Research Studies

c) Hourly Distribution

Tornadoes across the Lake Charles CWA have occurred at all times of the day and night (Figure 16), but strike most often between noon and 7 p.m. LST.

Hourly tornado reports during March-May again show that tornadoes occur at all hours peaking in the afternoon and early evening hours (Figure 17).

Like damaging wind events, there is nearly an identical secondary peak in the early morning hours from 2 a.m. to 5 a.m. LST. This again appears to be related to the NSSO which shifts from the Central Plains and the middle Mississippi River Valley during the warm season to the south-central and southeastern U.S. during the winter and spring months. (Fike, 1993). This nocturnal phenomenon is not evident in the June-September period (Figure 18) but is somewhat apparent in the October-February period (Figure 19).

During June-September, tornado events are strictly diurnal with development beginning shortly after sunrise. As mentioned, earlier, these are generally F0 and F1 tornadoes and occur early in the thunderstorm life cycle. During October-February , the activity mirrors the March-May period with an earlier peak in afternoon activity. Clearly, tornadoes in the summer months are associated with maximum diurnal heating while the remainder of the year seems to be a combination of large-scale weather systems and diurnal heating.

d) Intensity

Table 1 describes the Fujita Scale which categorizes tornadoes based on the strength of their winds and the damage produced. The categories range from an F0 to F5 in increasing order of an intensity. Most tornadoes in the Lake Charles CWA fall under the lower spectrum of the scale (Figure 20).

Of the tornadoes which have been recorded, 73 percent have been either an F0 to F1 rating, 19 percent have been classified as an F2, 7 percent as an F3, and less than 1 percent have been classified as an F4. The Lake Charles CWA has never reported an F5 tornado.

6. CONCLUSIONS

Temporal trends of severe weather found in the study reveal a "true" severe weather season for the Lake Charles CWA. Even though severe weather may occur in any month, the highest frequency is during the springtime, specifically from March through May. A second severe weather season was also noted in November. In addition, the study found several secondary diurnal peaks clearly indicating the presence of nocturnal severe weather outbreaks during the cool season.

While hail and damaging wind events are similar in occurrence diurnally, wind events are distinctively different during the warm season months. This is due to weaker synoptic scale forcing and strong low-level instability characterized by the pulse-type storm environment. These storms continue to produce damaging winds in the warm season, but due to the higher freezing levels much of the hail melts before reaching the surface.

Tornado occurrences also do not drop off as sharply after May as hail events. Weak F0 and F1 ratings comprise most of the tornado reports with more significant ratings of F2 or higher occurring during the springtime and autumn months. While the data point toward more destructive tornadoes in the autumn, this is greatly skewed by a few occurrences. Deadly, destructive tornadoes are just as likely in the springtime.

Through examining and understanding severe weather climatology across the Lake Charles CWA, the authors hope that this will lead to improved forecasts and warnings. Emergency managers, utility companies, and severe weather spotters could also benefit from this study.

7. ACKNOWLEDGMENTS

The authors would like to thank Michael Vescio at SPC for providing the CLIMO program; Steve Rinard (MIC), and Felix Navejar (SOO) for suggestions, comments, proofreading, and especially time spent in helping prepare this paper.

8. REFERENCES

Fike, Paul C., 1993: A climatology of nocturnal severe local storm outbreaks. Preprints, 17th Conf. on Severe Local Storms, St. Louis, Amer. Meteor. Soc., 10-14.

Grazulis, Thomas P., 1993: Significant tornadoes 1680-1991. Environmental Films, St. Johnsbury, Vermont, 1326 pp.

Hales, John E., 1993: Biases in the severe thunderstorm data base: ramifications and solutions. Preprints, 13th Conf. Wea. Forecasting and Analysis, Vienna, VA, Amer. Meteor. Soc., 504-507.

Kelly, Donald L., Joseph T. Schaefer and Charles 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, 1972: Storm data, 14, Nos. 1-12, National Climatic Data Center, Asheville, North Carolina.

Ostby, Frederick P., 1993: The changing nature of tornado climatology. Preprints, 17th Conf. Severe Local Storms, St. Louis, Amer. Meteor. Soc., 1-5.

Vescio, Michael D., 1995: Climo - Software to generate severe weather statistics for NWS County Warning Areas. NSSFC, February 1995.