States in the plains such as Oklahoma and Kansas have a reputation for having numerous tornadoes and very intense tornadoes. These states and surrounding states are in what is commonly called tornado alley. Tornadoes tend to be more frequent and sometimes more damaging in this region of the country. Is this true and if so why? The overly simplified explanation for the numerous tornadoes in the plains is that this is a region in which cold air clashes with warm air. While there does tend to be a strong temperature gradient along fronts in the plains this explanation only touches the surface to why tornadoes are common in the plains. A tornado requires a parent thunderstorm. Thus, an environment conducive to tornadoes must also be conducive to thunderstorms. Thunderstorms are much more common in certain locations. Thunderstorms are not nearly as common to the west of the plains as they are across the plains and to the east of the plains. The Gulf coast region has the most thunderstorms. For a location to have thunderstorms on a regular basis there needs to be adequate low level moisture and instability. The Gulf of Mexico is a major supplier of low level moisture to the plains and to the east of the plains. This moisture however is not able to work its way very well into the western U.S. The western U.S. can be influenced by the Pacific Ocean but that ocean water is relatively cold and the mountains block much of the moisture from penetrating very far inland. The low level dewpoints supplied by the Gulf of Mexico are a major factor in thunderstorm development. These high dewpoints are one factor that can promote instability. Instability is promoted through warm surface temperatures and a rich moisture supply near the surface. Cooling of air high aloft such as from a shortwave also contributes to instability. The richness in moisture supply tends to decrease when moving north. Throughout a long portion of the year the Northeast U.S., Great Lakes region and northern U.S. will be cool to cold in temperature thus that air can not support high dewpoints. It is the Southeast U.S. that has the highest dewpoints over the course of the year. This region will have thunderstorms year round. Since a tornado requires a thunderstorm we have narrowed the region most likely to have tornadoes to east of the Rocky mountains and in the southern half of the U.S. Keep in mind also that in the warm season Gulf moisture and heat can advect into the northern U.S. bringing thunderstorms and severe weather to those areas. The two main influences that create a thunderstorm environment in which a significant tornado could occur is a significant amount of instability and a significant amount of low level wind shear. In the overly simplified explanation for tornadoes in the plains it was stated this region often has cold and warm air clashes. Strong temperature gradients tend to produce strong winds. The flow associated with a low pressure system, a trough and a front will generate wind shear. If the winds in the low levels are weak and unorganized then a tornado is not as likely. The wind flow within the middle of a maritime tropical airmass tends to be weak and unorganized. This is why areas near the Gulf coast such as Florida can have many thunderstorms but little or no tornadoes in July for example. Where maritime tropical air meets up with cool air coming from the north, the low level wind field increases in speed and organization. Fronts, troughs and low pressure systems move through the plains more than they do along the Gulf coast over the course of a year. In the summer for example, weeks can go by along the Gulf coast before a cold front can penetrate this region. When tornadoes do occur in the Southeast U.S. and along the Gulf coast it tends to be in Spring and Fall when the troughs, low pressure systems and fronts do start to penetrate into that region. The plains states are routinely influenced by fronts in all seasons. The plains is also a region where extremely dry air and extremely moist air can clash. The high plains is the home of the dryline. If these dry and moist air masses clash in the right way it can set up a sounding profile that can further enhance instability. Essays on potential or convective instability are relevant to this concept. In conclusion, the plains states most often have tornadoes due to a more frequent combination of low level high dewpoints, instability, low level wind shear and lifting mechanisms (troughs, low pressure systems and fronts). Tornadoes can occur in any part of the country these factors come together. It is just that in the Great Plains these factors come together more often and at times to a particularly intense magnitude. |