THE TORNADO CLIMATOLOGY OF THE ST. LOUIS WEATHER FORECAST OFFICE COUNTY WARNING AREA
Mark F. Britt* and Fred H. Glass
NOAA/NWSFO St. Louis
St. Charles, Missouri
1. INTRODUCTION
Clearly the most important task facing a NWS field office forecaster is the prediction and warning of severe local storms. At the very foundation of the forecast and warning decision process is a knowledge of the climatology of the severe weather events. Knowing the annual, monthly, and hourly distribution of tornadoes can give a warning decision maker greater insight into the commonalities and probabilities of an event occurring, even before the first thunderstorm forms.
County Warning Areas (CWAs) were redrawn in the 1990s as part of the Modernization and Reorganization of the National Weather Service (NWS). The newly reorganized CWAs are larger and encompass geography never previously linked together. This paper examines the climatology of tornadoes of the National Weather Service Forecast Office in St. Louis, Missouri (NWS LSX). The NWS LSX CWA encompasses 46 counties (Figure 1, below) in central and eastern Missouri and portions of western Illinois, with an approximate population of 3.4 million.
Figure 1. WFO LSX County Warning Area
2. DATA
2.1 Data Sources and Methodology
The Storm Prediction Center's (SPC, formerly the National Severe Storms Forecast Center) severe weather database was used for this study (Schaefer and Edwards, 1999). This database contains tornado statistics from 1950 to 1995. Statistics for 1996 to 1999 were extracted from Storm Data (NOAA 1959-1999) for inclusion in this study.
Since tornado warnings are issued and verified by county, the SPC database contains information on tornado segments rather than tornadoes. A segment is counted each time a tornado touches down or crosses a county line, therefore a single tornado which moved across three counties would be recorded as three separate tornado segments. After close examination of all tornado segments using the Severe Plot software (Hart 1993) and Storm Data, the authors were able to merge individual segments back into individual tornadoes.
Tornado intensity is determined by using the F-Scale (Fujita 1981), as listed in Storm Data. This paper follows the accepted nomenclature that F2 and F3 tornadoes are strong and F4 are violent. Tornado times are in Central Standard Time (CST) and refer to the hour in which they occurred. For example, 14 refers to any tornado that occurred between 1400 to 1459 CST. A tornado day is a traditional 24-hour day (0000 to 2359 CST) in which at least one tornado occurred.
2.2 Data Limitation
Kelly et al. (1985) and Hales (1993) have documented a number of limitations inherent to the SPC database. They found that the number of severe weather reports increases in areas with greater population density and/or a greater concentration of trained spotters and well-educated people. Highway distributions, distance to an official reporting station, time-of-day, the warning office's motivation in verifying warnings, lack of appropriate measuring devices, intervening clouds, and topographic features are additional factors which affect severe weather reporting. Finally, the perception and motivation of the person witnessing the event must be considered. For example, a farmer may not feel obligated to report a tornado that touches down in an open field.
Ostby (1993) found that the occurrence of weak tornadoes (F0-F1) has shown a dramatic increase since 1980, while the number of strong and violent tornadoes has either remain steady or decreased. Reasons for this include improved verification efforts by local NWS offices and the marked increase in storm chasing. Since strong and violent (or significant herein) tornadoes produce a more stable long-term data set, these categories were the main focus of this study.
3. RESULTS/DISTRIBUTIONS
3.1 Overall and Yearly Distribution
During the 50 year period from 1950 through 1999, a total of 539 tornadoes (all intensities) were identified in the LSX CWA. Significant tornadoes accounted for 168 of the tornadoes (or 31% of the total), resulting in an average 3.32 strong or violent tornadoes per year. Significant tornadoes have occurred in the LSX CWA in 8 out of every 10 years (Figure 2, below)). The most active year was 1957 with 12, mainly because of two days with large outbreaks across the CWA: May 21 and December 18. The database indicates there has been a gradual decline in the number of significant tornadoes in the past 50 years. The 1950s had a total of 44 tornadoes, while the 1990s only had 24.
Figure 2: Yearly significant tornado distribution
The LSX CWA has experienced 106 significant tornado days (days with one or more strong or violent tornadoes) from 1950-1999, an average of 2.12 days per year (Figure 3, below). The number of days has also shown a gradual decline.
Figure 3. Significant tornado days distribution
3.2 Monthly/Seasonal Distribution
The most volatile time of year for tornadoes is the middle to late spring. Figure 4 (below) depicts May having the highest incidence of significant tornadoes at 38, with April a close second at 35. The four-month period from March to June contains 65 percent (109 out of 168) of all the significant tornadoes. Summer and early Autumn are relatively inactive, with August only having two strong tornadoes observed in the 50 year period. There is sound evidence of a secondary maximum from late autumn into early winter as the number of significant tornadoes increases to a peak of 16 in December; 21 percent of all significant tornadoes occur between October and December.
