Tropical Storm Nate gained strength as it barrelled toward popular Mexican beach resorts and ultimately the US Gulf coast after dumping heavy rains on Central America that left at least 28 people dead.
Nate, which currently has winds of 95 kilometres (60 miles) per hour, is forecast to reach hurricane strength by the time it makes landfall in the United States late today or early tomorrow, on the north coast of the Gulf of Mexico.
New Orleans, which was ravaged by Hurricane Katrina in 2005, leaving hundreds dead, and other cities on the US Gulf coast were under hurricane watch.
The US National Hurricane Center (NHC) warned of possible "hurricane conditions" by last night on Mexico's Yucatan Peninsula, home to Cancun and other Caribbean resorts.
Governor Carlos Joaquin said the storm was expected to pass just east of the peninsula, but warned that torrential rains could trigger an emergency anyway.
Authorities cancelled school in seven coastal towns and declared an orange alert for the northern half of Quintana Roo state.
In the United States, the storm will bring "potentially life-threatening" storm surge of 1.2 to 2.1 meters (four to seven feet) in some coastal areas, according to Mark DeMaria of the NHC.
New Orleans issued a mandatory curfew for Saturday from 6:00 pm (2300 GMT), and mandatory and voluntary evacuation orders were issued for certain low-lying areas.
"Our greatest threat... Is not necessarily rain, but strong winds and storm surge," said New Orleans Mayor Mitch Landrieu.
Unlike Hurricane Harvey, which dumped record amounts of rain while hovering over neighbouring Texas for a week, fast- moving Nate was expected to quickly pass along a northerly path.
Louisiana Governor John Bel Edwards declared an emergency, and warned that while Nate was forecast to be a weak hurricane, it could still cause unexpected damage.
"Anyone in low-lying areas... We are urging them to prepare now," he said.
In the neighbouring state of Mississippi, lines formed at gas stations in areas within the potential path of the storm.
Some offshore oil and gas rigs in the Gulf of Mexico were evacuated ahead of the storm's advance.
Nate was still wreaking havoc in Central America, where heavy rains continued causing floods.
Authorities in El Salvador, Guatemala and Honduras have declared a maximum or red alert.
On Thursday, intense rains forced thousands from their homes, uprooted trees, knocked out bridges and turned roads into rivers across a swath of Central America.
"We were drowning. Thank God (emergency workers) helped us. The river swelled so much it swept away our house, our pigs, our chickens - it swept away everything," said Bonavide Velazquez, 60, who was evacuated from her home in southern Nicaragua.
Nicaragua bore 13 of the deaths, according to Vice President Rosario Murillo.
In Costa Rica, where a national emergency was declared, 10 people died, including a three-year-old girl, after they were hit by falling trees and mudslides. An alert was issued for people to be wary of crocodiles that might be roaming after rivers and estuaries flooded.
Three other people were killed in Honduras, and two in El Salvador.
More than 30 people are listed as missing across the region.
The United States is recovering from two major hurricanes: Harvey, which tore through Texas in August, and Irma in September.
Another powerful storm, Hurricane Maria, ripped through the Caribbean in late September, devastating several islands, including Dominica and Puerto Rico.
Central America, the Caribbean, Mexico and the southern United States suffer an Atlantic hurricane season every year that runs from June to November.