Tornadoes without NWS warning

During a five-year period of study from 2000 to 2004, slightly more than 26% of all reported tornadoes across the U.S. occurred without a National Weather Service (NWS) warning being issued. This study examines some of the reasons behind why no warning was issued with these tornadoes, and what climatological, storm classification, and sociological factors may have played a role in the lack of a warning. This data set of tornado records is sorted by F-scale, geographically by region and weather forecast office (WFO), hour of the day, month of the year, tornado path length, tornado-to-radar distance, county population density, and number of tornadoes by day and order of occurrence.

Results show that the tornadoes most likely to strike when the public is least likely to be aware are also those tornadoes with the greatest chance of not being warned. Singular tornado events (one tornado report per day within a WFO County Warning Area) and the first tornado report of the day were the most difficult scenarios on which to warn, with over half of all single tornado events not warned. Weak, isolated, and nocturnal tornadoes had a much higher ratio of not being warned. Geographic areas that experience a significant proportion of weak, isolated, and/or nocturnal tornadoes, such as Florida and the western U.S., had a much higher ratio of missed warnings. In general, the stronger the tornado, as estimated from its F-scale and/or track length, the greater chance it was warned. However, many weak tornadoes were not warned, and the overall ratio of missed tornado warnings to reported tornadoes actually increased over more densely populated regions, likely due to more complete post-event verification. The tornado distance from radar had a significant impact on tornado warning statistics. Due to a small sample size, storm classification results were inconclusive, but suggest that linear events may lead to more missed warnings.