The storm, which early Wednesday was about 25 miles north-northwest of Houston, is expected to drop 6 to 12 inches of rain in the next two to three days across portions of eastern Texas, the National Weather Service's Weather Prediction Center said.
Imelda is slowly moving inland and has the potential to be among the more destructive storms in the US in recent months because of the amount of rainfall, CNN meteorologist Pedram Javaheri said.
After drenching coastal Texas and southwestern Louisiana through Wednesday, the storm system is expected to douse eastern Texas and western Louisiana on Thursday, the hurricane center said.
As much as 9 inches of rain had fallen by Wednesday morning in some areas southeast of Houston, according to the Harris County Flood Warning System.
Several schools in the Houston-Galveston area announced they would cancel activities for Wednesday because of Imelda.