WRAL WeatherCenter Blog

Trend in Twisters?

Posted March 23, 2010 10:05 a.m. EDT

A recent comment on a WeatherCenter blog post asked what kind of trend had occurred in North Carolina tornadoes over the past ten years. I answered by hunting up the numbers for the state over that time span (2000-2009) and found it difficult to detect much of a meaningful trend given the high variability from year to year, and the high dependence on very short-time scale localized conditions that determine whether a tornado, or multiple tornadoes, will form in a given scenario.

I did make a simple graph a little later, and thought I'd share that here. It shows the total number of reported tornadoes for the state (blue line) and the number of "significant" tornadoes, those reaching intensity of EF2 and up and therefore classified as "strong" (EF2 and EF3) or "violent" (EF4 and EF5) in red. Although an "eyeball trend line" might show a bit of an upward slope through the decade, this impression is at least somewhat offset by noting that both 2002 and 2007 were rather quiet, with less than 15 total tornadoes, while both 2004 and 2008 were very active, with over 60 twisters in the database. 2008 was also notable with regards to intensity, as 13 of the tornadoes ranked at EF2 or higher.

The small number of years involved here also made it difficult to relate the totals to the state, for example, of El Nino or La Nina. Historically, whether the El Nino Southern Oscillation is in an El Nino, La Nina or Neutral phase doesn't correlate in a statistically significant manner to the number of tornadoes in our state, although La Nina does correlate positively to an increase in hail and wind damage reports during the spring, while El Nino hail and wind damage reports are about the same as Neutral years. During the span covered by this graph, for example, the low activity in 2002 was in a year that started neutral and then trended toward a strong El Nino, while the rather inactive 2007 started neutral and trended into a moderate La Nina. Likewise, the very active 2004 and 2008 were characterized by a neutral start trending into a weak El Nino ('04) and then a moderate La Nina followed by a shift to neutral conditions ('08).

All in all, knowing that we're coming out of a moderate to strong El Nino that is expected to trend toward a neutral pattern by this summer doesn't help very much in assessing the threat of tornadoes for the year as a whole!