National Weather Service United States Department of Commerce


Terry W. Murphy
National Weather Service, Peachtree City, GA
Written in 1995 (Note: The severe criteria for hail has since changed to 1 inch)

1. Introduction

As a result of National Weather Service (NWS) modernization, warning responsibility for NWSFO Peachtree City was expanded to include a large portion of north and central Georgia. This area, known as the County Warning Area (CWA), is now more than twice its pre- modernization size. Included in the area being added to Peachtree City's original CWA are population centers such as Macon and Athens, as well as major recreational areas (e.g. Lake Lanier and the mountains). Providing severe thunderstorm and tornado warnings for the CWA will continue to be a fundamental task. (A "severe thunderstorm" is defined as one that produces wind damage or wind gusts of at least 50kt, tornadoes, and/or hail with a diameter of at least 3/4 inch.) Forecasters responsible for issuing these warnings may find it helpful to examine temporal and other characteristics of these storms. Toward this end a climatology of severe weather over the new CWA has been developed.

2. Data

Much of the data was extracted from a database maintained by the National Severe Storms Forecast Center (NSSFC) using the CLIMO software (Vescio 1995). This data set is the same as the one accessed by the SVRPLOT program (Hart 1993). Tornado data date back to 1950, while hail and wind events are archived as far back as 1955. Data relating to each type of event are maintained within the database on a county-by-county basis. This approach results in the breaking down of data relating to tornadoes that cross county boundaries into "segments". Therefore, combining data from the various counties that make up a CWA does not yield the total number of tornadoes, but rather the total number of "segments", or the total number of counties affected.

Damaging wind and hail data for the year 1972 is missing from the NSSFC database. To fill this void, data from the "Storm Data" publication (NOAA 1972) was used.

Data on population density was taken from the United States' 1990 Census Data. Local Climatological Data (LCDs) provided information on the number of thunderstorm days.

A number of authors have pointed out aspects of severe weather reporting that lead to problems in developing a severe storm climatology (e.g. Doswell 1985, Hales 1993, Grazulis and Abbey 1983). Such factors as population and roadway density, weather sensitivity of the media and citizenry, increased awareness of the citizenry, and increased efforts to seek out reports in the wake of a severe weather episode have all been cited as sources for skewing data on which a climatology can be based. Biases are frequently seen in spatial distributions and in yearly frequency data. However, Ostby (1993) suggests the data can be made more reliable (biases reduced) by focusing on events on the stronger end of the intensity spectrum. This approach was used to examine spatial distributions of tornadoes and large hail in Peachtree City's CWA. A comparison with population density yields results that appear minimally biased toward overrepresentation in areas with the greatest population. Keeping that in mind, one can make certain inferences on the spatial characteristics of these storms. For example, it appears that tornadoes are not common in the rough terrain of extreme northeast Georgia. Also, the incidence of tornadoes is somewhat greater in what is known as the "I-85 corridor" (an axis that extends from central Alabama northeast into extreme northwest South Carolina) than elsewhere in the region.

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