A Look at the Tornado Report and Watch Climatology for the Continental United States from 1986-2005

While the long-term annual number of tornadoes averaged around 910 from 1950-2005, the number of tornado reports has increased dramatically in the past 20 years with the annual number at 1140 per year. To determine a more recent monthly and yearly tornado climatology, the tornado reports were gridded and examined for the most recent 20-year period (1986-2005), with this data also broken down into two 10-year periods (1986-1995 and 1996-2005).

While the tornado climatologies for the two 10-year periods show only small changes from October through March, the area of maximum tornado activity from April through June has shifted north and east from Texas and extends from central Oklahoma northward into eastern Nebraska and Iowa, with another maximum from Arkansas northeastward into Illinois.

Tornado numbers along Gulf of Mexico and Atlantic coasts were higher in August and September in the 1996-2005 period than the previous ten years, primarily as a result of more hurricanes with tornadoes affecting these regions. While the tornado watch climatology shows a similar picture to the tornado numbers, subtle differences are noted. This may be due to the fact that the tornado number climatology, in some locations, is influenced by a few big tornado outbreaks. On the other hand, the watch climatology provides a forecaster view of where environmental conditions are favorable for tornadoes and a greater number of watch issuances would suggest that these locations may have more days when tornadoes are possible. The combination of the tornado and tornado watch climatology will inform all those interested in weather safety where and when the tornado threat is the greatest for their local area.