According to the National Weather Service, southern Texas has received nearly 600 percent more rainfall than normal for this time of year.
And because the ground is already saturated, the potential for dangerous flash flooding increases.
Houston residents have been told to stay in their homes until the water recedes.
More than 20 million people from southeastern Texas into most of Louisiana and Mississippi are under a flash flood watch through Saturday evening.
Over the weekend, the heavy rain will shift eastward toward Alabama and the southern Appalachians.
But as the storm moves east, clusters of storms could produce more flash flood threats for those areas.