On Thursday, 160 flights had been cancelled at airports on the gulf coast, primarily at Houston Intercontinental and Houston Hobby. George Bush Intercontinental Airport in Houston is expected to open Monday afternoon and William P. Hobby Airport is slated to stay closed through Wednesday. No inbound or outbound flights are occurring at this time. All told, more than 5,000 flights have now been canceled since Friday - most attributable to Harvey.
Both major airports in Houston have been closed amid severe flooding blamed on Tropical Storm Harvey.
Hobart said United put a travel waiver in place earlier last week when it was clear Hurricane Harvey would be bearing down on Houston and that a lot of customers took advantage and re-arranged their travel plans. The discount airline said it recommends that passengers "shelter in place" at Hobby.
Five of the airline's stranded Boeing 737s flew out of the flooded airport Sunday evening back to Love Field in Dallas, according to one of the sources.
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Southwest did not immediately respond to CNN's request for comment.
At both Austin and San Antonio, more than a quarter of all flights had been canceled on both Saturday and Sunday, according to FlightAware's calculations.
Hobart said there are about 200 customers stuck, for now, at the airport. United has more than 4,000 weekly departures from IAH; Southwest has almost 900 weekly departures from HOU. More than 30,000 people are expected to be placed temporarily in shelters, FEMA Administrator Brock Long said at a news conference on Monday.
Southwest said all of its flights at HOU are canceled at least through August 29.