The annual tornado trends chart is a result of the following methodology applied to the SPC observed tornado dataset by Harold Brooks, NSSL and Greg Carbin, SPC. As 2009 tornado reports come in and are reviewed, the actual, or "smoothed", tornado numbers will be added to this chart.

Details: A simple linear regression equation is fit to the 1954-2007 annual tornado totals. This equation is then used to compute the delta, or difference, between the original/observed annual tornado total and the smoothed, or "adjusted" annual total represented by the point on the linear trend line for that year.



Using 2007 as the "baseline" year, we apply each year's delta value from 1954 to 2007 to the linear trend value of 1283.3 for 2007. Each year is thereby adjusted, or standardized, to the 2007 annual tornado numbers. (Note that applying the delta of -185.3 to the 2007 adjusted value of 1283.3 results in the original/observed "baseline" total for 2007 of 1098 tornadoes.)

When these annual adjusted values are plotted, we see that the linear upward trend is removed from the data. 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 answer becomes the adjusted average across the 54-year period, or 1283.3 tornadoes per year. This value is also the max trend line value at 2007 that was combined with the individual delta value for each year to adjust all annual totals in the data set.

The daily tornado running totals for each year are then adjusted by multiplying the original observed daily running total by the annual adjustment factor:


This modifies the daily running total values for each year so that the annual total for each year matches the adjusted annual totals arrived at in the steps above. A significant drawback to this methodology is that we cannot add tornadoes to past days with zero tornado reports when tornadoes actually did occur but were never reported. These "unknowns" still introduce considerable uncertainty with respect to what constitutes an average year.

Once the daily running totals are adjusted, the quantile curves are computed by sorting the daily adjusted values across all years and binning those values into lower 25%, lower 50%, upper 50%, and upper 25% bins. The extreme values are calculated the same way by taking the adjusted maximum/minimum running total values on any day in any year. Note: only the extremes, 90th percentile, 50th percentile, and lowest 10% curves are shown on the graph to reduce clutter. Essentially, these curves represent "benchmarks" for the annual running total and as the year progresses we can make some assessment about whether the running total is above or below the long-term adjusted trend.

Finally, the current tornado running total for this year is added to the chart. The latest value is an adjusted value from the preliminary Local Storm Report (LSR) tornado count. This is done in an attempt to reduce the number of duplicate reports. See: this post for a full description of the preliminary vs. final tornado count issue.