Torrential rainstorms battered Louisiana on Sunday, leaving thousands without power, after pounding southeastern Texas as the remnants of Hurricane Patricia converged with a second storm.

The heaviest band of rain moved over the Gulf of Mexico, triggering coastal flood warnings and flash flood watches in southwest Louisiana and soaking New Orleans, according to the National Weather Service (NWS).

About 22,000 people were left without power in the greater New Orleans area. Some streets were flooded, while a high tide surge brought some coastal flooding as well.

Rainfall has totaled as much as 18 centimetres since late Saturday night, and forecasters predicted another 13 cm could fall. The NWS said waterspouts over lakes and tornados over land were both possible into the early morning hours.

"Most of the heavier rain to the west of New Orleans will taper off in the evening ... and for far eastern Louisiana it will probably end closer to midnight," said NWS forecaster Gavin Phillips.

The NWS issued a tornado watch for southeastern Louisiana and coastal Mississippi into early Monday, and warned that severe thunderstorms could develop in the region.

A tornado touched down near the community of Larose, about 45 minutes south of New Orleans, though no serious damage was reported.

Tides along the southern coast of Louisiana were expected to be a few feet above normal at high tide due to sustained winds, likely flooding roads in lower-lying areas, Phillips said.

More than 23 cm of rain swelled rivers and flooded roads around Houston, but no injuries or deaths were reported as flash flood warnings ended.

Petroleum refineries along the U.S. Gulf Coast, which make up more than 40 per cent of U.S. capacity, also appeared to be largely undamaged.

In the Eagle Ford and Permian Basin oil fields of south and west Texas, no major production cuts were reported. While the rains were heavy in Houston, they came after a month-long dry spell so flooding was relatively limited.

Texas takes pummelling

The storms over the past two days drenched a large swath from south of Dallas to the southeast coast, triggering flash flooding in Navarro County, about 80 kilometres south of Dallas, on Saturday.

A Union Pacific freight train carrying cement derailed in Navarro County after a creek overflowed, washing out the tracks. Locomotives and rail cars were pushed on their sides, and a two-person crew was forced to swim to safety.

Repair teams cleared the derailed cars by Sunday morning, but they were not expected to be righted for several hours and the rail line was not due to reopen until Monday at the earliest, Union Pacific spokesman Jeff DeGraff said.

Navarro County was one of the hardest-hit areas. The tiny town of Powell got 50 cm of rain over 30 hours, said meteorologist Brett Rathbun of Accuweather.

Navarro County Sheriff Elmer Tanner reported dozens of rescues from vehicles, homes and businesses since Friday.

In San Antonio, a woman reported her boyfriend was swept into a drainage ditch as he walked his dog early Saturday.

The force of the water washed him out of the underground ditch and he passed out, the San Antonio Fire Department said on Twitter. He later came to and called authorities.

The rain was strengthened by the remnants of Patricia, which was downgraded to a tropical depression after crashing into Mexico's west coast on Friday as a powerful hurricane.