BEAUMONT, Texas -- Residents in counties ravaged by Hurricane Rita three years ago began leaving Saturday as Gustav strengthened into a powerful Category 4 storm.

Authorities say a mandatory evacuation order is likely forthcoming.

Governor Rick Perry called on Texas to prepare for an estimated 45,000 evacuees fleeing Louisiana, where roads began clogging with traffic as a million people took to Gulf Coast highways.

Jefferson and Orange counties in Texas planned issuing an evacuation order at 6 a.m. Sunday, and officials urged people to pack up pets and 10 days worth of clothes and supplies in case Gustav delivered another Rita-like punch to the area.

The National Hurricane Center issued a hurricane watch Saturday stretching from east of High Island, located just south of Beaumont, to the Alabama-Florida border.

A hurricane watch means hurricane conditions are possible within the watch area, generally within 36 hours. Forecasters say Gustav is a powerful Atlantic storm taking aim at Louisiana and will likely become a Category 5 with sustained winds of 160 mph or more by Sunday.

Mandatory evacuations already began Saturday for special needs residents, such as those without transportation or medical needs, in Jefferson, Orange and Hardin counties. More than 100 buses and 200 ambulances had either arrived or were en route to transport residents inland.