1982 was a particularly prolific year, with more than 150 tornados reported across the country. Tornados usually form during thunderstorms, but not always. Some question whether global warming is responsible for events like the recent tornado in Selly Oak, but there is no evidence for this. Eyewitnesses A BBC Online story with eyewitness accounts of the Selly Oak tornado. The record breaking Oklahoma City tornado.
Weather forecasting is an extremely difficult mathematical problem.
It's also important to take the effect of the oceans into account, as they have a major influence on air currents. In short, to predict the weather with any degree of accuracy requires a computer model of the surface of the entire Earth. But it's possible for weather conditions to be completely different in two places just a short distance apart. For example, the Selly Oak tornado in July was only seen in a few streets. Trying to reconcile the computer model of the whole Earth, which is necessarily on a very large scale, with local weather patterns, which are on a much smaller scale of 1km or less, is one of the major difficulties involved in weather prediction.