The combination of heavy rain, "life-threatening" storm surges, flooding and strong winds could leave wide swaths of South Texas "uninhabitable for weeks or months," the National Weather Service in Houston said. Such daunting language hasn't been seen by CNN's experts since Hurricane Katrina, which left more than 1,800 people dead in 2005.
Harvey strengthened to a Category 3 storm midday Friday, with maximum sustained winds reaching 120 mph. The storm is expected to make landfall around Corpus Christi.
A hurricane warning is in effect for about 1.5 million people, with another 16 million under a tropical storm warning, the weather service said.
"Texas is about to have a very significant disaster," said Brock Long, director of the Federal Emergency Management Agency.
Gov. Greg Abbott warned of record-setting flooding and called on people to flee the area before the storm hits.
"My top goal is to be able to make it through this storm in a way in which we lose no lives," Abbott said. "Put your life first and your property second."
TRACK THE STORM
Residents were urged to evacuate. A mass exodus from the coast caused extensive traffic jams along the state's highways, while other people boarded up windows and stocked up on food and water ahead of the storm, the effects of which are expected to last for days.
After landfall, the storm will stall and dump rain on South Texas and parts of Louisiana into the middle of next week, forecasters predicted.
"All indications from the hurricane center are that this is going to be the first major hurricane the nation has dealt with since 2005," Long said.