According to the National Climatic Data Center, Illinois was on the list twice for billion-dollar weather disasters in 2013. In both cases, some damages occurred elsewhere but Illinois took the brunt of the losses. These numbers are based on reports from government agencies, media reports, and insurance industry estimates.
The first was the flooding in the Chicago area in April 2013. I had posts on this event here, here, and here. This event produced an estimated $1 billion in damages and led to 4 deaths. They described it as:
A slow-moving storm system created rainfall totals of 5 to 10 inches across northern and central Illinois including the Chicago metro. This resulted in damage to many homes and businesses. There was also severe weather damage from wind and hail across Indiana and Missouri.
Late-season outbreak of tornadoes and severe weather over the Ohio Valley (IL, IN, MO, OH, KY, MI) with 70 confirmed tornadoes. Most severe impacts occurred across Illinois and Indiana.