The crucial role of tornado watches in the issuance of warnings for significant tornados

The tornado warning is one of the single most important severe weather products that the National Weather Service issues. Considerable effort is made to provide the information needed to issue a tornado warning in an accurate and timely manner. The tornado watch is a crucial ingredient in this support. The objectives of this paper are to evaluate the service provided by the Watch/Warning program during significant tornado occurrences and to point out the important role tornado watches play in support of the tornado warning program. This study used tornados that resulted in fatalities, plus those rated F3 or greater on the Fujita scale, from the period 1982-1988. For this paper only, the term significant will be used to describe these tornados. Each of these tornados were checked to determine their inclusion 1) in or close to a tornado watch and 2) in a tornado warning. Whereas only about 40 percent of all tornadoes were in or near tornado watches, over 80 percent of the significant tornados in this study were in or near tornado watches. At the same time only about 5 percent of the significant tornados occurred in severe thunderstorm watches, even though the number of severe thunderstorm watches issued annually exceeds tornado watches. This indicates considerable skill in 1) discriminating between tornadic and non-tornadic severe weather threats and 2) forecasting for the more intense tornados. Another surprising finding was that the mode of tornado watch lead time to significant tornado occurrence is 4 to 5 hours. Significant tornados were preceded by tornado warnings in about 30% of the cases studied. However, of these warned tornados, over 90% were preceded by tornado watches. This confirms the critical nature of the tornado watch in setting the stage for timely warnings. This is accomplished by creating a proper mind set in the field forecaster by stressing the meteorological parameters driving the tornado threat. Also, the watch can serve to activate spotter networks, which provide important feedback to the warning meteorologist and possibly increase tornado sightings. The findings of this study suggest that for the optimum public service it is critical that a tornado watch be in place prior to the occurrence of a significant tornado.