Research Studies

1. INTRODUCTION

The National Weather Service (NWS) is nearing completion of its Modernization and Associated Restructuring (MAR). Once this process is complete, the Lake Charles NEXRAD Weather Service Office (NWSO) will assume forecast and warning responsibility for sixteen Louisiana parishes, six Texas counties, and the adjacent coastal waters (Figure 1). This paper will serve to address this new responsibility by enhancing the forecasters knowledge of the local severe weather climatology in the Lake Charles County Warning Area (CWA). The basic assumption is that increased knowledge will lead to more accurate forecasts and warnings.

The National Weather Service defines a severe local storm as meeting any one of the following criteria: tornado, hail equal to or greater than 3/4 inch in diameter, convective wind gust greater than or equal to 50 knots, or significant convective wind damage. This study will take each criteria on an individual basis and address the severe weather climatology trends in hourly, monthly, and annual distributions.

2. DATA

The NWS Storm Prediction Center (SPC) maintains an extensive database of severe weather records dating back to 1955 for damaging wind and hail data and back to 1950 for tornado data. From this database, the CLIMO program (Vescio, 1995) was modified to include only parishes and counties in the Lake Charles CWA. The authors of the paper did an objective analysis of the data to generate severe weather statistics and quantify the results. Damaging wind and hail data from 1972 were not included in the SPC database and were therefore supplemented by Storm Data records (NOAA, 1972).

There are however limitations to the data as described by Ostby (1993). These include population biases, increases in population densities, increased emphasis on storm spotting and chasing, and non-uniform event gathering procedures. Hales (1993) further noted that there has been a significant growth in the severe storm database since the National Weather Service began verifying severe storm events in 1980. To account for these non-meteorological influences, data were examined on the hourly and monthly spectrum. Since all the data was subject to the same biases, it would seem one could safely derive trends in severe weather climatology.

3. HAIL CLIMATOLOGY

a) Yearly Distribution

There were 404 hail reports in the Lake Charles CWA between 1955-1995. Overall, hail reports began to trend upward in the late 1970's (Figure 2), with a decrease in the mid and late 1980's, and then a substantial increase in the 1990's. In fact, 70 percent of the documented hail reports have occurred since 1980. As stated earlier, the NWS began formally verifying severe thunderstorm and tornado warnings in 1980; thus, this increase appears to be non-meteorological in nature.

In the spring of 1995, several troughs and cut- off lows over the Mississippi River Valley and Gulf Coast brought an unprecedented amount of severe weather reports. That year there were 76 hail reports, more than doubling the second highest year on record, 1991. The decrease in the mid 1980's is more likely a result of meteorological factors than in a disruption of event gathering techniques. Prior to the mid 1970's, reports were nearly uniform as the seek-and-search method for storm reports was not a priority.

b) Monthly Distribution

By examining Figure 3, we find that severe hail events across the Lake Charles CWA can occur during any month.

However, the March-May period exhibits the peak months of severe hail. During the springtime peak, optimal freezing and wet- bulb zero levels are attained. Additionally during this time, the transient influence of the westerlies brings fronts through the region. Sufficient moisture off the Gulf of Mexico and warming of the low levels of the atmosphere result in larger CAPE (Convective Available Potential Energy) values; thus, the needed instability and stronger updrafts required for hail development are present. In fact, 67 percent of hail reports occur during March-May time period. May is the peak month with 105 reports correlating to 26 percent of all reports.

The decreasing trend in hail reports for the June-September period can be best explained by the higher freezing and wet bulb zero levels and northernmost extent of the westerlies. During this time, fronts reaching the Gulf Coast are a rarity and cold pockets in the mid levels reside over the northern half of the country. A bit of a re-emergence or a secondary peak occurs in the autumn months. The southward migration of the westerlies brings a few strong fronts into the region at this time. Furthermore, there is a lag effect as the low levels remain warm and moist due to the proximity of the Gulf of Mexico.

To further support the seasonal distribution of hail reports, the study divided the reports into three groups based on the size of the hail reported:

Large Hail:  0.75-1.74" in diameter

Giant Hail: 1.75-2.74" in diameter

Enormous Hail: 2.75" or > in diameter

According to Hales (1993), the actual severe thunderstorm climatology can be best determined by only considering the most significant events. This eliminates the biases associated with collection techniques used to verify severe weather events. Using this technique, Figure 3 continues to show that the peak months of severe hail are March-May. In fact, 71 percent of the giant hail reports and 66 percent of the enormous hail reports occur during this period. In addition, almost half the reports, 45 percent, are either giant or enormous hail.

c) Hourly Distribution

From Figure 4, we can see that hail occurrences are diurnal with the peak occurring from 2 p.m. to 8 p.m. Local Standard Time (LST). The relatively high number of reports in the late evening and early morning between 10 p.m. and 4 a.m. can be best described by Fike (1993) as nocturnal severe local storm outbreaks (NSSO). This phenomenon is most prevalent along the Gulf Coast during the winter and spring months when the westerlies transport large-scale weather systems through the region at all times of the day.

By examining Figures 5 and 6, we see that October-February and March-May tend to be influenced by either diurnal heating or large - scale weather systems. However, June- September (Figure 7) occurrences are predominantly in the late afternoon and early evening hours with very few events during the morning hours. The warm season (June - September) events are mainly driven by low- level instability which is greatest during the hours of maximum heating.

*  Resigned from National Weather Service

** NWS's National Centers for Environmental Prediction - Hydrometeorological Prediction Center, Camp Springs MD
 

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