The deadly early-season tornado outbreak of March 2 - 3 that hit Indiana, Kentucky, and surrounding states, killing 41 people, may have been the 10th largest two-day tornado outbreak since record keeping began in 1950. NOAA's Storm Prediction Center now lists 132 preliminary tornado reports for March 2, and 11 for March 3. It typically takes several months to finish damage surveys and verify all the tornadoes that occur in a large tornado outbreak.
10th Largest Outbreak in History
Sixty-one tornadoes have been confirmed so far, according to Wikipedia's tally of the outbreak. The two-day total of 143 tornadoes from March 2 - 3 is probably an over count of about 15%, based on historical levels of over counts. This would give the March 2 - 3 outbreak around 120 tornadoes, making it the tenth largest outbreak since record keeping began in 1950. Assuming this is true, the past two tornado seasons would hold four of the top ten spots for largest tornado outbreaks in recorded history. Below are the top two-day tornado outbreaks since 1950. Several of these two-day totals were taken from outbreaks that lasted three or more days; the highest two-day period of activity was selected for this list, so that the outbreak would not be mentioned multiple times. The numbers from the 2011 outbreaks are still preliminary:
262, Apr 26 - 27, 2011
169, Apr 3 - 4, 1974
160, May 29 - 30, 2004
141, May 24 - 25, 2011
135, Jan 21 - 22, 1999
130, Apr 15 - 16, 2011
125, May 4 - 5, 2003
123, Jun 15 - 16, 1992
121, May 4 - 5, 2007
120 (still counting), March 2 - 3, 2012
120, May 3 - 4, 1999
Counting Tornadoes is an Inexact Science
While there's no question that having four top-ten tornado outbreaks in just two years is highly unusual, the quality of our tornado data base is poor, and there are probably outbreaks that occurred prior to 1990 that were significantly under-counted and would have made the top ten list, had they occurred today. The number of tornadoes being reported has increased in recent decades, and this increase may be due entirely to factors unrelated to climate change:
1) Population growth has resulted in more tornadoes being reported.
2) Advances in weather radar, particularly the deployment of about 100 Doppler radars across the U.S. in the mid-1990s, have resulted in a much higher tornado detection rate.
3) Tornado damage surveys have grown more sophisticated over the years. For example, we now commonly classify multiple tornadoes along a damage path that might have been attributed to just one twister in the past.
However, it is difficult to make solid conclusions on how tornadoes may be changing, since the quality of the historical tornado data set is so poor. This is largely due to the fact that we never directly measure a tornado's winds -- a tornado has to run over a building before we can make an EF-scale strength estimate, based on the damage.
