Dolly became a hurricane on Tuesday and bore down on southern Texas, but forecasters don't expect it to pack too much of a punch when it comes ashore near the Mexican border on Wednesday.

Dolly intensified from a tropical storm and became the second hurricane of 2008 Atlantic storm season after gathering strength over the warm waters of the Gulf of Mexico and reaching nearly 80 miles per hour (130 km per hour).

The U.S. National Hurricane Center issued a hurricane watch for the southern Texas coast as far north as Corpus Christi.

At 11 p.m. EDT (4 a.m. British time), Dolly was 110 miles (175 km) east-southeast of the border town of Brownsville, Texas, where it was due to come ashore on Wednesday afternoon as a low-grade hurricane. Tropical-force winds will begin to lash the region's coast this evening.

The storm's predicted landfall and strength are unlikely to jeopardize sensitive offshore drilling rigs and production platforms in the U.S. and Mexican waters of the Gulf of Mexico. U.S. crude oil prices hit 6-month lows before recovering.

The National Hurricane Center has said Dolly is unlikely to become a major hurricane prior to landfall, but could dump as much as 15 inches (38 cm) of rain on South Texas and northeastern Mexico in coming days.

Though Dolly will be the weakest category of hurricane, officials in low-lying South Texas counties are worried that torrential rains could overcome levees holding back the Rio Grande River and cause flooding.

The United States has largely escaped the past two Atlantic hurricane seasons, with just one hurricane - Humberto in November 2007 - making landfall on its coasts.

But it was pummelled in 2004 and 2005, when a series of powerful hurricanes, including the catastrophic Katrina, ravaged Florida and the U.S. Gulf Coast.

AHEAD OF SCHEDULE

The 2008 Atlantic hurricane season is already a month ahead of schedule, but is unlikely to see a repeat of the devastating 2005 season. On average, the fourth tropical storm of the six-month season does not occur until August 29. Dolly, this year's fourth, formed on July 20.

"I definitely think that the activity we have seen so far this year is a harbinger of things to come," said Jeff Masters, co-founder of meteorological Web site The Weatherunderground.

In Texas, Gov. Rick Perry put 1,200 National Guard troops on alert and issued a disaster declaration for 14 low-lying counties in the southern part of the state, although no mandatory evacuations were ordered.

Some 250 buses stood by in the inland city of San Antonio to evacuate coastal residents if needed.

In the Rio Grande Valley, Veronica Mascor was one of hundreds of volunteers filling sandbags in an attempt to keep the flooded Rio Grande out of nearby neighbourhoods.

"They gave us 20 each," Mascor said. "I hope it will be enough."

Brownsville residents scurried to hammer plywood over windows and shore up their homes.

"I'm worried," said Angel Rivera, who was boarding up his home. "It's a wooden house. Can't take any flooding."

In the Mexican city of Matamoros, across the border from Brownsville, authorities evacuated 23,000 people from flood-prone areas and urged residents to seek temporary shelter.

"If they wish to leave for a shelter, we will gladly transport them," said Saul Hernandez, head of civil protection.

At Mexico's Playa Bagdad beach near the U.S. border, police put up road blocks to keep people away from increasing choppy waters.

Meanwhile, Tropical Storm Cristobal strengthened on Tuesday as it sped away from the U.S. East Coast but forecasters said it was likely a "last hurrah" before the third named storm of the 2008 hurricane season faded over cool Atlantic waters.

By 11 p.m. EDT (4 a.m. British time) Cristobal was around 230 miles (370 km) east of Halifax, Nova Scotia, and moving northeast over open waters at 35 mph (56 kph), the Hurricane Center said. Top sustained winds were near 50 mph (85 kph).