Scattered thunderstorms developed along the front on Wednesday afternoon, producing hail up to golfball size over the northern part of the St. Louis Metropolitan area. Additional storms farther north over southwest Illinois produced heavy downpours. As the surface low began to approach the region during the evening hours, southerly winds developed in lower levels of atmosphere around 5,000 feet above ground level. The warm and humid air rode up above cool air near ground, providing the needed lift for widespread thunderstorm development. Storms continually developed north of the front over Missouri and moved quickly into Illinois, causing training of thunderstorms.
Training occurs when thunderstorms continually moved over the same area.
The doppler radar rainfall estimates indicate that between 4 and 7 inches of rain fell over parts of Greene, Macoupin, and Montgomery counties in Illinois. This area received heavy rainfall from both the afternoon and evening rounds of storms. These estimates are confirmed by cooperative observe rainfall reports of 5.15 inches in Greenfield and 4.20 in Nokomis. The radar also indicated between 2.5 and 5 inches stretching from western St. Charles county, Missouri to Marion county, Illinois. However, this is a good example of overestimation of precipitation by the radar because of hail contamination.