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 with sustained winds near 75 mph, and some strengthening of the Category 1 storm is forecast.

The National Hurricane Center forecast Tuesday that landfall would occur about noon today.

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

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 Johnny Cavazos, emergency management coordinator for Cameron County.

The first bands of rain began to pass over South Padre Island and Reynosa, Mexico, Tuesday afternoon, and the surf continued to get rougher.

Forecasters predicted Dolly would dump 15 to 20 inches of rain and bring coastal storm surge flooding of 4 to 6 feet above high tide levels.

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 killed 58 people and causing more than $1 billion in damage.

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

DAVID J. PHILLIP/AP

Cars leaving South Padre Island crowd the outgoing side of the bridge as Tropical Storm Dolly approaches the South Texas coast on Tuesday.

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"We could have a triple-decker problem here," Mr. 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."

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

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

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

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.

In Mexico, Tamaulipas Gov. Eugenio Hernandez said officials are planning to evacuate 23,000 people to government shelters.

Guard readied: Maj. Jose Rivera of the Texas Army National Guard said troops were preparing at armories in Houston, Austin and San Antonio. Gov. Rick Perry called up 1,200 troops to help and issued a disaster declaration in 14 South Texas counties.

Oil rigs: In the Gulf of Mexico, Shell Oil evacuated workers from oil rigs but said it didn't expect its production to be affected.

In Mexico:

From wire reports