This discussion I will be focusing on the severe weather events of March 12, 2006 and the supercells, tornadoes and resulting damage that occurred in the areas of Missouri, Illinois as a result of them. Attached here are some damage reports that show the destructive power of these storms they were provided by the NWS Office in Lincoln IL.  Attached in the following link are some very striking visual examples of damage done by these storms and the resulting tornadoes in Springfield IL, again these were taken by the NWS Office in Lincoln IL.

This brings up the question of what was the cause of this massive outbreak of severe weather? The answer lies in the fact that on that day the weather in the central part of the country was almost ideal for the formation of severe thunderstorms and quite possibly tornadoes. In the following sections I will discuss the reasons behind this with the help of a number of charts produced by the Storm Prediction Center's Mesoscale Analysis Archive. In any event, the threat was significant enough for the Storm Prediction Center to start off the 430pm CST day one severe weather briefing with the following words (click the link for the full text of the briefing) "This is an extremely dangerous situation". Shortly after that discussion there were a series of mesoscale discussions issued (again click each  MD255 and MD256 for full details and descriptive graphics) concerning the potential for severe thunderstorm formation and possible tornadoes in the near future.

So what was the large scale, big weather picture that was setting up to bring a massive outbreak of storms to the central United States on March 12, 2006? As you can see on this Surface chart from 18Z (Click on image to enlarge) there was a warm front extending from Kansas eastward through Missouri and into Illinois and Indiana. This front was separating an area of extremely warm and humid air to the south from cooler drier air north of the front. 

(This March 12, 2006 1800Z surface chart image is courtesy of the NWS Hydrometeorological Prediction Center)

At the time this chart was valid the temperature differences on either side of the front were almost 30 degrees as shown on this detailed synoptic observation map. Also near that time there was a small vorticity maximum located over Oklahoma (as shown by the X on this 500mb chart (Courtesy of the NWS Hydrometeorological Prediction Center)) taken at 6pm CST March 12) which most likely was the trigger to start upward motion of the atmosphere that led to the formation of these huge, intense long-lived supercells.