Research Studies

4. WIND CLIMATOLOGY

a) Yearly Distribution

Wind events account for nearly half of the severe weather reports in the Lake Charles CWA. From 1955-1995 there were a total of 1151 events (Figure 8).

The temporal trends are similar to the hail events in that we see an upward trend beginning in the late 1970's. Once again, non- meteorological factors such as population density and verification techniques appear to be skewing the database. The similarity in data suggests that both wind and hail events appear together. Specifically, the Gulf Coast region experiences most of its severe weather from pulse-type storms. These storms develop in a highly unstable, but weakly sheared environment. The severe pulse storm has a more intense updraft which often favors hail development than an ordinary thunderstorm. However, the updraft is short-lived, giving way to an intense downdraft and subsequent period of damaging winds at the surface.

b) Monthly Distribution

Damaging winds can occur during any month as shown in Figure 9, but the March-May period is once again the seasonal peak.

In fact, 44 percent of the damaging wind reports occur during the springtime peak. May is the peak month with 221 reports correlating to 19 percent of all reports. However, unlike hail, wind events do not drop off as sharply after May. Wind events are more evenly distributed through the warm season months than hail events. This is a result of the previously discussed pulse-type thunderstorms.

Although pulse-type thunderstorms produce hail, in the warm season much of the hail melts due to higher freezing levels. Additionally, the environment is weakly sheared during this time inhibiting significant tornado development. However, pulse-type thunderstorms driven by highly unstable environments continue to produce damaging winds. By extending the seasonal peak (March-May) through July, this period now accounts for 63 percent of the wind events.

A secondary peak is also noted in November and is consistent with results found by Kelly et al. (1985). This peak will also be seen in the tornado climatology section. The influence of the westerlies becomes more pronounced at this time of year allowing for frontal systems to push to the Gulf Coast region and collide with warm, unstable air still in place.

c) Hourly Distribution

From Figure 10, we can see that wind events are diurnal with the peak occurring from 2 p.m. to 7 p.m. LST. Another significant feature are the two secondary peaks noted during the morning hours. These can be seen from midnight to 5 a.m. LST and 9 a.m. to noon LST. The first secondary peak may be best explained by the previously discussed NSSO (Fike, 1993), while the second peak also appears to be driven by large-scale weather systems in the cool season (October- May).

Again, damaging wind data are similar to hail data with the June-September months being strongly dependent on diurnal heating, while the October-February and March-May events are a combination of large-scale weather systems and diurnal effects (Figures 11, 12, and 13). This results in a more equitable distribution of wind events during the cool season months.

For October-February, the primary peak is between 9 a.m. and 4 p.m. LST with a secondary peak from 9 p.m. to 2 a.m. LST; for March-May, the primary peak is between 2 p.m. and 5 p.m. LST with a secondary from midnight to 5 a.m. LST. Thus, a conclusion can be drawn that wind events can occur almost any time of the day during the cool season months, with the primary peak during the afternoon hours.

5. TORNADO CLIMATOLOGY

a) Tornado Frequency

There were 576 tornadoes reported in the Lake Charles CWA (Figure 14), the annual average is 13, between 1950-1995. Every county/parish in the CWA reported at least one tornado during this period.

The upward trend of reports after 1980 in hail and damaging wind occurrences is not as clearly recognizable in the tornado trends (Figure 14). A possible explanation is that some damaging events such as microbursts were classified as tornadoes before 1980 (Grazulis, 1993). Reported U.S. tornadoes which showed a steady increase for several years after the establishment of the National Severe Storm Forecast Center (NSSFC) in the early 1950's, leveled off in the 1970's. Since then, the annual tornado fluctuation seems to be increasingly a function of the shifting weather patterns rather than the reporting system (Hales, 1993).

b) Monthly Distribution

Total tornado reports by month (Figure 15) show a primary peak from March-June and a secondary peak from September-November. The primary peak March-June comprises 44 percent of the events. May is the peak month with 90 reports correlating to 16 percent of all reports. The secondary peak month is November with 60 reports accounting for 11 percent of all reports. The least active months are February and August, with only 5 percent of the annual total in each month.

Similar to the wind events, the distribution of tornadoes does not drop off after May like the hail events. This can be best explained by the development of weak F0 and F1 tornadoes during the summer months. These type of tornadoes form during the rapid development of the updraft in highly unstable environments, often along a pre-existing boundary. This is in contrast to springtime tornadic activity, when strong wind shear and instability are both present.

Some of the deadliest and most damaging tornadoes have occurred during November and December. In fact, around 65 percent of all fatalities and injuries have occurred during the secondary peak. The data point toward more violent tornadoes occurring in the fall or early winter. However, most fatalities and injuries in the secondary peak occurred during a few events which have greatly skewed the data. For example, a strong cold front moved through the Lake Charles CWA on November 7, 1957 spawning numerous tornadoes resulting in 180 injuries and 12 fatalities. Total injuries and fatalities in November from 1950-1995 are 209 and 13, respectively. Appendix I details tornado occurrences, injuries, and fatalities in each parish/county in the Lake Charles CWA.

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