By Heide Brandes OKLAHOMA CITY, May 31 (Reuters) - Violent thunderstorms on Friday produced tornadoes in central Oklahoma that killed five people including a mother and her baby and menaced Oklahoma City and its hard-hit suburb of Moore, authorities said. The mother and baby were killed while traveling on Interstate 40, just west of Oklahoma City, when their vehicle was picked up by the storm, said Betsy Randolph, spokeswoman for the Oklahoma Highway Patrol. The interstate was shut down due to the storm, with multiple crashes and injuries. Two of the deaths occurred in Union City and one was in El Reno, west of Oklahoma City, said Amy Elliott, a spokeswoman for the state Office of the Chief Medical Examiner. She could not immediately confirm where the other deaths occurred. Some 40 to 50 people were being treated for storm-related injuries, including five patients in critical condition, among them a child, according to the Integris Health hospital system. National Weather Service meteorologists had earlier declared a tornado emergency for parts of the Oklahoma City metropolitan area, and a twister touched down in Moore, which was hit by a massive EF-5 twister last week that killed 24 people. A tornado also rampaged down Interstate 40 toward Oklahoma City, tipping over dozens of trucks, one witnesses said. Television images showed downed power lines and tossed cars as the storm systems dumped heavy rains, stranding motorists in flood water. "For reasons that are not clear to me, more people took to the roads, more than we expected. Everyone acted differently in this storm, and as a result, it created an extremely dangerous situation," Oklahoma City Mayor Mick Cornett said. "I think we are still a little shaken by what happened in Moore. We are still burying children and victims, so our emotions are still strong," he added. Brandi Vanalphen, 30, was among the hundreds of drivers trapped on traffic-snarled roads as she attempted to flee the tornado system menacing the suburb of Norman. "What got me scared was being stuck in traffic with sirens going off," she said. "I started seeing power flashes to the north, and I said 'screw this.' I started driving on the shoulder. People started driving over the grass." Oklahoma Governor Mary Fallin told CNN that motorists stuck on any freeway in the path of a twister should try to go in the opposite direction to where the twister was coming from. "What we saw from the tornadoes that came through Moore and the other ones last week was that people who were in cars on the Interstate were killed," Fallin told CNN. Moore Mayor Glenn Lewis told CNN it was "unbelievable" that Moore had been hit again. CITYWIDE FLOODING Tim Oram, meteorologist for the National Weather Service, said it was difficult to know exactly how many tornadoes had touched down, but three major thunderstorms were potentially producing tornadoes throughout the center of the state. The service later lifted a tornado warning for Oklahoma City and surrounding areas, as flash floods in the wake of the storms dunked parts of the sprawling metropolitan area - home to more than 1.3 million people - under water. Will Rogers Airport in Oklahoma City was shut down as it sheltered 1,200 people, local station KWTV reported. Storms also swept into neighboring Missouri, where Governor Jay Nixon declared a state of emergency. "Much of Missouri is experiencing dangerous severe weather tonight, on the heels of several days of heavy rain," Nixon said. "I urge Missourians to closely monitor weather conditions, so they can take shelter or move to higher ground if needed." The St. Louis Post-Dispatch newspaper reported downed trees, road closures and damage in the metro St. Louis area, but no deaths or serious injuries. Power utilities Oklahoma Gas and Electric and Ameren said 171,000 customers were without power in Oklahoma, Missouri and Illinois, which had been under a tornado warning on Thursday. On Thursday, storms in Oklahoma and Arkansas killed at least three people, including Scott County Sheriff Cody Carpenter, whose body was recovered early on Friday, said Keith Stephens, a spokesman for the Arkansas Game and Fish Commission. Authorities continued to search for a missing game warden on the Fourche La Fave River. A man also died in Tull, Arkansas, when a tree fell on his car, and a woman's body was found in flood waters in Scott County on Friday. Large, long-lasting thunderstorms known as supercells are responsible for producing the strongest tornadoes, along with large hail and other dangerous winds. Tulsa, Oklahoma, as well as Springfield, Missouri, may all be buffeted by Friday's severe weather and possible tornado touchdowns, said Rich Thompson, a lead forecaster at the National Weather Service's Storm Prediction Center in Norman, Oklahoma. The danger zone included Joplin, Missouri, he added. Joplin was hit by a monster tornado that killed 161 people and destroyed about 7,500 homes two years ago.