The weather can be significantly different just in the span of a couple of kilometers. This is certainly the case when it comes to storms. One location can experience heavy rain, high wind and hail while another location nearby misses the experience of these weather elements. Because of the high variability of experiences over short distances, forecasting who exactly will experience the storm conditions is not feasible. This is why weather events are often referenced in probability. For example, “there is a 40% chance of thunderstorms” and “there is a 15% chance of a tornado occurring within 25 miles of your location”. Severe weather is commonly forecasted in this probabilistic fashion. Forecasters tend to focus on a region that weather event(s) will occur.

The use of probability removes the problem of having to give a yes/no forecast, such as there will be severe weather at your house. Probability ranges from 0 to 100%. The higher the probability, then the greater the likelihood is of a particular weather event occurring. Some weather events are fairly rare at any one location but are more common over a region. This is why the probability forecasts (such as severe weather damage within 25 miles of your location was developed). For example, a tornado has only a tiny probability of striking any one location but that probability is higher if it is considered as the probability of a tornado striking anywhere within 25 miles of a location (which is about a 2,000 square mile area).