From the National Weather Service office in St. Louis:
The April 10, 2001 historic hailstorm and supercell
During the afternoon and evening hours of April 10, 2001, a long-lived, high-precipitation supercell thunderstorm traversed portions of Missouri and southwest Illinois, producing catastrophic hail damage. The HP supercell produced a swath of large hail approximately 245 miles in length and up to 22 miles in width as it moved east through the highly populated Interstate 70 corridor from southeast of Kansas City through St. Louis.
Most of the hail ranged in size from 1 to 3 inches in diameter. However, south of the largest hail, marginally severe hail (0.75 to 1 inch) also caused considerable damage as it was propelled by 70-plus mph downburst (rear flank downdraft) winds. This storm has been named the “Tristate Hailstorm” ... and is considered the most costly hailstorm in U.S. history with insured losses of $1.5 billion.
Known Missouri insurance claims consist of 120,000 home claims, 65,000 auto claims, and 8,000 commercial claims. It is believed nearly every home and business in northern St. Louis County suffered hail damage. All of the SUVs parked outside at the Ford Motor Company assembly plant in Hazelwood were damaged (hundreds), while in the adjacent community of Florissant, every home was estimated to have received damage. Twenty-four commercial and military aircraft at Lambert St. Louis International Airport were also damaged.
Uninsured losses are unknown. Largely overshadowed by the devastating hail were the tornadoes produced by the HP supercell. This single storm produced a total of 9 weak tornadoes (6 F1, 3 F0) with path lengths ranging from 1 to 10 miles. The F1 tornado which struck Fulton (southeast of Columbia) destroyed a mobile home, producing the first tornado fatality in Missouri since 1994. With $12 million damage reported from the tornadoes, the total damage from the tornadoes paled in comparison to the hail damage.
More at: http://www.crh.noaa.gov/lsx/?n=04_10_2001