CHARACTERISTICS OF HAIL-PRODUCING RADAR ECHOES IN ILLINOIS

Abstract

Data from 103 hail echoes on 24 days in 1967 and 50 no-hail echoes from the same days were analyzed to describe hailstorm characteristics and to provide information useful in operational detection and forecasting of hail-producing echoes. Echo characteristics investigated included locations of echo formation and dissipation, echo reflectivities, echo-top heights, echo duration, direction of motion, speed, time of occurrence, and associated synoptic weather conditions. A single hail-echo model could not be derived because of the extreme variability found in all characteristics. However, distinctive echo models could be developed for the three predominant hail-producing synoptic weather conditions, cold fronts, stationary fronts, and low-pressure centers. The frontal hailstorms were faster moving, longer lived, and had taller echoes than those with low-pressure systems. Hail production after echo inception varied from an average of 32 min for low conditions to 59 min for cold frontal echoes. The average hail-echo top exhibited a 5,000-ft growth in the 15-min period prior to the average time of hail, suggesting that a major updraft surge was the prime producer of hail. The no-hail echoes occurring on hail days had characteristics of speed, direction of motion, reflectivity, and location that were very similar to the hail-producing echoes. The only distinct consistent difference between the hail and no-hail echoes in all synoptic situations was that the hail-echo tops averaged between 2,000 and 4,000 ft higher throughout their entire durations.