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Weather Almanac for June 2013COUNTING TORNADOES
The link to last month's Almanac came when I read Henry Margusity's Accuweather blog Meteorological Madness. In it, he makes the interesting statement concerning the 2013 dearth of tornadoes: “Keep in mind, while the lack of tornadoes has been a good thing, it does show us how this pattern rates extreme in a different manner then one would think. We tend to think of extremes being weather that causes death and destruction, but in this case, the lack of a specific weather event over a long period is just as extreme.” If a Tornado Strikes in the Open Plains, Is It Severe?The heading title here paraphrases the old quandary of a tree falling in the wilderness, but it also makes a technical point. Since the two Fujita scales base the strength of the tornado on the damage it caused, a twister churning up dirt and grass would rate low on a damage scale almost without regard to its true strength. The same storm striking a residential area would likely do considerable damage and rate much higher. This is not a condemnation of the methodology, but our current inability to accurately measure the force/energy of all tornadoes in real time. (If you have followed shows such as Storm Chasers, you know the difficulty of getting real-time measurements of winds generated in tornadic vortices, unless the portable doppler radars get close. I emphasized all because there are some caught by the dopplers that can be measured.)  The ability to catch the fleeting nature of tornadoes has changed dramatically since I was a kid growing up in Northeastern Illinois. For the most part, tornadoes were then reported based on the damage they left behind, and many only tore down vegetation or tore up crop land and were largely uncounted. I recall one in 1959, I believe, that rumbled through the newly planted cornfields that less than a decade after would have rumbled through a community of more than 10,000. It tore up some corn and did no other damage, and so is unlisted in Grazulis' book Significant Tornadoes. But today, with advanced radar, mesoscale observational networks, a cadre of well-trained local storm spotters, citizens with a variety of photographic and video devices, and an army of storm chasers following nearly every cumulonimbus in sight, it is hard for a tornado to miss detection. Add to the mix an increase in population in areas that may fall under a tornado event, and thus more eyes to see them.
The US National Oceanic and Atmospheric Administration (NOAA) recognizes this difficulty. In their discussion on counting tornadoes, they state: “One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. Unlike rainfall or temperature, which may be measured by a fixed instrument, tornadoes are ephemeral and very unpredictable. If a tornado occurs in a place with few or no people, it is not likely to be documented. Unfortunately, much of what we know as tornado alley was very sparsely populated until the 20th century, and so it is possible that many significant tornadoes may never have made it into the historical record.” “Today, nearly all of the United States is reasonably well populated, or at least covered by NOAA's Doppler weather radars. Even if a tornado is not actually observed, modern damage assessments by NWS personnel can discern if a tornado caused the damage, and if so, how strong the tornado may have been. This disparity between tornado records of the past and current records contributes a great deal of uncertainty regarding questions about the long-term behavior or patterns of tornado occurrence. Improved tornado observation practices have led to an increase in the number of reported weaker tornadoes, and in recent years the number of EF-0 and EF-1 tornadoes have become more prevelant in the total number of reported tornadoes. In addition, even today many smaller tornadoes still may go undocumented in places with low populations or inconsistent communication facilities.” “With increased national Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the true variability and trend in tornado frequency in the U.S., the total number of strong to violent tornadoes (EF3 to EF5 category on the Enhanced Fujita scale) can be analyzed. These are the tornadoes that would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports.”
The National Weather Service (NWS) has a rule of thumb that for every 100 preliminary tornado reports, at least 65 tornadoes are typically confirmed by the assessment teams. Thus, the initial number of storms reported by the Storm Prediction Center (SPC) takes some time to investigate, and the final count published by the SPC may not be determined for several months when all reports have been investigated. Thus, final monthly numbers are typically less than the preliminary counts reported at the end of a calendar month. Are the Number of Tornadoes Increasing?This statement can be interpreted in two ways. First, it is obvious from the above discussion that the number of tornadoes reported has increased dramatically over the past half century in the United States (and elsewhere around the world for many of the same reasons). And as NOAA said above, that increase in seen most in the weaker storms: “the number of EF-0 and EF-1 tornadoes have become more prevelant in the total number of reported tornadoes.” The second interpretation to the question is concerned with the true trend in tornado events. This is an important question in the climate change debate as well as concerns over public safety. To look toward an answer, NWS scientists have looked at the observation numbers for a preliminary answer. To do so, they first must find a way to account for obvious trends in the numbers for reasons cited above. Again, as stated above, they look to “better understand the true variability and trend in tornado frequency in the U.S., [using] the total number of strong to violent tornadoes (EF3 to EF5 category on the Enhanced Fujita scale).”  Annual Total of Tornadoes rated F3 or Higher: 1954-2012 |
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This month, I want to look at tornadoes from a climatologist's point of view, and in some respects, continue on with last month's discussion on normals and rare events. As of this writing, the United States was finishing a stretch with one of the lowest number of tornado occurrences in modern records.
Given that a good number of tornadoes are viewed by dozens, and perhaps hundreds of eyes, how do we count the actual number of storms given that observers may be seeing the same storm from a different angle, different storms in a cluster, or perhaps misinterpreting an oddly shaped cloud, downburst, or rainshaft as a tornado?
As a result, the number of storm reports generated during bouts of severe weather often greatly overstate the actual number of tornadoes. Assessment teams comb the reports and seek out the areas of storm damage to determine if indeed it was a tornado. Once the damage is established as from a tornadic storm, its EF rating must be determined along with any information that may shed light on its path, width, or duration.
