Wide swaths of southeast Texas have been devastated by Tropical Storm Harvey, with some areas drowning in nearly 40 inches of water — and Lone Star State residents are bracing for another beating with up to 20 more inches of rain.

“This is, if not the largest, it has to be categorized as one of the largest disasters America has ever faced,” Texas Gov. Greg Abbott declared at a press conference Monday afternoon.

Forecasters say Harvey, which made landfall in Texas as a Category 4 hurricane Friday night, will move slowly to the northeast throughout the week and shower some parts of the state with another 15 to 20 inches of rainfall by the end of Thursday.

Certain areas to the west of Houston could see as much as 50 inches of rain by the time the storm is over — which would be the largest recorded total in Texas history.

By Monday, more than 2,000 victims in Houston had been plucked from the catastrophic flooding, which left roads, homes and cars across Texas under water.

Abbott activated the entire Texas National Guard — about 12,000 members — to aid in rescue efforts.

Citizen rescuers also jumped in to help fellow residents, using private motorboats and even kayaks to ferry stranded people to safety as thousands poured into shelters around the state.

With waters continuing to rise, some people panicked as they waited for rescue.

“They’re making it difficult for us to rescue them,” said Clyde Cain, a member of the Louisiana-based Cajun Navy rescue force. “You have people rushing the boat. Everyone wants to get in at the same time. They’re panicking. Water is rising.”

FEMA officials said they expected to see at least 30,000 people show up to shelters by the end of the slow-moving storm.

“It is imperative that we do everything possible to protect the lives and safety of people across the state of Texas as we continue to face the aftermath of this storm,” Abbott said.

Joe Garcia, wearing a Mets hat, fled his flooded home in Spring, Texas, with his German shepherd, Heidi, in tow, wading through chest-deep water until he was rescued by a boat.

But some were not so lucky, with the storm’s death toll rising to at least eight people.

A family of six is believed to have been killed when their van was swept away by floodwaters Sunday afternoon as they tried to escape the storm. Four children — Daisy Saldivar, 6, Xavier Saldivar, 8, Dominic Saldivar, 14, and Devy Saldivar, 16 — were in the white van, along with their great-grandparents, Manuel Saldivar, 84, and his wife, Belia, 81.

Officials hadn’t confirmed their deaths, but Houston Police Chief Art Acevedo said he’s “really worried about how many bodies we’re going to find.”

One woman was killed near Houston when an oak tree toppled onto her mobile home Monday afternoon while she was napping inside. Porter Fire Department firefighters had to evacuate her husband and remove her body from the home.

In other storm-related news:

President Trump will touch down in Corpus Christi, Texas, at noon Tuesday to survey the flood damage with his wife, Melania. “Protecting the lives of our people is my highest priority,” he said at a press conference Monday. “Every asset at my command is at the disposal of local officials.”

Former President George H.W. Bush and his wife, Barbara, who live in Houston, were riding out the storm in Maine but said Monday that “our hearts are” in Texas. “We are praying for all of our fellow Houstonians and Texans affected by Harvey, and truly inspired by the flotilla of volunteers — Points of Light all — who are answering the call to help their neighbors,” the former first couple said in a statement.

The Army Corps of Engineers warned residents that the flooding in some areas could get worse as the Corps releases water from two Houston-area dams. “If we don’t begin releasing now, the volume of uncontrolled water around the dams will be higher and have a greater impact on the surrounding communities,” said Col. Lars Zetterstrom, Galveston District commander of the Corps.

Officials said that all 350 patients at Ben Taub Hospital, a busy trauma center in Houston, would have to be evacuated after its basement was flooded. The floodwaters also forced one of the world’s most famous cancer institutes, MD Anderson Cancer Center, to cancel all appointments and surgeries.

Meanwhile, FEMA Administrator Brock Long said he expects about 450,000 people will file for disaster relief, adding that the agency will “be here for several years helping you guys recover.”

Former FEMA Director Michael Brown — who ran the George W. Bush administration’s botched rescue efforts during Hurricane Katrina — said Harvey will surely be worse than the devastating 2005 storm.

Brown said flooding in population-dense Houston, the country’s fourth-largest city, is likely to bring unprecedented suffering and damage.

“There are several factors that make it worse than Katrina. For one, there is the scope of the flooding. Harris County and the surrounding areas are so saturated,” Brown told the Houston Chronicle. “Also, the amount of damages will continue to grow. There will be mold and structural damages adding up.”

Brown said that when Tropical Storm Harvey finally goes away, it will leave an incredible bill for taxpayers to pick up.

“This will be unfathomably expensive for both the private sector and taxpayers,” Brown said. “This will be easily the most expensive natural disaster in American history.”