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Utica IL Tornado

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This tornado outbreak, including a handful of significant tornadoes with the strongest hitting Utica, was particularly unique in that a severe weather environment was not markedly evident just a few hours prior to the event.  However, a rapid change occurred.  Below to the left is a collection of surface observation maps from three hour intervals during April 20, 2004 (12Z = 7 am, 00Z = 7 pm).  Note the cool conditions and rain early in the afternoon and then the significant change that follows in North Central Illinois.  To the below right is a hand analyzed graphic at 7 pm showing the warm air approaching the south Chicago metro, where a tornado would occur as far north as Joliet.


Utica Tornado
Surface Map


Weeks after the event, meteorologists in the weather enterprise that included NWS Chicago forecasters researched the quickly changing and deceptive tornadic environment.  Their technical assessment can be found to the right. 
In layman's terms what had happened was that the warming and moistening that so rapidly occurred during the mid to late afternoon across north central Illinois created quick instability (or ascent) that was sufficient for thunderstorms. The focus for these storms was along an evolving warm front, which basically was surging northward.  This type of boundary is particular ripe for rotating storms and low cloud bases, both ingredients for potential significant tornadoes. Much of the computer models struggled with this destabilization however, and indicated basically zero instability and thus no lift for any precipitation to develop. Observations early in the afternoon matched this evolution.  However, conditions in southern Illinois raced northward through central Illinois.  With the rain earlier in the day, the lower atmosphere was likely moistened even further which supported even more rapid instability development as well as low cloud bases, again all better for tornadoes.
Local Research


Radar Imagery

Utica Tornado
Utica, IL Radar Imagery Stills