Gaining strength with frightening speed, Hurricane Rita swirled toward the Gulf Coast a Category 5, 175-mph monster Wednesday as more than 1.3 million people in Texas and Louisiana were sent packing on orders from authorities who learned a bitter lesson from Katrina.

"It's scary. It's really scary," Shalonda Dunn said as she and her 5- and 9-year-old daughters waited to board a bus arranged by emergency authorities in Galveston. "I'm glad we've got the opportunity to leave. ... You never know what can happen."

With Rita projected to hit Texas by Saturday, Gov. Rick Perry urged residents along the state's entire coast to begin evacuating. And New Orleans braced for the possibility that the storm could swamp the misery-stricken city all over again.

Galveston, low-lying parts of Corpus Christi and Houston, and mostly emptied-out New Orleans were under mandatory evacuation orders as Rita sideswiped the Florida Keys and began drawing energy with terrifying efficiency from the warm waters of the Gulf of Mexico.

Forecasters said Rita could be the most intense hurricane on record ever to hit Texas, and easily one of the most powerful ever to plow into the U.S. mainland. Category 5 is the highest on the scale, and only three Category 5 hurricanes are known to have hit the U.S. mainland — most recently, Andrew, which smashed South Florida in 1992.

The U.S. mainland has never been hit by both a Category 4 and a Category 5 in the same season. Katrina, at one point became a Category 5 storm, weakened slightly to a Category 4 hurricane just before coming ashore.