Tulsa is the second-largest city in the state of Oklahoma and 47th-largest city in the United States. The city serves as the county seat of Tulsa County, the most densely populated county in Oklahoma, and extends into Osage, Rogers, and Wagoner counties. Located in Tornado Alley, the city frequently experiences severe weather. It is situated on the Arkansas River at the foothills of the Ozark Mountains in northeast Oklahoma, a region of the state known as "Green Country".