HOUSTON — Tropical Depression Imelda brought driving rains to Louisiana and southeast Texas on Thursday, killing at least one person, prompting widespread evacuations and inundating many of the same communities ravaged by Hurricane Harvey two years ago.

As much as 40 inches of rain could fall in the region on Thursday and Friday as "significant and life threatening flash flooding is ongoing across portions of far southeast Texas," the National Hurricane Center said.

The Jefferson County Sheriff's Office said Thursday that Hunter Morrison, 19, was moving his horse from flooded waters to higher ground when an electrical storm hit.

An official cause of death wasn't reported pending an autopsy, but his family said in a statement released through the sheriff's office that Morrison was electrocuted and drowned.

Crews had rescued more than 1,000 people in the Houston area, mostly in eastern Harris County, because of rising waters, officials said Thursday.

"What we need is for folks to stay calm ... and stay home," Harris County Judge Lina Hildago, the county's chief executive, told reporters. "The best thing you can do is remain indoors, wherever you are, to not go outside."

All bus and rail service was shut down in Houston, Texas' biggest city and the fourth largest in America, the area's public transportation agency announced.

More than 57,000 homes and businesses in Texas, almost all of them in the southeast corner in the Houston area, were without power late Thursday afternoon, utilities reported to Poweroutage.us.

Both of Houston's airports were affected by extreme rain.

Hobby Airport announced shortly after noon that departing flights would be allowed to take off but that arrivals were being turned away. George Bush Intercontinental Airport issued a full ground stop late in the morning, before resuming flights with significant delays.

The roof of a Postal Service distribution facility on Aldine Bender Road in Houston collapsed at about 10:30 a.m., and three people were transported with minor injuries, Houston firefighters said. It wasn't immediately clear whether the collapse was directly connected to the heavy rains, a Postal Service spokeswoman said.

HFD is responding to a building collapse on Aldine Bender Road. The flat roof of a post office facility mail distribution area collapsed. The building was occupied at the time but everyone made it out. 3 people are being transported with minor injuries. @FireChiefofHFD pic.twitter.com/X6exlPtAY1 — Houston Fire Dept (@HoustonFire) September 19, 2019

Flood waters forced the hasty evacuation Thursday of Riceland Medical Center in Winnie, about 60 miles east of downtown Houston.

"It's as bad as I've ever seen it. Right now, I'm in an absolute deluge of rain," Chambers County Sheriff Brian Hawthorne said Thursday morning as he took cover under a carport at an auto dealership.

Cars drive through a flooded street in Sargent, Texas, on Wednesday, Sept. 18, 2019. Mark Mulligan / Houston Chronicle via AP

"Right now, as a Texas sheriff, the only thing that I really want is for people to pray that it will quit raining," he said, adding the town "looks like a lake."

James Revia, 40, of the Chambers County community of Hankamer, and his four children were rescued from their flooded trailer park home by a passing firetruck.

Revia, a mobile DJ who owns a lawn service, said he feared that all of his music equipment, kept inside his truck, had been lost to floods.

"This storm grew into a tropical depression within four hours. It caught everyone by surprise," he told NBC News.

Erika Zamora, who was stranded with her five children and her husband inside their home, said the rain in Winnie was unrelenting.

"I opened the door, and the water was to our door," Zamora said.

A neighbor with a rescue boat fetched them to safety, but the Zamoras believe they've lost almost all of their belongings.

"This is my family. These are my kids, and I'm pregnant. It was scary," a tearful Zamora said Thursday at a school cafeteria-turned-evacuation center.

In Beaumont, about 85 miles northeast of Houston, flood waters were above and beyond what Hurricane Harvey unleashed in August 2017, officials said.

"It's bad," Jefferson County Judge Jeff Branick said. "Homes that did not flood in Harvey are flooding now."

By midday, a rain gauge just outside Beaumont reported a two-day rainfall total over 38 inches, with 34 inches coming down in the previous 24 hours.

Gov. Greg Abbott on Thursday declared a state of emergency in counties suffering from the heavy rains and floods: Brazoria, Chambers, Galveston, Hardin, Harris, Jasper, Jefferson, Liberty, Matagorda, Montgomery, Newton, Orange and San Jacinto.

Annie Rose Ramos reported from Houston. David K. Li reported from New York.