A Severe Weather Climatology for NWSFO Peachtree City's County Warning Area

A SEVERE WEATHER CLIMATOLOGY FOR NORTH AND CENTRAL GEORGIA

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The occurrence of large hail is a seasonal phenomenon. Figure 12 shows that nearly all hailstorms strike between February and August. April is the most favored month. During the spring and summer months, the vast majority of hail events have occurred during the afternoon and evening hours (Figs. 13 and 14 respectively). This points to the importance of diurnal heating in the formation of the Georgia hailstorm. In the autumn and winter months (Fig. 15) we find not only a much lower frequency, but a broader distribution in the time of occurrence. Only in the morning hours from predawn to around noon do these storms rarely occur.

c. Damaging Wind

By far the most common severe weather event in the CWA is damaging wind. The average number of damaging wind days during the 1955 - 1993 period was 19 per year. The data show a dramatic upsurge in the number of events in recent years. However, this is thought to be the result of increased reporting rather than a true increase in occurrence.

Unlike hail and tornadoes, damaging windstorms are just as common in summer as spring (Fig. 17). In fact, more occur in July than in any other month of the year. One explanation for this relates to the nature of summer thunderstorms in Georgia. Nearly all are of the "pulse" type that develop with afternoon heating. They often collapse abruptly, sometimes resulting in brief damaging microbursts. Hail is less frequent with these storms because wetbulb zero heights are usually quite high (greater than 12,000 ft). Figure 18 offers some evidence of this scenario. Note how well these summertime events correlate with the hours of maximum heating.

The afternoon maximum seen in figure 19 suggests that afternoon heating is an important ingredient in springtime events as well. However, some spring windstorms occur at night and during the morning hours. This suggests that some events are driven by something other than heating, probably large-scale forcing from fronts or features aloft.

During autumn and winter, the times of events are more evenly distributed (Fig. 20). The two maxima, one just prior to sunrise and the other just after sunset, are not representative of the entire cool season. The morning maximum is unique to the month of February, whereas the evening maximum is largely made up of events from the month of November. Causes for either of these maxima are not obvious. However, despite these aberrations it is apparent that diurnal heating does not play a major role in the occurrence of damaging wind events during the cool season.

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