Houston - The National Weather Service issued a dire warning on Sunday as massive floods inundated Houston in the aftermath of Hurricane Harvey, saying the severe weather conditions were "beyond anything experienced.""This event is unprecedented & all impacts are unknown & beyond anything experienced. Follow orders from officials to ensure safety. #Harvey," it said on Twitter.The city's only other airport, Hobby International, had earlier stopped all flights "due to standing water in runways."At least three deaths have been officially confirmed, and the toll appears certain to rise.Meantime, with flood waters pouring into the basement of Houston's Ben Taub Hospital, one of the largest medical facilities in this fourth-largest US city, authorities began evacuating patients to other hospitals, moving critical patients out first, authorities said.They said the rising water and severe power outages had compromised the safety of food and medical supplies. The area's main airport, George Bush Intercontinental Airport, announced its closing to commercial traffic in the early afternoon."All commercial flight operations have been stopped until further notice," the airport said on Twitter, adding that both inbound and outbound roads were "closed due to flooding."William P Hobby Airport similarly announced on Twitter that, with roadways in and out of the facility flooded, it was ceasing all commercial operations "until further notice."