A very strong cold front swept across the region Monday afternoon, and was accompanied by scattered severe thunderstorms over portions of northeast Missouri, southeast Iowa, and west central and northwest Illinois.
The surface map below at 1800z (12:00 pm CST) shows the potent cold front moving in from the west. Note the nearly 50 degree temperature contrast across the state of Iowa! Unseasonably warm, moist and unstable air was drawn northward ahead of the front on gusty south winds. Moline, Illinois set a new record high for the date at 71 degrees, while many other locations ahead of the front flirted with record highs. This potent cold front along with the unseasonable warmth and strong winds aloft combined to produce conditions favorable for rotating supercell thunderstorms with large hail and tornadoes.
The radar reflectivity image below depicts a pair of supercell thunderstorms at 2000z (2:00 pm CST) . One is located in Knox County near Altona and the second in Bureau County just north of Sheffield. In this image we are looking into the storms at roughly between 2,800 feet and 3,400 feet above ground level (AGL) , with the Quad Cities Doppler Radar located approximately 40-45 miles to the northwest of the storms. The purple and aqua blue shading in the core of the storms show very high levels of reflectivity of 70-76 decibels (dBZ), with values this high often a good indicator of hail. More intriguing, however, is the long "spikes" of reflectivity extending well to the southeast of both storms. These "spikes" are not a reflection of actual precipitation in the area, but rather a classic example of a radar artifact known as a "Three-Body Scatter Spike", or TBSS. The presence of a TBSS almost always indicates that a storm contains large hail. In simple terms, a TBSS is caused by the radar beam hitting the hail aloft, scattering to the ground below, then scattering back upward, and finally being scattered once again by the hail aloft. The 3 scatterings illustrate the triple reflection, thus the term "three-body scatter spike".