Story highlights

NEW: All exits to downtown Houston on I-45 closed due to high water

Up to 20 inches of rain have fallen over parts of Texas in a few days' time

Another 6 to 12 inches are on their way -- even more in isolated spots

CNN  — 

The math is simple, but the potential weather consequences are daunting.

Up to 20 inches of rain have fallen over parts of Texas over the last few days, breaking some local rain records. In southeast Texas, another 6 to 12 inches were on their way overnight – even more in isolated spots.

Rain should continue through part of the week.

Add coastal high winds that are driving in high waters on Sunday, and it all equals flood warnings of almost every type from the National Weather Service from Corpus Christi up the Gulf Coast to Morgan City, Louisiana:

– Rivers and creeks were predicted to overflow in Corpus Christi, in Waco and near San Antonio, Texas, and in Lake Charles, Louisiana.

– Flash flooding in Houston led to 50 confirmed high water locations, emergency management spokesman Michael Walter said. All exits to downtown on I-45 were closed.

– Coastal flooding was predicted to push inland from Beaumont east throughout much of the Louisiana coast.

Galveston County is predicted to become inundated on two fronts – up to a foot of rain is in the forecast, and at the same time tides pushed by howling winds are projected to rise up to five feet above normal. Residents of Bolivar Peninsula are advised to evacuate, an official said.

Wayward streams

In and around Houston, high water led to nearly a dozen road closings late Saturday, CNN affiliate KTRK reported.

When rushing water meets cars and people, danger often results. It only takes 12 inches of flowing water to sweep away a small car, the weather service has warned.

“Most flood deaths occur in automobiles. Never drive your vehicle into areas where the water covers the roadway,” it said.

Earlier in the day, flooding washed away cars, and derailed a freight train. Rescuers pulled motorists out of cars stuck in high water.

Train derails in Texas flood

One person disappeared in San Antonio, a homeless man who tried to rescue his dog from a drainage ditch. He was swept away in pre-dawn hours, a fire official said.

The dog was later found safe. The man hasn’t been.

Records broken

San Antonio broke its all-time single-day rain record Saturday, as did Camp Mabry, a military installation in Austin.

Dallas was in the thick of it Saturday, getting about eight inches of rain in 24 hours. As the Trinity River approached flood stage, a flood gate on nearby White Rock Lake opened to release water and keep it from overflowing.

On roadways in and around Austin, heavy rain prompted the closures of 66 low-water crossings. The city government, via Twitter, urged people to stay inside.

Rain on top of rain

Though much of the rain has shifted southeast since Saturday, previously inundated places will get hosed again. Navarro County saw a jaw-dropping 20-plus inches fall in the last two days. Heavy rain fell there again overnight, the weather service said.

Navarro is where the train, which was pulling dozens of cars, derailed in high water.

Two workers climbed out then swam to high ground, Union Pacific spokesman Jeff DeGraff told CNN.

“They are in good condition, no injuries, just a little wet and shaken up,” he said.

Patricia’s remnants

The torrents are coming from two weather systems. The first has hovered over Texas and neighboring states for the last few days, but it is being joined by remnants of the most powerful hurricane the National Hurricane Center has ever measured.

At its peak off Mexico’s Pacific Coast, Patricia was a mighty Category 5 with 200 mph sustained winds.

“The remnants of Patricia should be absorbed by a non-tropical low pressure system over southern Texas later tonight or on Sunday,” the weather service said. In other words, what’s left of the storm is feeding a lot of its moisture into Texas torrents.

Some flood warnings last late into the week, the weather service warned. Even after the rain stops falling, creeks and rivers may continue to overflow as runoff rushes through their banks.

Drought then flood

Much of Texas has suffered most of the year under the worst levels of drought on the U.S. Drought Monitor, kept by the University of Nebraska-Lincoln.

But the state has been inundated twice this year with extreme rainfall. Over the Memorial Day holiday in May, rain and flooding was blamed for at least 15 deaths in Texas and at least six in Oklahoma.

CNN’s Greg Botelho contributed to this report.