(CNN) Flooding has hit parts of the South hard this week, and forecasts suggest Texas and neighboring states remain under threat -- particularly in areas that have already seen heavy rainfall.

Nearly 20 million people were at risk of flash flooding, according to CNN meteorologist Dave Hennen, from southeast Texas into much of Louisiana and southern Arkansas.

The Houston metro area was forecast to receive heavy rainfall in the coming days, with a flash-flood watch set to go into effect Thursday afternoon and stretch into the weekend, said the National Weather Service's office in the Texas city.

Those affected could expect widespread rainfall totals of 5 to 8 inches, with 9 to 12 inches possible in other areas, the service said on Twitter.

A Flash Flood Watch is in effect starting today at 1 PM and continuing through Saturday 7 PM. Forecasted rainfall totals estimate about 5 to 8 inches widespread, with isolated 9 to 12 inches possible through the weekend. #txwx pic.twitter.com/hoSdwu6vbh — NWS Houston (@NWSHouston) May 9, 2019

Flash-flood warnings were already in effect Thursday morning from northern Louisiana into southern Arkansas, CNN's Hennen said. The same areas could see the threat of flooding continue into Friday and Saturday, and will likely receive additional rainfall totals of 3 to 8 inches.

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