HARLINGEN, Texas - Tropical Storm Erin formed Wednesday in the Gulf of Mexico and headed toward Texas, threatening to bring downpours to a state that already has had one of its rainiest summers on record.

Gov. Rick Perry ordered emergency vehicles and personnel, including National Guard troops, to the region in advance of the weather system.

"Because storms have saturated much of our state this summer, many communities in this storm's projected path are at high risk of dangerous flash flooding," Perry said in a statement.

Homeowners headed to hardware stores in the Rio Grande Valley for supplies to board up their houses, said Ruben Dimas, assistant store manager at a Home Depot in Harlingen.

"They know the drill, they're familiar with it," Dimas said.

The fifth depression of the Atlantic hurricane season formed late Tuesday and was upgraded to a tropical storm Wednesday when its maximum sustained speed was measured at 64 kph, the National Hurricane Center said. The threshold for tropical storm status is 62 kph.

At 11:30 a.m. EDT, the depression was centered 400 kilometres east of Brownsville, Texas, and about 466 kilometres east-northeast of La Pesca, Mexico, according to the National Hurricane Center in Miami. It was moving toward the west-northwest and was expected to continue following that track for at least 24 hours.

A tropical storm warning was posted for the Texas coast from Freeport, south of Houston, southward to the border. Mexico issued a tropical storm watch for its northwest coast from Rio San Fernando northward. A tropical storm watch means that tropical storm conditions are possible within 36 hours.

Eight to 20 centimetres of rain was possible along the middle Texas coast, the hurricane center said.

A series of storms earlier this summer poured record rainfall across Texas and parts of Oklahoma and Kansas, with Marble Falls in the Texas Hill Country getting as much as a foot. Flooding was widespread across all three states.

In the Pacific, meanwhile, Flossie was downgraded from a hurricane to a tropical storm after sideswiping Hawaii's Big Island with rain and heavy surf.

Flossie lost its hurricane status as its maximum sustained wind dropped below 119 kph. By 11 a.m. EDT, the storm's top sustained wind was down to 80 kph and it was expected to continue weakening. The storm was about 434 kilometres south of Honolulu, moving west at about 24 kph, and forecasters canceled a tropical storm warning for the Big Island.

Flossie delivered just 30 minutes of steady rain to the small town of Naalehu late Tuesday and there was light rain Wednesday morning at Hilo, but little significant wind as the storm spent most of its wrath at sea.

Elsewhere, Tropical Storm Dean got a little stronger but was far out in the Atlantic.

At 11 a.m. EDT, the storm was centered about 1,681 kilometres east of the Lesser Antilles in the Caribbean. Its top sustained wind speed had reached 96 kph, up from 64 kph on Tuesday. Some strengthening was expected within the next day and forecasters said it could become a hurricane by Friday. Hurricanes have sustained wind of at least 119 kph.

Dean was moving west at about 32 kph but meteorologists said it was too early to tell where the storm will go.

Hurricane specialists expect this year's Atlantic hurricane season to be busier than average. Last week, they said as many as 16 tropical storms are likely, with nine strengthening into hurricanes.

The season runs from June 1 to Nov. 30, but August typically is the start of the most active period. Ten tropical storms developed in the Atlantic last year, but only two made landfall in the United States.