Figure 4. Monthly significant tornado distribution
When the monthly distribution of all tornadoes (all F scales) is plotted versus significant tornadoes (Figure 5, below), some noteworthy observations are evident. While the four-month period from March-June contains the greatest percentage of significant tornadoes (65 percent; 109 out of 168) and of all tornadoes (66 percent; 355 out of 539), only 31 percent of all tornadoes which occur during this period are significant (109 out of 355). In contrast, the four-month period from November-February contains only 23 percent of the significant tornadoes (38 out of 168) and only 15 percent of all tornadoes (78 out of 539), however 49 percent of all tornadoes which occur during this period are significant (38 out of 78). Thus for any tornado occurrence, the greatest probability of it being significant, is during the late autumn and winter.
Figure 5. Monthly distribution comparison between all tornadoes and significant tornadoes
3.3 Hourly Distribution
Tornadoes are mainly a late afternoon and evening phenomena in the LSX CWA (Figure 6, below), with two thirds (113 out 168) of the significant tornadoes occurring between 300 and 1100 pm CST. Only 5 percent (8 out of 168) occurred between 300 and 1000 am.
Figure 6. Hourly significant tornado distribution
A scatter plot showing the date/time distribution of strong-violent tornadoes yields some additional interesting results (Figure 7, below). Tornadoes that occur in the late afternoon and evening hours (300 to 1100 pm CST) during the spring (March through June) account for 49% of significant tornado occurrences (82 out of 168). Occurrences outside this window tend to be spread out. What is particularly disturbing is only 4 of the 11 violent tornadoes (F-4) are in this cluster. Six of the remaining violent tornadoes occurred in the winter months (from four separate severe thunderstorm events) during the late evening and overnight hours. The most notable event struck the city of St. Louis at 140 am on February 10, 1959. This violent tornado resulted in 21 fatalities and 345 injuries.
Figure 7. Month vs. Hour distribution
3.4 Multiple Tornado Episode
Table 1 (below) lists the number of severe weather episodes that produced two or more significant tornadoes. An episode was defined similar to Galway (1977) in which individual tornadoes are not separated by more than 6 hours. For this study, being in the same CWA was considered spatially linked. Results indicate that 94 of the 168 significant tornadoes in the data base (56%) occurred in these events.
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
2 |
3 |
4 |
7 |
6 |
3 |
3 |
3 |
4 |
Table 1. Number of multiple tornado episodes per month
4. SUMMARY
The year-to-year comparison of tornado data shows a gradual decrease in the occurrence of significant tornadoes observed in the LSX CWA in the past 50 years. As expected, most of the tornadoes occurred in the late afternoon and evening hours of the spring months. Significant tornadoes however comprise a larger percentage of all tornadoes that occur during the late autumn through winter. Violent tornadoes appear to have the largest temporal disparity in the occurrence, with a cluster centered in the early evening hours and another cluster centered just after midnight. For the immediate St. Louis metropolitan area, the climatology shows a decided preference for violent tornadoes during the late evening-early morning hours of the winter.
The desired goal in developing this climatology is to give forecasters greater insight into the commonalities and probabilities of an event occurring, and prompt a higher state of awareness even before the first thunderstorm forms. This is the first step toward developing a synoptic climatology of severe weather events across central and eastern Missouri and portions of western Illinois.
5. ACKNOWLEDGEMENT
The authors would like to thank Doug Speheger from WFO Norman for his computerized help in deciphering the SPC database. Additional thanks are extended to Ron Przybylinski (SOO/WFO LSX) and Steven Thomas (MIC/WFO LSX) for support in this study.
6. REFERENCE
Fujita, T., 1981: Tornadoes and Downbursts in the context of generalized planetary scales. J. Atmos. Sci., 38, 1511-1534.
Galway, J. G., 1977: Some Climatological Aspects of Tornado Outbreaks. Mon. Wea. Rev., 105, 477-484.
Hales, J.E., 1993: Biases in the severe thunderstorm data base: Ramifications and solutions. Preprints, 13th Conf. Wea. Forecasting and Analysis, Vienna, VA, AMS (Boston), 504-507.
Hart, J.A., 1993: SVRPLOT: A New Method of Accessing and Manipulating the NSSFC Severe Weather Data Base. Preprints, 17th Conf. On Severe Local Storms, St. Louis, AMS (Boston), 40-41.
Kelly, D.L., J.T. Schaefer, and C.A. Doswell, III, 1985: Climatology of Nontornadic Severe Thunderstorm Events in the United States. Mon. Wea. Rev. 113, 1997-2014.
National Oceanic and Atmospheric Administration, 1959-1995: Storm Data. Vols. 1-37, Nos. 1-12, National Climatic Data Center, Asheville, NC.
Ostby, F. P., 1993: The Changing Nature of Tornado Climatology. Preprints, 17th Conf. On Severe Local Storms, St. Louis, AMS (Boston), 1-5.
Schafer, J. T. and R. Edwards, 1999: The SPC Tornado/Severe Thunderstorm Database. Preprints, 11th Conf. On Applied Climatology, Dallas, AMS (Boston), 215-220.
*Corresponding author address: Mark Britt, National Weather Service, 12 Research Park Drive, St. Charles, MO 63304; e-mail: mark.britt@noaa.gov
