(ABC NEWS) -- Hurricane Harvey is expected to hit Texas as a Category 3 storm — the likes of which haven't been seen since 1999's Hurricane Bret — when it makes landfall late Friday or early Saturday, forecasters say.

The storm strengthened to a hurricane, with maximum sustained winds of 80 mph, on Thursday afternoon.

The storm's surge could be life threatening, up to 10 feet, with waves as high as 20 feet above that. Rain is expected to range from 10 to 20 inches, and some areas could receive up to 30 inches. Perilous flash flooding and 115 mph gusts are possible.

Most of the flights in and out of Corpus Christi International Airport for Friday have been canceled, according to FlightAware.

United Airlines will operate its last flight out of Corpus Christi at 7 a.m. Friday, and Southwest Airlines will operate its last flight at 10:40 a.m., the airport announced on Facebook.

"Harvey is expected to landfall around 1 a.m. Saturday, based on the current track," ABC News meteorologist Melissa Griffin said, adding that more time over water could increase the storm's strength. "If it had another day or two, it could be a higher category."

The storm is being compared with not only Hurricane Bret but also the devastating Tropical Storm Allison, which in 2001 punished Houston with 40 inches of rain.

The biggest worry with Hurricane Harvey is that it's expected to stall over southeastern Texas, Griffin said.

"When it does make landfall, it is expected to ... not move for several days, which is why we expect the rainfall to be the most devastating," she said. "Some spots could receive over 30 inches."

At a press conference Thursday afternoon, Corpus Christi's newly elected Mayor Joe McComb told residents not to dismiss Harvey and to heed requests to voluntarily evacuate.

"We encourage the residents in low-lying areas, as they say, to get out of Dodge," he said.

McComb stressed that flooding alone could cut off electricity and clean water from homes in the area. "Go to a family, friend and get to higher ground," he said.

He left open the possibility of calling for mandatory evacuations, cautioning against trying to "ride [the storm] out."

"I'm not going to risk our fire people and police people to risk themselves to get somebody out of the home," he said. "They got relatives, and they got family, and we don't want to put them in harm's way because someone wanted to stay in their home."

Residents there were emptying shelves at the local Sam's Club, where staples like batteries, generators and milk were in high demand Thursday.

"We're out of bread, and we're out of water right now," Sam's Club employee Julie Canales told ABC News. "We've been stocked every day, but as soon as we get it, it's gone within an hour between truck [deliveries]."

She said topping locals' shopping lists is water. "We do have trucks en route, but the line is already over 100 people," she said.

Many customers are stocking up on alcohol as well, she added. "Beer and wine," she said. "It's something people are buying probably to calm everybody's nerves."

At Coastal Hardware, kerosene lanterns, kerosene, sandbags and sand were out of stock.

The store's manager, Jennifer Wimsett, said customers have been buying up duct tape and have cleared out her stock of kits to make window shutters hurricane-proof.

Since sand and sandbags are unavailable, she said, many people are improvising. "They can fill up trash bags and fill them with topsoil if they don't have sand," she said.

Texas Sen. Ted Cruz asked his Facebook followers to keep "all those in the path of this storm in your thoughts and prayers."

He urged Texans in the storm's predicted path to "heed warnings from local officials, know your evacuation route and avoid all high-water areas" and not to underestimate Mother Nature.

"This is a serious storm, with strong winds and what forecasters have predicted will potentially be a historic amount of rainfall that will significantly raise the risk of serious and life-threatening flooding," Cruz said.

On Wednesday, Texas Gov. Greg Abbott warned of "severe flooding" and declared a state of disaster for 30 counties along the coastline.

Lt. Craig Cummings of the Texas Department of Public Safety told ABC News that people need to take prepare for the worst.

"The best thing that I tell my own family but I also tell professionally is to prepare," he said, encouraging people to have a physical map on hand.

In neighboring Louisiana, Gov. John Bel Edwards signed a statewide emergency declaration Thursday. Southwestern Louisiana could see up to 10 inches of rain from the storm.

Edwards said that while plans to evacuate people are ready, "hopefully we don't have to execute those plans."