Tornadoes play a more significant role in the Great Plains of the United States than anywhere else on the planet, and Oklahoma is the epicenter. The spring season brings both beauty and restoration from the dormant winter months. Brown turns to green, birds sing loudly, and the aroma of Gulf moisture fills the air. But with the transition comes a darker side, a war between cold and warm as winter desperately tries to hold its ground. Supercells, rotating thunderstorms that produce the greatest number of and most intense tornadoes, unleash the most violent winds on earth with a frequency and ferocity like nowhere else.
While tornadoes have struck all 50 states, the statistics clearly show that the conditions necessary for supercell storms to form occur most often in what is known as Tornado Alley. Tornado Alley does shift from south to north to some degree as spring transitions to summer. As a matter of fact, the tornado season usually ends very abruptly in the middle of June for Oklahoma while increasing simultaneously for areas like northeast Colorado, Nebraska, and the Dakotas. The simplest way to explain why the eastern half of the U.S. experiences the most tornadoes in the world begins with topography.
The Rocky Mountains provide a steady source of cold air aloft while the Gulf of Mexico provides the warm moist air needed at the surface. Thunderstorms require the atmosphere to be unstable. In other words, air forced upward by surface heating, dry lines, cold fronts, warm fronts, etc. will continue to rise on its own due to buoyancy. In meteorology we refer to it as conditional instability because the air only realizes the instability if it is forced upward somehow. So, how does the atmosphere become unstable? Instability develops when cold dry air is displaced above warm moist air. The warm air is less dense than the cold air, and will rise until it reaches a point where the surrounding atmosphere is warmer than it is.
Supercell thunderstorms are most likely to develop in an unstable atmosphere where the winds change in both direction and speed with height. It just so happens that westerly winds aloft (18,000 to 40,000 feet) prevail in our part of the world. When storm systems move east out of the Rockies a surface cyclone develops which causes the surface winds to draw warm moist air north from the gulf underneath the colder air aloft. Bingo, you have the recipe for Tornado Alley.
The graphics I have built illustrate and included in this blog illustrate the probability of where tornadoes are most likely to strike between February and May. Notice the rapid increase in the chances of tornado in Oklahoma for April and May? Tornadoes can strike anytime, but it is important to remember that we are now entering the stretch where they are most likely. The legend on the right side of each graphic shows the percentage of any tornado (F0-F5) striking with 25 miles of a particular point. These graphics are based on a study conducted by Harold Brooks, a research meteorologist at the National Severe Storms Laboratory in Norman, OK. The data set used to produce these graphics was generated from confirmed tornadoes that occurred between 1980 and 1994.
In my next blog, I will show you the frequency of violent tornadoes that occurred between 1921 and 1995, where the bull's eye on Oklahoma becomes even more apparent. The bottom line is: have and plan and stay with News 9, we'll keep you advised!