McALLEN, Texas — Dolly spun into a hurricane today, heading toward the America-Mexico border and the heavily populated Rio Grande Valley, where officials feared heavy rains could cause massive flooding and levee breaks.

Dolly was upgraded from a tropical storm this afternoon with sustained winds near 75 mph, and some strengthening of the Category 1 storm was forecast before landfall Wednesday. At 8 p.m. EDT, the storm’s center was about 130 miles east-southeast of Brownsville, moving northwest at about 11 mph.

A hurricane warning is in effect for the coast of Texas from Brownsville to Corpus Christi and in Mexico from Rio San Fernando northward.

In Mexico, the Tamaulipas Governor, Eugenio Hernandez, said officials planned to evacuate 23,000 people to government shelters in Matamoros, Soto La Marina, and San Fernando.

Texas officials urged residents to move away from the Rio Grande levees because if Dolly continues to follow the same path as 1967’s Hurricane Beulah, “the levees are not going to hold that much water,” the Cameron County Emergency Management Coordinator, Johnny Cavazos, said.

The first bands of rain began to pass over South Padre Island this afternoon and the surf continued to get rougher. Intermittent heavy downpours began moving through Brownsville by late afternoon. Forecasters predicted Dolly would dump between 15 and 20 inches of rain and bring coastal storm surge flooding of between 4 and 6 feet above normal high tide levels.

Tropical storm warnings were issued for areas adjacent to the hurricane zone, and Governor Perry declared 14 South Texas counties disasters, allowing state resources to be used to send equipment and emergency workers to areas in the storm’s path.

The storm, combined with levees that have deteriorated in the 41 years since Beulah swept up the Rio Grande, pose a major flooding threat to low-lying counties along the border. Beulah spawned more than 100 tornadoes across Texas and dumped 36 inches of rain in some parts of South Texas, killing 58 people and causing more than $1 billion damage.

“We could have a triple-decker problem here,” Mr. Cavazos told a meeting of more than 100 county and local officials today. “We believe that those (levees) will be breached if it continues on the same track. So please stay away from those levees.”

Around Brownsville, levees protect the historical downtown as well as preserved buildings that were formerly part of Fort Brown on the University of Texas at Brownsville campus. Outside the city, agricultural land dominates the banks of the Rio Grande, but thousands of people live in low-lying colonias, often poor subdivisions built without water and sewer utilities.

The International Boundary and Water Commission, which operates a series of levees, dams, and floodways in the lower Rio Grande Valley, put its personnel on standby alert. If needed, the IBWC will begin patrolling the levees around the clock looking for seepage and erosion, a spokeswoman, Sally Spener, said.

The IBWC made significant improvements to the levee system after Beulah and its studies showed that a 100-year flood in Cameron County would not top the levees, Ms. Spener said. Levees upstream at Hidalgo County are in the midst of improvements, but the river could spill over sections in a 100-year flood, a flood so big that it has only a 1% chance of happening in any given year.

Much of the damage to New Orleans from Hurricane Katrina was from levee breaks instead of wind.

Lines grew today at centers giving out sandbags in the Rio Grande Valley.

In inland Hidalgo County, officials called for volunteers at five shelters the county planned to open for fleeing coastal residents.

The Navy began flying 104 of its aircraft out of Naval Air Station Corpus Christi to bases inland. Other aircraft will be sheltered on base in hangars and no evacuation was planned.

Major Jose Rivera of the Texas Army National Guard said troops were preparing at armories at Houston, Austin, and San Antonio, after Mr. Perry called up 1,200 Guard members to help.

Immigration and Customs Enforcement was evacuating its Port Isabel Detention Center, a spokeswoman, Nina Pruneda, said. Fewer than 1,000 people were being sent to other detention centers in Texas.

In the Gulf of Mexico, Shell Oil evacuated workers from oil rigs, but said it didn’t expect production to be affected. It also secured wells and shut down production in the Rio Grande Valley, where it primarily deals in natural gas.

Residents of northern Mexico were taking the impending storm in stride.

A 62-year-old builder at Reynosa, Blas Garica, was taping up his windows and putting sandbags in front of his porch to prepare.

“I’m not afraid because we flood frequently around here,” he said. “If my house floods, we’ll just run to the roof.”

At Reynosa, restaurants, businesses, and maquilas, the import-export plants along the border, were also getting sandbags ready.

On South Padre Island, vacationers packed up their camps and headed for the mainland.

“We’re not taking any chances with these kids,” the director of the Lubavitch Camp Gan Israel, where about 40 children and staff were heading north to San Antonio, Rabbi Asher Hecht, said.

Just across the causeway in Port Isabel, Larry Haines pulled out the plywood for the first time in years, boarding up his waterside art gallery.

“We’re just worried about flying debris breaking through the windows,” Mr. Haines said. “We’re not too worried about storm surge and other things you get from a bigger storm, but we’re going to board up anyway.”

Other parts of Texas, stricken by drought, watched Dolly expectantly, with as much as 4 inches forecast to fall by the time the storm’s eastern edge sweeps across the region, said Texas A&M University’s John Nielsen-Gammon, the state’s climatologist.

About 20 counties in the northern part of South Texas – which includes San Antonio and nearby counties to the north, south and east – are behind in annual rainfall by between 12 and 16 inches, he said.

“If you get that much (rain) in two days there’d be flooding,” he said. “Weather never gives you ideal stuff. This is certainly not going to be an exception to that. The best to hope for from this is a temporary reprieve from the dry conditions.”