The relentless downpour from tropical storm Imelda has flooded parts of Texas and Louisiana, with rescue crews and volunteers with boats scrambling to reach scores of stranded drivers and families trapped in their homes.
Officials in Harris County, which includes Houston, said there had been a combination of at least 1000 high-water rescues and evacuations to get people to shelter.
More than 900 flights were cancelled or delayed in Houston.
Further along the Texas Gulf Coast, authorities at one point warned that a levee could break near Beaumont in Jefferson County.
A 19-year-old man drowned and was electrocuted while trying to move his horse to safety, according to a message from his family shared by the Jefferson County Sheriff's Office.
Crystal Holmes, a spokeswoman for the department, said the death occurred during a lightning storm. Authorities elsewhere had reported no loss of life or major injuries.
The National Weather Service said preliminary estimates suggested that Jefferson County was deluged with more than 100 cm of rain in a span of just 72 hours.
Even when Houston was rid of the worst, downtown highways remained littered with abandoned cars submerged in water. Thousands of other drivers were at a standstill on narrowed lanes near flooded banks.
Ahead of the evening rush hour, Houston officials urged commuters to stay in their offices rather than embark on flooded and already jammed highways.
A massive Houston furniture store became a shelter for evacuees. Live television footage showed firefighters rescuing stranded truckers on major highways. On social media, people posted that water was quickly seeping into their home and asked for help.
Even as the intensity of the storm weakened, Harris County officials warned that some of their 4.7 million residents might not see high waters recede in their neighbourhoods until the weekend.
The flooding from Imelda came as Hurricane Humberto blew off rooftops and toppled trees in the British Atlantic island of Bermuda, and Hurricane Jerry was expected to move to the northern Leeward Islands on Friday and north of Puerto Rico on Saturday.
Australian Associated Press