National Weather Service United States Department of Commerce

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Figure 6 shows the monthly distribution of tornado occurrence in terms of the number of counties affected. Several things to note include:
  • More counties report tornadoes and tornado-related deaths in April than in any other month of the year.
  • The primary tornado season is March - April - May.
  • Strong tornadoes (F2+) have occurred in every month of the year.
From the data used to generate figure 6 one finds that of all counties reporting tornado occurrences, 37% are affected by storms with an intensity of at least F2. This is somewhat higher than the 32% ratio reported nationwide (Grazulis, 1993).

Figures 7, 8, and 9 show favored times of the day for tornado occurrence for each of 3 "seasons". Figure 7 shows data for the warmest months (June - September) when the westerlies aloft are weak, and transient weather systems are at a minimum. Note the strong correlation of events with the afternoon and early evening hours. This implies that diurnal heating is a major ingredient in the development of tornadic thunderstorms during these months.

Figure 8 shows data for the coldest months (October - February) when the westerlies are often strong and progressive weather systems are common. Note that tornadoes occur at any hour of the day, suggesting that large-scale forcing is an important ingredient in the formation of tornadoes during these months. However, the most favored time for occurrence is mid afternoon through early evening, again pointing to diurnal heating as a contributing element.

Figure 9 shows data for the main "tornado season" (March - May). This is a time of transition when the strength of the westerlies varies, and strong cold fronts occasionally displace warm and unstable air masses. The hour of arrival of fronts and any impulses embedded in the westerlies varies. Since tornado occurrence is well represented in all hours of the day (Fig. 9), there is a suggestion that fronts and transient features aloft are important ingredients in some springtime tornadic episodes. However, once again we see the best correlation with the hours of maximum heating.

b. Hail (3/4 inch diameter or larger)

Hailstorms involving hailstone diameters of at least 3/4 inch are more common than tornadoes across the CWA. Between 1955 and 1993, the average number of hail days per year was seven, as compared to about six tornado days. The data show a dramatic increase in the number of hail events in the 1980's and 90's. This is likely related to increasing efforts to verify warnings and increased public awareness as opposed to being associated with any meteorological phenomenon.

When looking at size distribution we find that most severe hailstorms (59%) in the CWA involved hailstone diameters of less than 1.75 inches. Hailstones larger than 2.75 inches in diameter are truly rare (Fig. 11), making up only 2% of the total.

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