Tropical Storm Harvey has intensified into a hurricane and forecasters said it would become a major hurricane to hit the middle Texas coastline.
Sustained winds reached 80mph more than a day before landfall was expected late on Friday between Port O'Connor and Matagorda Bay, a 30-mile stretch of coastline about 70 miles north-east of Corpus Christi.
Forecasters said a "life-threatening" storm surge along with rains and wind were likely as Harvey was intensifying faster than previously forecast.
A major hurricane means winds greater than 110mph.
As of midday on Thursday, Harvey was about 340 miles south-east of Corpus Christi, moving to the north-northwest at about 10mph.
Once Harvey makes landfall, it is possible the storm could then just stall inland for as many as three days, exacerbating the threat of flooding brought by tropical downpours, the National Hurricane Centre said.
Some forecasts indicated rain totals over several days extending into next week could exceed 24in.
Harvey would be the first major hurricane to hit Texas since Ike in September 2008 brought winds of 110mph to the Galveston and Houston areas and left damage costing 22 billion dollars (£17 billion).
Numerous cities were shipping in sandbags, extra water and other items ahead of the storm.
In South Padre Island, Dave Evans took advantage of the free sandbags, noting he and his fiance live in an older house prone to flooding.
"The master bedroom floods every time the rain gets very strong. I think our home is below sea level," Mr Evans told The Brownsville Herald.
Alex Garcia, of Corpus Christi, was buying bottled water, bread and other basics in Sugar Land, a Houston suburb, after dropping his daughter off at college in Houston.
The beer distributor salesman said grocery items were likely to be more available in Houston than back home, where he said shops were "crazy".
"We'll be selling lots of beer," he laughed.
Mr Garcia's house is about three miles from the bay.
While a seawall offers some protection, "it floods in Corpus all the time", he said.
"Harvey is looking like it's going to just meander around the area," National Weather Service meteorologist Penny Zabel said.
"It's going to hang out for a few days, and that's why we're looking at such high rainfall amounts."
Texas governor Greg Abbott has ordered the State Operations Centre to elevate its readiness level, making state resources available for possible rescue and recovery actions.
Mr Abbott also pre-emptively declared a state of disaster for 30 counties on or near the coast, to speed deployment of state resources to any areas affected.
Nearly all of the state's 367-mile coast was under a hurricane or tropical storm warning or watch as of Thursday.