McALLEN, Texas — Texas mobilized National Guard troops and residents along the Gulf Coast near the Mexican border were buying plywood, flashlights and gasoline as Tropical Storm Dolly gained strength Tuesday over the Gulf on its way to becoming a hurricane before it hits land.

Hurricane warnings were in effect for parts of the Texas and Mexico coasts, meaning hurricane conditions were expected in those areas by the end of Tuesday.

Forecasters at the National Hurricane Center in Miami said Dolly's winds were expected to strengthen before landfall to hurricane force, which would mean at least 74 mph.

At 11 a.m. EDT, data from a NOAA plane indicated maximum sustained winds had increased to near 70 mph with higher gusts.

Dolly was expected to make landfall later this week and bring with it high winds and up to 15 inches of rain and coastal storm surge flooding of 4 to 6 feet above normal high tide levels.

Emergency officials feared major flooding problems and urged coastal residents to prepare. Gov. Rick Perry activated 1,200 National Guard troops and other emergency crews and Shell Oil said it was evacuating workers from oil rigs in the western Gulf of Mexico. Shell said it didn't expect its production to be affected by the storm.

Business was brisk Tuesday morning at a Wal-Mart in Edinburg, 15 miles from the Mexican border, but didn't have the wall-to-wall shoppers like the night before. Kerri Urdaz, 31, of McAllen loaded ice, water and batteries into her car, while her 2-year-old daughter Claire watched from the shopping cart.

"It wasn't too bad," Urdaz said of the last-minute shopping. "That's why we woke up and came in early before the rush."

Urdaz said they're clearing everything that's loose out of the yard at home.

"We're just expecting lots of rain."

Jesus Gil was lifting large coolers into the back of his pickup truck and had bought flashlights and batteries, bracing for the storm at both work and home.

"I'm just trying to be prepared," said Gil, who was in Houston for the Hurricane Rita evacuation. He doesn't plan to leave this time, but bought extra gas just in case.

In Hidalgo County, the call went out for volunteers to man shelters that were scheduled to open Tuesday afternoon for residents of neighboring coastal counties.

Even as far up the coast as the Houston area, Harris County officials told residents to be ready in case the storm changes course and heads their way.

The National Hurricane Center issued a hurricane warning from Brownsville north to Port O'Connor. Meanwhile, a tropical storm warning was issued from north of Port O'Connor to the San Luis Pass, a strait south of Galveston.

Mexico also announced a hurricane warning from Rio San Fernando north to the U.S. border. A tropical storm warning and a hurricane watch were also in effect from La Pesca to Rio San Fernando.

Forecasters said Dolly was expected to make landfall late Tuesday or early Wednesday as a Category 1 hurricane, which has with sustained winds of 74 mph to 95 mph.

Texas officials said they wouldn't order evacuations along the coast unless Dolly strengthens to a Category 3, with sustained winds of at least 111 mph.

At 11 a.m. EDT Tuesday, the center of Tropical Storm Dolly was located over the Gulf about 230 miles southeast of Brownsville. It was moving west-northwest at about 12 mph. Tropical storm-force winds extended up to 160 miles.

There are about 2 million people in the Rio Grande Valley, which includes popular summer beach resort South Padre Island. Officials readied to evacuate residents in flood-prone areas and urged RV owners on South Padre to head for higher ground.

"That amount of rain will present a big flooding problem for us," said Cameron County Emergency Management Coordinator Johnny Cavazos.

Mindful of the disastrous evacuation before Hurricane Rita hit the Texas Gulf Coast in 2005 — when far more people died from heat-related injuries and auto accidents fleeing the storm than from the severe weather — Perry also ordered 250 buses to be staged in San Antonio. He also ordered fuel teams to be ready to keep gas stations supplied and to help stranded motorists.

Mexican border towns near the Gulf coast were setting up shelters for those who want to leave areas that flood easily. Soldiers were also being sent into Matamoros to protect against looting, in case the storm does strike the Mexican border town on the Gulf Coast, forcing many residents to flee.

Even as far up the coast as around Houston, Harris County officials told residents to be ready in case the storm changes course and heads their way. Harris County Judge Ed Emmett asked residents of the state's most populous county to keep their gas tanks full, stock up on supplies and make sure they have plans ready to either evacuate or ride out a storm.

The federal government was trying to decide whether they could begin construction on a new border fence, which was to be combined with levee improvements along the Rio Grande in Hidalgo County. While project supervisors met with emergency officials about the storm, large cranes unloaded steel beams and other supplies at a staging area near the levee Monday. Concrete walls will be incorporated into the river side of the levees to keep floodwaters, illegal immigrants and smugglers out.

The county is upgrading other levees and informed contractors Monday they should activate plans to prevent flooding, said Godfrey Garza, head of Hidalgo County Drainage District 1.

Meanwhile, Tropical Storm Cristobal was moving toward the northeast at about 21 mph, away from the U.S. Cristobal was located about 485 miles northeast of Cape Hatteras, N.C., with maximum sustained winds near 60 mph. Forecasters said the storm, which dumped rain on the coast of the Carolinas, was no longer an immediate threat to the U.S.

In the Pacific, Tropical Storm Genevieve strengthened slightly off Mexico's coast, but forecasters said the storm was not expected to threaten land. Tropical Storm Fausto, which had been a hurricane, also was weakening and moving out to sea.