A dataset maintained by NOAA’s Storm Prediction Center (SPC) indicates 2011 has likely seen more tornadoes through April 19 than any other season on record (since 1954).
570* tornadoes have been recorded year to date (on the order of 140 from the April 14-16 outbreak alone), more than twice the average of 253 and about 75 more than previous maximum through April 19. These historical tornado numbers are approximate, as they are adjusted based on tornado numbers in 2007. The adjustment is made to smooth a long-term upward trend which is not real. SPC explains further:
Removal of this upward trend is desirable because the increase in tornado reports over the last 54 years is almost entirely due to secular trends such as population increase, increased tornado awareness, and more robust and advanced reporting networks. By removing the upward trend and making the broad assumption that 2007 represents something closer to reality for annual tornado numbers, we can attempt to answer the question, “what constitutes a normal year with respect to modern-day tornado reports?”
The hyper-active tornado season shows no signs of stopping. A persistent dip in the jet stream over the center part of the country is driving cold air south and southeastward, while at the same time warm, humid air from the Gulf of Mexico surges northward. That volatile combination is creating a fertile breeding ground for tornadic storms where these contrasting airmasses intersect. Weather Bell lead forecaster Joe Bastardi writes this pattern supports another outbreak next week: “We are liable to see another 100 to 200 tornadoes before the month is out, making it the most active April ever.”
That this season has supported an abundance of severe weather should not come as a surprise. Spring outlooks were pretty consistent in predicting this kind of season.
(H/T to @wxbrad who posted this graph on Twitter)
* The count of 570 is approximate as it is multiplied by 0.85 to account for possible duplicate local tornado reports. The unadjusted number is 671.