With every tornado outbreak comes a bevy of tornado statistics. Often times these stats will arrive within a day of the outbreak ending, and they are almost always based on preliminary information. They also are sometimes missing that important “preliminary” caveat. In order to understand the difference between tornado reports and actual tornadoes, you need to be familiar with the process of how a tornado gets confirmed by the National Weather Service (NWS).
- A tornado, funnel cloud, and/or damage is sighted and reported to the NWS, or NWS doppler radar detects a debris signature. This can either get logged as a “tornado” or “thunderstorm wind damage” event type, depending on the confidence level as to whether or not a tornado actually occurred, and sent out as a preliminary Local Storm Report product. The main purpose is to transmit information about damage and severe weather occurrences, rather than an exact tabulation of the number of tornadoes.
- Damaged areas are surveyed and reports are further scrutinized after the event is over. This will lead to an official determination as to whether or not a tornado occurred. If a specific area is impacted by numerous and/or long-tracked tornadoes, this process can take some time.
- Several months after the event, the tornadoes that were determined earlier are officially certified in NWS Storm Data. The tornadoes and associated statistics that are put into Storm Data are the basis for the official US tornado and severe weather record.