An unrelenting Tropical Depression Imelda is causing serious flash flooding in parts of southeastern Texas on Thursday morning, covering roads, trapping people in vehicles and homes, and sending rescuers scrambling to get to those most in need.

Flooding is most serious in counties just to the north and east of Houston -- in places like Chambers and Jefferson counties, and its communities such as Beaumont and Winnie. Thursday is the third straight day Imelda is deluging the region.

Floodwaters intruded onto Interstate 10 early Thursday near Winnie east of Houston, trapping motorists like Steve Castle.

"I was supposed to be driving back to Houston. I don't think I'm going anywhere soon," Castle told CNN from his truck on the highway.

Imelda could still drop 5 to 10 more inches of rain on the Upper Texas Coast region Thursday, bringing three-day totals to as high as 25 to 35 inches in some places.

In Chambers County, which includes Winnie, at least 200 homes were taking on water, Sheriff Brian Hawthorne said.

In Beaumont, about 80 miles east of Houston, police told people to "shelter in place and seek high ground."

"DO NOT drive a vehicle on BMT (Beaumont) roads. Most access roads are under water," Beaumont police said on Twitter Thursday morning. "Rescue/Evacuation requests are being prioritized. Please only call 911 for life threatening situations."

Imelda has brought intense rain to southeastern Texas since coming ashore from the Gulf of Mexico on Tuesday. Some places already have received more than 21 inches.

In Baytown, about 26 miles east of Houston, a tornado added to the damage on Wednesday.

The twister picked up a hundred-gallon propane tank, launching it into the house he was in and sending everything flying, Albert Elizondo said.

Jeremy Franklin, left, with Mitchell Historic Properties, unloads bags of sand at Texas Scuba Adventures, in Galveston, Texas. Photo: Jennifer Reynolds/AP

"It lasted about three minutes. Boom. I went outside, no porch -- nothing," Elizondo said.

More rain to come Thursday
Imelda is currently in East Texas and slowly moving to east and then north, meaning eastern Texas, western Louisiana and portions of Arkansas could be hit by its rain, according to CNN Meteorologist Pedram Javaheri.

Those areas might not get relief until Friday afternoon, CNN Meteorologist Michael Guy said.

Flash flood watches are in effect Thursday more millions of people across East Texas and western Louisiana.

In Louisiana, Imelda could bring 3 to 5 more inches of rain -- with isolated amounts as high as 10 inches -- on Thursday, the National Weather Service said.

Houston itself -- long a place with flooding problems due in part to vast concrete sprawl -- may escape the worst of Imelda's flooding.

Still, the city may receive more than 12 inches of rain before the storm is over, and some streets flooded Wednesday.

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