A tropical storm moving over the Gulf of Mexico is expected to grow into a hurricane, threatening the southern United States, including the coastlines of Texas and Louisiana, the National Hurricane Center (NHC) said.

Tropical Storm Francine is expected to travel over warm water that will serve as fuel to strengthen it before it makes landfall as a Category 1 hurricane on Wednesday with winds of 90mph (145km/h).

The storm is expected to stay just offshore of the coast of northeastern Mexico on Tuesday as it moves northeast towards the US.

Coastal residents are being warned of life-threatening storm surge of up to 10 feet (three metres) in places, as well as potential tornadoes and dangerous winds, forecasters said. Residents were encouraged to evacuate some low-lying areas.

As of Tuesday afternoon, Tropical Storm Francine was about 380 miles (610km) southwest of Morgan City, Louisiana, about 85 miles (136km) west of New Orleans. With maximum sustained wind speeds of 65mph (105km/h), it is 9mph (14.5 km/h) short of hurricane status.

Schools closed

The governor of Louisiana, Jeff Landry, urged residents “not to panic, but be prepared” and heed evacuation warnings. Some schools and colleges in the state closed on Tuesday through Wednesday as a precaution.

In Baton Rouge, Louisiana’s capital, long lines began forming as people filled petrol tanks and stocked up on groceries.

The Louisiana coast is still recovering from a battering in 2020 from hurricanes Laura and Delta, followed a year later by Hurricane Ida. The state recently marked the 19th anniversary of Hurricane Katrina, a major Category 5 storm that caused 1,392 deaths and damages estimated at almost $200bn.

US oil and gas producers on the Gulf of Mexico, including Exxon Mobil and Shell, had evacuated staff and in some cases paused drilling in preparation for the storm.

Warmer seas

Francine is the sixth storm to be named in 2024. Its arrival comes as meteorologists have been scratching their heads over a quiet August and early September, normally the peak of the annual hurricane season, which typically lasts from June until November.

Experts had predicted a more active than usual season this year due to a confluence of factors, including warmer than usual seas.

Water temperatures are about 31 degrees Celsius (87 degrees Fahrenheit) where Francine is located, said Brian McNoldy, senior research associate at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science.

“The ocean heat content averaged over the entire Gulf is the highest it’s been on record for the date,” McNoldy wrote on his blog.

Forecasters are also monitoring two other weather systems in the mid-Atlantic that have the potential to grow into larger storms.