Meteorological Synopsis
The outbreak occurred when the warm front of a deep storm system moved north and east out of Missouri. April 19 started off cool and skies were overcast ahead of the warm front. Meteorologists were trying to figure out if the warm front would move into Illinois that afternoon. As the day wore on, temperatures warmed, dew points rose, and thunderstorms started to explode in Iowa during the mid-afternoon hours. Although there were some doubts on specifics, the potential significance of the outbreak was rather foreseeable, with storm chasers traveling from the Great Plains and the Storm Prediction Center issuing a high risk early on.
The outbreak across the south-central U.S. on the 21st was produced by a different weather system.
Read more about this topic: April 1996 Tornado Outbreak Sequence