1. BP-77 The Correlation of El Niño-Southern Oscillation to Tornado Occurrence in the Mid-South Thomas L. Salem Jr. and Preston Bradley National Weather Service Memphis, Tennessee Introduction Protection of lives and property is the mission of the National Weather Service. For the NWS forecast office in Memphis this includes providing timely and effective information in outlooks, forecasts, briefings, and warnings for severe weather including tornadoes. Ashley (2007) shows the Mid-South is the center for the highest frequency of killer tornado events in the country from 1950 to 2004. Also, Grady et al. (2011) showed that the tornado risk extends from the Great Plains (the traditional “tornado alley”) into the southeast U.S., sometimes called Dixie Alley (not shown). This study looked to find if there is a correlation between tornadoes in the Mid-South and El Niño-Southern Oscillation (ENSO). Background Information The Mid-South has had tornadoes in every month and just about every month there has been a deadly tornado with records dating back to 1873 (NWS 2015). The Climate Prediction Center has data for ENSO categories only since 1950 (CPC 2015). Using these two databases we will look at tornadoes in the Mid-South in comparison to the associated ENSO category or episode. The CPC defines El Niño (warm) periods when there are five consecutive months with Oceanic Niño Index (ONI) greater or equal to 0.5 over the Niño 3.4 region. La Niña (cold) periods are when ONI is less than or equal to ‐ 0.5 for five consecutive months. All other months are considered neutral. Mid-South Tornadoes compared to ENSO A comparison of Mid-South Tornadoes and ENSO episodes results in the following graphs for the number of tornadoes and the number of deadly tornadoes in each month. These images show that neutral episodes stand out. However, the number of times months are in neutral periods are typically more than either El Niño or La Niña. Normalizing the information provides a better look at which months are more active in comparison to ENSO episodes. The winter months during La Niña years are more active than other phases of ENSO. Neutral years have no deaths in January and February. Since 1950, there have been an average of about 15 tornadoes per year in the Mid-South; with 51 tornadoes occurring in 1999 and only 2 tornadoes occurring in 1960 and 2007. A box and whisker plot of tornadoes for each month shows that during La Nina episodes the winter months are active, meanwhile Neutral episodes the Spring months are more active. The phase of ENSO does not guarantee a tornado in any month, since all months have had times with no tornadoes for the month. The number of tornado outbreak days--a day with 6 or more tornadoes in the Mid-South, a modified definition used by Grazulis (1993)—during Neutral episodes of ENSO are mainly in the Spring months. And during La Nina episodes the tornado outbreaks are more in the winter months. Summary Climatologically, the months of March through June have the most tornadoes in the Mid-South. The ENSO Neutral episodes result in the highest frequency of tornadoes during this period including the highest occurrence of deadly tornadoes. The correlation of these ENSO patterns to tornado frequency can help the forecasters and preparedness officials anticipate active tornadic months and communicate heightened seasonal tornado threats. Reference Ashley, W.S., 2007: Spatial and temporal analysis of tornado fatalities in the United States: 1880-2005. Wea. Forecasting, 22, 1214-1228. Climate Prediction Center, cited 2015: Cold and Warm Episodes by Season. [Available online at http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml.] Dixon. P. G., A. E. Mercer, J. Choi, and J. S. Allen, 2011: Tornado Risk Analysis: Is Dixie Alley an Extension of Tornado Alley? Bull. Amer. Meteor. Soc., 92, 433–441. Grazulis, T.P., 1993: Significant Tornadoes: 1680-1991. Environmental Films, St. Johnsbury, VT, 1326pp. National Weather Service, cited 2015: NWS Memphis Tornado Database. [Available online at http://innovation.srh.noaa.gov/tors/index.php?cw=meg.] The authors would like to thank Tim Janhsen, Southern Region Headquarters, for printing this poster. Percent of months with different ENSO Episodes and the percent of tornadoes