Transcript for Several towns evacuating before the expected Category 3 hurricane
with that hurricane rapidly growing in size and strength. Barrelling through the gulf and headed toward the U.S. Tonight. Harvey becoming a category 1 hurricane just this afternoon. 85 mile-an-hour winds already. And still growing. Expected to be a category 3 when it hits. Urgent warnings going out. Evacuations under way at this hour. Flights already being canceled. Families stocking up for what could be nearly 3 feet of rain, and if the forecast is right, it would be the first category 3 to make landfall in the U.S. In more than a decade. Chief meteorologist ginger zee has been in the news room with us all afternoon tracking this, and ginger, where is Harvey right now? It's less than 300 miles southeast of Corpus Christi. We go to the maps to see it moving. Moving at 10 miles per hour in the next 24 hours. We'll see again, rapid intensification, and that's why the track takes it into that hurricane-warned area as a category 3. Major hurricane. Those hurricane watches and warnings have been up and now you see the track taking it through the weekend, staying at category 2, through the afternoon on Saturday, making a u-turn through the gulf of Mexico, and then going north and east. We could be talking about the storm through the next midweek or beyond. That's why the 35 inches is up, and it's a 10 plus region, and why rainfall and flooding are the number one impact as far as life and property loss, and then you see storm surge, up to 20 feet in the wind warning. They are saying low cases could be uninhabitable for weeks or months. It's the rain. Up to 35 inches of rain they are talking about. Correct. Days on end. Ginger, thanks. Texas in the path tonight. The first significant rains begin tomorrow morning. Outer bands already being seen tonight. Evacuations under way. Also a state of emergency in parts of Louisiana at this hour. ABC's senior national correspondent, Matt Gutman, is already in the storm zone. Reporter: With hurricane Harvey's outer bands already gouging the coastline, the gulf coast is in a frenzy of hammering. Sandbagging and fleeing, a mass exodus now underway in several Texas towns including port aransas where there's a mandatory evacuation tonight. The storm whipping itself from a tropical depression into a full blown hurricane in just a few hours. Triggering alarm bells from Corpus Christi, Texas to new Orleans, and some very stark warnings. We're going to in the strongest possible terms encourage residents in the low lying areas as they say, get out of dodge. Reporter: Helping folks get out of dodge, airline waiving change and cancellation fees for those in the storm's path. Some are buying sand by the pallet, but this line goes around the block and is it's 3 1/2 hours long. Inside Southerland's hardware, clerks scrambling -- We're out of wood. Reporter: The line for plywood 30 minutes long, customers on edge -- And everybody just went into code red, and we all came and basically just to prepare ourselves. I'm 65, and I don't want to get out -- and I don't want to die at 65. Reporter: In Houston where the storm is forecast to dump a biblical deluge shelves picked clean. There's no more water, the bread's already running out as well. Reporter: Cars lining up for hours at gas stations. This is the line to get gas. Absolute insanity over this hurricane. Reporter: In New Orleans early they are month, a summer squal, flooded streets and there's now concern the city's pumps won't be able to keep up. We're in a more vulnerable space than we should be in. Reporter: Millions of people will feel the direct impact of the storm, but the entire country could be affected. Also in the storm's path, some 20 refineries producing 4 million barrels of oil. It doesn't take a hurricane to wreak destruction. Tropical storm Allison in 2001 caused $5 billion in damage. Let's get to Matt Gutman live from Corpus Christi, and they are talking about a storm surge of up to 12 feet? Reporter: That's right, David, and just over the past couple of hours, we have also felt this wind pick up here. You may hear it on my microphone. Initially, they were talking about 4 to 6-foot storm surge, and now it's 12 feet meaning the water in the bay here would go over my head, and the waves during the storm would be crashing well above that lamp post, David. Matt Gutman already there for us. Matt, thank you. We'll have much more on the hurricane throughout the night at abcnews.com, and first thing in the morning on "Good morning America."
This transcript has been automatically generated and may not be 100% accurate.