The forecast for the deadly storm Harvey that has inundated the Texas city of Houston is "uncertain," a top US weather official said Monday.

National Weather Service Director Louis Uccellini told a news conference in Washington that Harvey, now a tropical storm, "is creating its own circulation even aloft, so this track forecast still represents a difficult forecast for us. We have to pay attention as we move forward."

At the moment, Harvey was expected to move east over the next five days toward the Texas-Louisiana state line.

As of Monday morning, there were reports of five to six inches (12.7 to 15.2 centimeters) of rain per hour and unofficial reports of up to eight inches in a band of heavy rainfall.