Visual Comparison: 3-4 April 1974 and 27-28 April 2011

Posted by on 5 Jul 2011 | 3 comments

By the afternoon of 28 April 2011 it was fully apparent that the unthinkable had happened. In an era of unprecedented communication abilities, a single tornado outbreak took the lives of more people than all the tornadoes over the past several years combined – in broad daylight no less. In the days the followed, many tried to place this event into historical context. Nearly every one defaulted to the 3-4 April 1974 “Super Outbreak”.

The Super Outbreak was nothing short of impressive from a meteorological point of view. 148 tornadoes, 319 fatalities, over 13-states, in 24-hours. Never before, and not until this April, had anything even close to the scale of this tornado outbreak had ever been recorded. By comparison, the tornado outbreak of 27-28 April 2011 has an unofficial count (undertaken by several of us at the Storm Prediction Center) of over 174 tornadoes (done via Public Information Statements) and 259 fatalities attributed to these tornadoes. (Unfortunately, the death toll is considerably higher, I simply have been unable to place all the fatalities to the corresponding tornado at this time.)

From the standpoint of the number of tornadoes recorded and the number of fatalities, these two tornado outbreaks are in a class by themselves (in the “modern” tornado database starting in 1950). In the days that followed, I created a set of figures for internal NWS/SPC/NSSL use to compare the two tornado outbreaks. The images show all reported tornado tracks, color coded based on intensity and the counties are color-filled based on the number of fatalities that occurred within that county’s boundaries. A simple, quick look through the two events shows that the 3-4 April 1974 event covered a much larger area than the 27-28 April 2011 event, although there is considerable overlap between the two events. Several counties experienced fatalities in both events; in fact, Marion County, Alabama was unfortunate to have had a F/EF-5 tornado, and large loss of life, in both of outbreaks (1974: Guin, AL; 2011: Hackleburg, AL). Lastly, each figure has a table of the number of tornadoes and corresponding fatalities, broken down by EF-Scale (the 2011 event is still “preliminary” and subject to change). (Note, higher resolution images, for “zooming” are available by request.)

Meteorologists (and others) can, and will, debate for years as to which event was “more impressive”. I know what my thoughts are, but I’ll spare you those. However, please feel free to leave your thoughts in the comments.

3-4 April 1974 Super Outbreak 27-28 April 2011 Super Outbreak

The two images above are on the same background. This means if you download both of them and flip back and forth between the two, the only things that should change are the county colors and tornado tracks. Below is a zoomed in version of the 27-28 April 2011 event, complete with NWS County Warning Areas and County Names denoted.

27-28 April 2011 Super Outbreak (Zoom)

Keeping Perspective – Preliminary Vs. Final Tornado Reports

Posted by on 17 Apr 2011 | 6 comments

Over the last few days, the southern United States has endured a significant severe weather event that took the lives of a still increasing number of people. While many communities are still trying to sift through the wreckage, meteorologists, “meteorologists”, chasers, and other weather enthusiasts have taken to Facebook and Twitter to discuss what has happened. Unfortunately, a lot of misinformation is floating around. Here’s my quick attempt to clarify some of this information.

As of this writing, 248 preliminary tornadoes have been reported via the Storm Prediction Center’s Preliminary Storm Report webpage over the three days 14-16 April 2011. Much has been made about this number. Unfortunately this number contains many duplicate tornadoes, and potentially even some tornadoes that never were. Hence the label “Preliminary”. Over the next few weeks, National Weather Service Offices throughout the south will be conducting damage surveys to determine the number of actual tornadoes to the best of their ability. There is no doubt that this has been a significant three-day tornado outbreak. However, until the official numbers are released via the National Weather Service’s Storm Data publication, people should exercise extreme caution in trying to quantify where this week’s severe weather outbreak ranks in history.

It used to be the case that the preliminary number of tornadoes underestimated the number of actual tornadoes. However, near March of 2006 the pattern reversed itself with the number of preliminary tornadoes typically overestimating the number of actual tornadoes. For more information regarding preliminary vs. final tornado reports, please read this blog post on the topic by Harold Brooks, which can be found on the United States Severe Weather Blog.

In 2008, the final tornado count was roughly 75-80% of the preliminary count. Based on the number of spotters now reporting tornadoes, it’s not out of the question to assume this ratio is now closer to 70%, if not lower. Based on this an estimated guess to the number of final tornadoes might be closer to 198 (80%), 186 (75%), or even 173 (70%). Since I know people want to know below are the top three-day tornado counts on record using the final tornado numbers. The date given is the last day of the three days used in the count.

Three Day** Final Tornado Counts (1950 – 2010)

  • 20040530: 184
  • 20030506: 183
  • 20040531: 172
  • 19740403: 166
  • 19920617: 152
  • 20030508: 148
  • 19740404: 147
  • 20070506: 136
  • 20070505: 135
  • 20030510: 135

** The day listed is the end of the three days used in the counting.

To download the complete list, please click here.

Also, a disclaimer to the above list: This is a pure count of tornadoes over a three day period. A four-day period with tornado counts of 10, 85, 80, and 0 would show up twice even though for many applications, it probably should only count once. If you wish to throwout duplicate days, please refer to the dataset linked above.

Update 1PM CDT 18 April 2011:
It should be noted that the 75% ratio mentioned above was compute prior to the SPC removing a simple time and space filter. As such, the current ratio will most likely be even lower.