A weakened but still powerful Hurricane Rita altered its course Thursday to threaten residents in southwest Louisiana and southeast Texas, where highways were overloaded with frustrated evacuees.

Max Mayfield, director of the National Hurricane Center in Miami, Florida, said Rita will hit a deep trench of warm water and could intensify overnight, possibly to Category 5 status.

Mayfield said forecasters expect Rita to make landfall Saturday morning likely between Galveston, Texas, and the Louisiana border.

"This is still a very, very dangerous Category 4 hurricane. We'll likely see some fluctuations. ... but the wise thing to do is to go ahead and plan for a Category 4 hurricane making landfall," Mayfield said.

With the hurricane generating winds of tropical storm force extending 205 miles (335 kilometers) from the center, some areas in Louisiana already were feeling Rita's outer bands. (Watch a report on the science behind monster storms -- 3:50)

Highways in Texas were crammed with cars creeping north, the blistering heat adding to the discomfort of the hundreds of thousands fleeing the storm.