Authorities passed out sandbags, evacuated inmates and opened emergency operations centers in a region still soaked from the remnants of Tropical Storm Erin, which caused severe flooding Sunday and at least 13 deaths from Texas to Minnesota.
Meanwhile, the federal government encouraged people in the southern part of the state to be ready for the worst.
"If I was a Texas resident, particularly along the southeast coast, I would make sure I was ready," R. David Paulison, the head of the Federal Emergency Management Agency, told reporters in Washington. "This is not a time to be complacent."
A state of emergency was declared in the resort town of South Padre Island. About 3,300 jail and prison inmates in the area were to be bused to correctional facilities elsewhere by Sunday night.
Paulison said up to 100,000 people might have to be evacuated from the state's southeastern coast and its immigrant shantytowns near the Mexican border. The storm is on course for northern Mexico, but could shift and hit the region around Brownsville, Texas, Paulison said.
Flooding from what was left of Erin forced about 1,000 people to evacuate homes in Abilene on Sunday and was blamed for at least 13 deaths in Texas, Oklahoma and Minnesota.
The level of preparation for Dean was influenced by memories of two destructive hurricanes that hammered the Gulf Coast region in 2005.
"In part, it is because of the unfortunate events from Rita and Katrina," Johnny Cavazos, the chief emergency director for Cameron County at the southern tip of the state.
During Rita, the evacuation quickly turned into a nightmare of clogged highways, stalled traffic and sweltering heat, as motorists from the coast ran into residents fleeing Houston. Gas stations ran out of fuel and supplies, and drivers sat for hours on gridlocked evacuation routes.
Dean was a Category Four storm late Sunday, threatening to pour as much as 20 inches (50 centimeters) of rain on Jamaica. The National Hurricane Center in Miami said it was projected to reach the most dangerous hurricane classification, Category 5, with wind of 160 mph (257 kilometer) before crashing into the Mexican coastline near Cancun on Monday night or Tuesday.
The storm was forecast to make landfall Wednesday, likely somewhere along the coast of northern Mexican or southern Texas, the hurricane center said.
Even if Mexico gets the brunt of the storm, Texas could still get soaked by Dean's outer bands of heavy rain, Cavazos said.
Texas Gov. Rick Perry mobilized the National Guard and search and rescue teams, shipped 60,000 to 80,000 barrels of gasoline to gas stations in the Rio Grande Valley, and got a pre-emptive federal disaster declaration from President George W. Bush.