DALLAS — A powerful storm system rumbled through Texas on Saturday, flooding roads and causing a freight train to derail as parts of the state braced for the remnants of Hurricane Patricia.

Many parts of Texas, including its biggest cities, were under flash flood watches through Sunday or Monday. The remnants of Patricia, which was downgraded to tropical depression status and was expected to reach northern Mexico by Saturday night, may add to the rain falling in South and Central Texas, said Jesse Moore, a National Weather Service forecaster in Fort Worth.

The storm system already moving through Texas dumped more than a foot of rain on parts of the state on Friday, causing flooding that blocked several major roadways.

In San Antonio, a man walking his dog before dawn early Saturday was swept into a flooded drainage ditch and disappeared, fire officials said. Firefighters searched for two hours but were forced to stop by the severe weather. They planned to resume as soon as possible. The dog is safe.

A driver in the Central Texas town of Temple heading to work Saturday morning was saved after he was able to exit his car that was floating in floodwaters and grab a tree. Temple police say the man called 911 from his cellphone to summon help. Firefighters retrieved him and walked him to safety.

A Union Pacific freight train derailed before dawn on Saturday near Corsicana, about 50 miles south of Dallas, because a creek overflowed and washed away the tracks, said Jeff DeGraff, a railroad spokesman. The two crew members swam to safety and nobody was hurt, DeGraff said.

One locomotive and several rail cars loaded with gravel went into the water and were partly submerged, DeGraff said. He couldn’t say how many cars derailed because crews couldn’t reach the site.

Authorities on Saturday morning reopened a section of Interstate 45 near Corsicana that was closed overnight due to flooding, backing up traffic for 12 miles. Moore said that since Friday morning, Corsicana has received 18 inches of rain and Powell has received 20 inches.

Flight tracker flightaware.com reported that about 100 Saturday flights had been canceled at Dallas-Fort Worth International Airport.

A flash flood watch was in effect for the Dallas, Fort Worth, Waco, Austin and San Antonio areas through Sunday morning, according to the National Weather Service. Corpus Christi, Laredo and Brownsville were under a flash flood watch through Sunday night.

Forecasters say Houston and Galveston remain under a flash flood watch through Monday morning.

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