McALLEN, Texas -- Forecasters said Dolly has become a hurricane and is heading toward southern Texas and northeastern Mexico.

Special Section: Hurricanes | Texas Town Preps For Dolly

Dolly became a Category 1 hurricane Tuesday and some strengthening is forecast before landfall.

At 5 p.m. the center of hurricane Dolly was located about 165 miles east-southeast of Brownsville, Texas.

Dolly is moving toward the northwest near 10 mph. This motion should bring the core of Dolly near northeastern Mexico or southern Texas on Wednesday.

Maximum sustained winds have increased to near 75 mph.

The National Hurricane Center issued a hurricane warning from Brownsville to Corpus Christi, Texas.

A tropical storm warning was issued from north of Corpus Christi to San Luis Pass, Texas. Brazoria and Matagorda counties are included in the warning.

Hurricane Dolly is expected to gain strength and strike between northern Mexico and Corpus Christi on Wednesday morning. The center of the forecast track is near Brownsville.

The storm has really started to get stronger here at the center," KPRC Meteorologist Anthony Yanez said. "The other thing we're seeing is the outer rain bands. All along, the greatest convection of the storm has been to the north of the dirty side of the system."

Forecasters said Dolly could bring so much rain that floodwaters will break through the levees.

They're urging people to move away from the levees. An emergency management official said if Dolly continues along the same path as Hurricane Beulah from 1967, the levees won't "hold that much water."

Dolly is expected to dump 15 to 20 inches of rain.

In the Rio Grande Valley, people have been waiting in lines to get sandbags.

People planning to ride out the storm have been picking up supplies -- including ice, water and batteries -- at a Wal-Mart about 15 miles from the Mexican border.

Meanwhile, Tropical Storm Cristobal continued moving out to sea. Forecasters said the storm was about 485 miles northeast of Cape Hatteras, N.C. and is 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.