hurricane centre warned that Nate could raise sea levels by 4ft to 7ft from Morgan City, Louisiana, to the Alabama-Florida border. It had already had caused deadly flooding in much of Central America.
A White House statement said Mr Trump has authorised the Federal Emergency Management Agency and the Department of Homeland Security to co-ordinate all federal disaster relief efforts.
In New Orleans, the city's pumping system remains fragile but is working.
Two flash floods this summer led to revelations about personnel and equipment problems at the agency that runs the system that drains the city.
New Orleans Mayor Mitch Landrieu said 109 of its 120 pumps are functioning, which is 92% capacity.
"We are ready for whatever Nate brings our way," Mr Landrieu said of forecasts that Nate could dump 3-6in of rain on the region - with isolated totals of up to 10in.
Officials ordered the evacuation of part of coastal St Bernard Parish east of New Orleans ahead of the storm.
Earlier, a voluntary evacuation was called in the barrier island town of Grand Isle south of New Orleans.
On Alabama's Dauphin Island - a barrier island south of Mobile, Alabama - owners hauled boats out of the water ahead of the storm's approach.
The major concern was the storm surge was projected to coincide with high tide. Shelters were being open for coastal residents.
In neighbouring Mississippi, Governor Phil Bryant declared a state of emergency in six southernmost counties.
State officials warned storm surge was the biggest danger in that state's low-lying coastal areas, as well as high winds that could damage mobile homes.
The Mississippi Emergency Management Agency also announced the opening of shelters on the coast.