McALLEN, Texas—Dolly spun into a hurricane Tuesday, heading toward the U.S.-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 Tuesday afternoon, and sustained winds later reached about 80 mph. Some strengthening of the Category 1 storm was forecast before landfall Wednesday. At 11 p.m. EDT, the storm’s center was about 110 miles east-southeast of Brownsville, moving northwest at about 9 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, Tamaulipas Gov. 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,” said Cameron County Emergency Management Coordinator Johnny Cavazos.

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

Tropical storm warnings were issued for areas adjacent to the hurricane zone, and Gov. Rick 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,” Cavazos told a meeting of more than 100 county and local officials Tuesday. “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, said spokeswoman Sally Spener.

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, Spener said. Levees upstream in 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 percent 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 Tuesday 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.