Bush Airport will be closed until noon Thursday, McCarthy said, and United is offering waivers to passengers who need to change their travel plans.

While Harvey has been downgraded to a tropical storm, with waters rising and a record rainfall of 50 inches forecast for some areas of southeast Texas through Thursday, the catastrophic flooding is expected to continue for days, the National Weather Service said.

While United passenger flights are grounded, the airline has been flying relief missions into Houston, providing emergency supplies to flood victims, McCarthy said.

United said it was expecting to get its remaining passengers out of Houston on Monday night on a relief flight to Chicago.

Weather waivers, which let passengers reschedule flights in and out of Houston before the storm hit, helped minimize the number of people stuck on the ground, the airline said.