In May of 2013, the U. S. Storm Prediction Center sent out a tornado warning sixteen minutes before a devastating twister touched down in Newcastle, OK, killing twenty-four people and destroying dozens of homes and buildings. Despite the destruction and loss of life, the Newcastle tornado represented a milestone in tornado prediction, as the usual warning time is between ten and thirteen minutes. In Oklahoma that day, residents had the benefit of an additional three to six minutes of lead time as they scrambled to find shelter. Compared to tornado prediction in the 1980s, when the storms could be predicted just five minutes before touching down, the sixteen minute warning was truly remarkable. Scientists believe that even longer warnings are possible, but the prediction of these types of storms remains very challenging.
In an era when meteorologists can forecast a hurricane a week in advance, it can be difficult to imagine that there remain weather events that cannot be accurately predicted with hours or days of advanced notice. Unfortunately, unlike hurricanes or blizzards, tornadoes are very short-lived weather events and their paths are much shorter. While a hurricane often begins as a storm cell off the African coast and is tracked for days, even weeks, using satellite imagery, as it moves thousands of miles east and makes landfall in North America, a tornado can appear suddenly as a result of an instant synthesis of multiple weather features.
Tornadoes form suddenly, and meteorologists admit that there seem to be some elements of chance involved. A tornado forms when moist, warm air combines with faster-moving, cooler air and creates with wind shear. Wind shear is a significant difference in wind speed and direction over a short distance. Wind shear can be horizontal or vertical and can result in a rotation of wind currents. This rotation, combined with the different air temperatures can, but does not always, result in a tornado.
The Midwest of the United States provides the perfect environment for these chance weather events. In an area known as Tornado Alley, which lies in the middle of the United States between the barriers of the Rocky Mountains and the Appalachian Mountains, artic winds moving out of Canada and tropical winds moving north from the Gulf of Mexico collide. As early as 1870, the United States government noted the frequent proliferation of storms in this area, and began to track conditions likely to produce tornados. The first warnings, or probability predictions, provided residents with general information about a tornado “season” when it was more likely a storm would form. They did not predict specific storms, or even specific days on which a storm might occur.
In 1953, the National Weather Service began a program that provided warnings of tornado-forming conditions as they emerged. While these warnings still did not predict specific storms, they were able to identify a time of day when conditions would be conducive to tornadoes. Prior to 1953, tornadoes killed at least 250 a year. With the warning system, that number dropped to 120 in 1953 and to 28 in 1962. It seemed, for a time, that a network of weather centers, by combining data and maintaining communication, were developing a successful means of giving residents some sort of advanced warning and, in some rare cases, even predicting a tornado touchdown a few minutes before it occurred. The accuracy, however, was spotty and communication between weather centers, in an era before satellite and cellular communication devices, was unreliable. Therefore, no consistent success in prediction was possible.
With the advent of high-speed computing technology from the late 1960s to the present, tornado prediction has improved significantly. Scientists have developed mathematical models based on past tornado events. By using these models and measuring real-time atmospheric conditions, they can now identify more discrete areas that are likely to see tornado activity, and, with growing accuracy, pinpoint tornados as they begin to form, providing more precise probability models and advanced warnings of touchdowns.
According to the passage, what accounts for the rotation within a tornado?
