Harvey blocks major air travel artery through Houston

(CNN Money) — Houston’s airports took a direct hit by Hurricane Harvey, leaving commercial air service at a standstill and a major air transit artery blocked just days before one of the busiest travel weekends of the year.

George Bush Intercontinental Airport was shut down Sunday to all but military and relief flights for the storm-battered city. The airport is expected to remain closed at least until Thursday, August 31 at noon CT, according to the Federal Aviation Administration.

IAH, shorthand for the Houston’s biggest airport, is the 14th busiest airport in America with 20 million passengers flying through in 2016. It’s the second largest hub for United Airlines.

Houston’s Hobby Airport, a hub for Southwest Airlines, also remained closed Monday and wasn’t scheduled to reopen to commercial flights until Wednesday at 8 am CT.

With more rain expected to pound the city in coming days, it was unclear when the airports might reopen. A decision to reopen ultimately falls to the city.

Travelers at United’s other hubs said flights were operating with long lists of standby passengers who would’ve otherwise flown through Houston. A United spokeswoman said it was adding capacity elsewhere in its network to account¬†for the Houston closure.

United customers who were flying from Houston on Friday ahead of the Labor Day holiday weekend were already receiving cancellations notices from the airline.

Airlines will move planes around and add flights to accommodate passengers who would’ve flown through Houston. Carriers will “rob Peter to pay Paul to pad their schedule,” said Ken Jenkins, principal crisis response strategist at NavAid Crisis Consulting Group.

But already congested airports mean open gates in other cities are scarce. “It’s a huge undertaking, especially when you’re a huge airline like United and Southwest,” said Jenkins.

The impact of Houston’s closure is also likely to ripple across airline operations throughout the country in coming days as pilots and flight attendants based in Houston, country’s fourth largest city, aren’t able to get to their assignments.

Once back online, don’t expect airports to return to immediately return to normal, he said.

“The number of passengers is going to be exponentially higher” at the airports as airlines catch up from days of backed up travelers returning to the airport, he said.

Related: Southwest flies stranded Houston customers out of flooded airport

Despite the flooding across the city, infrastructure at IAH appeared to be largely functioning Monday. A live webcam stream showed the automated trains that connect the terminals functioning and relief flights continued to land throughout the day.

IAH sits 96 feet above sea level, compared to 46 for Hobby.

At Hobby, the status was less certain. Photos on social media Sunday showed the airport partially under water. By Sunday evening, Southwest was able to fly out five of its 737s from its Hobby hub, airlifting around 500 stranded customers to Dallas.

The FAA said instrument landing systems for arriving planes were offline at both airports.