The climate varies from the semi-tropical in the Mississippi Delta, South Louisiana, and Southeast Texas, to the dry Chihuahuan desert in West Texas. The southeastern portions include the Appalachian mountains in Alabama and Tennessee and the Piney Woods of East Texas, Louisiana, and southern Arkansas and the Mississippi Delta. A large portion of the northeastern quarter of the region is mountainous, with the Ozark and Ouachita Mountains of Arkansas and eastern Oklahoma. The northwest quarter of the region is dominated by the Great Plains which become progressively drier west of 100° W, forming the North American Llano Estacado. The southwestern portions border the Rio Grande, and are generally drier than other areas of the South Central United States.