(CNN) Highways in Texas filled with cars Thursday into the night as coastal residents made their way north and out of the path of a hurricane that forecasters say will bring "life-threatening" amounts of rain.

Some areas could get as many as 35 inches of rain from Hurricane Harvey and many will get more than a foot, forecasters from the National Hurricane Center said. Harvey has strengthened to a Category 2 storm, with maximum sustained winds of 100 mph, according to the National Weather Service.

As Harvey gained strength Thursday, residents along the Texas coast made the choice to evacuate or stay. Those staying in place aren't taking any chances; they're filling sandbags, stocking up on water and boarding up windows.

Harvey is rapidly becoming more powerful and is forecast to become a Category 3 hurricane with winds of at least 111 mph by the time it hits the middle Texas coast late Friday or early Saturday, the hurricane center said Thursday. After hitting Corpus Christi, the storm is expected to stall over the state, forecasters say.

In Corpus Christi, where Harvey could make landfall and Mayor Joe McComb has issued a voluntary evacuation order, Walmart shelves were clearing quickly, as the city canceled Friday's dockets in city court and provided self-serve sandbags to residents.

Hurricane-force winds are a concern as the storm builds strength in the Gulf of Mexico, and a potential deluge and subsequent flooding may be a big danger as well, according to meteorologists.

Timelapse of WP-3D Orion #NOAA42 flying a pass through the eye of #HurricaneHarvey from west to east. Advisories at https://t.co/3phpgKMZaS pic.twitter.com/b5lyqqkW9x — NOAAHurricaneHunters (@NOAA_HurrHunter) August 24, 2017

"Harvey is likely to bring multiple hazards, including heavy rainfall, storm surge and possible hurricane conditions to portions of the Texas coast beginning Friday," the National Weather Service said.

"Those conditions can happen far away from the landfall," hurricane specialist John Cangialosi said Thursday afternoon.

People shouldn't focus on where the eye will come ashore but should listen to local authorities and be prepared for rain through the weekend and possibly into next week, he added.

"This is really going to stretch the emergency services in the state of Texas in the days ahead," CNN meteorologist Tom Sater said.

Making preparations

Northbound Interstate 37 out of Corpus Christi was filled as people evacuated. The highway, which goes to San Antonio, was backed up for miles while the southbound lanes were mostly empty, a photo from Mari C. Rivera shows.

Rose Yepez told CNN it took her twice as long as normal to drive from Corpus Christi to San Antonio. Yepez, who was continuing her travels to the Texas Hill Country, said traffic was constantly slowing down and coming to a stop during the 140-mile drive.

Northbound traffic was using the shoulder and three regular lanes to get out of Corpus Christi.

Not everyone is leaving. Rebecca Esparza of Corpus Christi said her family is prepared to stay in their now-boarded up house.

"I want to leave, but I refuse to leave my mom behind," she said, noting that her mom rode out Hurricane Celia in 1970.

"We have lots of bottled water, satellite radio, lanterns, bagged ice," she said.

Amanda Weldy is staying put, too.

Amanda Weldy plans to ride out the storm with her family.

She said the family is freezing lots of bottled water to keep perishable food cold. She also plans to have everyone shower Friday morning then fill up the tub in case the water lines get contaminated.

"Mostly we are just making sure our 2- and 1-year-old have everything they need," she said.

Other people around Corpus Christi started stocking up Wednesday on food, bottled water and other essentials.

"We know that if anything as far as flooding happens, that's the one thing everyone runs to, we have to have water," Mariah Barter told CNN affiliate KZTV . "It's a big deal. It scares the hell out of people. It's better to be prepared."

Many residents of South Texas readied sandbags, fueled up power generators and bought plywood to board up their windows, CNN affiliate KRIS reported.

Slow storm means more rain

At 11 p.m. ET, Harvey was a Category 1 hurricane with 85 mph winds. It was about 250 miles southeast of Corpus Christi, and was moving northwest at 10 mph.

It's expected to bring 15 to 25 inches of rain to the Texas coast, with isolated instances where rainfall totals could reach 35 inches, the hurricane center said.

"Rainfall from Harvey will cause devastating and life-threatening flooding," the center warned.

Rainfall amounts increase exponentially when a storm moves at a slower speed, as Harvey has been doing.

The National Hurricane Center has warned Harvey will slow down when it reaches the coast and there will be days of heavy rain and flooding across portions of Texas. Louisiana and Mexico will also be affected by the storm.

Compounding potential problems is the tidal cycle. If peak storm surge arrives during high tide, parts of the coast could see 1 to 8 feet of flooding, with the potential of 6 to 12 feet between Padre Island National Seashore and Sargent, Texas.

"Historically, water from tropical cyclones is more deadly" than from damage done by the powerful winds of the storm, Cangialosi said.

While it has been nine years since Texas last saw a hurricane, the state is no stranger to devastating flooding from tropical systems. In 2001, Tropical Storm Allison was a multibillion-dollar disaster for the state, specifically Houston. Allison became nearly stationary for days, dropping more than 30 inches of rain across portions of the city.

A hurricane warning is in effect from Port Mansfield near Mexico to Sargent, with a storm surge warning stretching from Port Mansfield to High Island in Galveston County.

Harvey could be the first hurricane to hit Texas since 2008 when Hurricane Ike smashed the coast near Galveston. The storm killed 21 people in Texas, Louisiana and Arkansas, and caused widespread destruction.

'It scares the hell out of people'

As Harvey churned toward Texas, Gov. Greg Abbott declared a state of disaster Wednesday in 30 counties along the Gulf of Mexico. The governor's action allows agencies to "quickly deploy resources for the emergency response."

"Texans believe in taking action and always being prepared in the event of an emergency," Abbott said. "That is why I am taking every precaution prior to ... Harvey making landfall."

On Thursday the governor activated 700 members of the Texas Army Guard, Texas Air National Guard and the Texas State Guard.

Energy companies are evacuating personnel from off-shore production platforms, the US Bureau of Safety and Environmental Enforcement announced.

In San Antonio, Mayor Ron Nirenberg issued a local disaster declaration warned residents about going out in the rain.

There are evacuation orders in many counties and cities, including Victoria, near where the eye of the storm may hover for days.

At a Thursday afternoon White House briefing, Press Secretary Sarah Huckabee Sanders said President Donald Trump has been briefed on Harvey. She downplayed concerns about preparedness, given that there is no permanent Homeland Security secretary, and said the White House is in "great shape," given that chief of staff John Kelly was previously in charge of the office.