DALLAS (AP) — 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 to arrive.

Many parts of Texas, including some of its biggest cities, were under flash flood watches through Sunday or Monday. The remnants of Patricia, which was downgraded to a tropical depression 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 nearly two feet of rain on parts of the state since Friday, causing flooding that blocked several major roadways.

Galveston County Judge Mark Henry on Saturday issued a voluntary evacuation for Bolivar Peninsula, just northeast of Galveston Island, after forecasters predicted that the area would get 8 to 12 inches of rain and tides that are 4 to 5 feet high.

The judge warned that residents who don't leave might find themselves cut off from emergency services as the heaviest winds and rains come ashore. The storm was expected to reach the area by around 6 p.m. Saturday.

"Elderly residents and people with medical conditions should make plans to leave the area before dark," Henry said.

County spokeswoman Brittany Rainville said there were buses waiting to evacuate people, but no one had yet shown up to leave as of early Saturday afternoon. She said about 4,000 people live on the peninsula.

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 had to stop due to bad 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 get out of his car that was floating in floodwaters and grabbed 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, he said.

"They (crew) escaped the train after it stopped and swam to high ground," DeGraff said. "A Navarro County rescue team was able to get in and pull them to safety, they are back safe on dry ground."

One locomotive and several rail cars loaded with gravel went into the water and were partly submerged, DeGraff said. He had no specifics on how many cars derailed because crews couldn't reach the site due to flooding.

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 Waco, Austin, San Antonio, Corpus Christi, Laredo and Brownsville areas through Sunday morning, according to the National Weather Service. The flash flood watch for Dallas and Fort Worth was lifted Saturday afternoon.

Forecasters say Houston and Galveston remain under a flash flood watch through Monday morning, with possible coastal flooding concerns.

Patricia was the most powerful hurricane on record in the Western Hemisphere and made landfall Friday along Mexico's Pacific Coast as a Category 5 storm. It quickly lost power as it moved inland and appeared to have caused remarkably little damage.

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Associated Press writer Diana Heidgerd contributed to this report.

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This story has been corrected to reflect that Patricia is no longer considered a tropical storm and has been downgraded to a tropical depression.