An overview of the devastating high precipitation (HP) supercell event which affected Missouri and southwest Illinois during the afternoon and evening hours of 10 April 2001 is presented. Evolving during the mid-afternoon over extreme eastern Kansas, the single storm produced eight weak short-lived tornadoes as it tracked eastward over a seven hour period. While the spawning of eight tornadoes (including one deadly tornado) alone is noteworthy, the HP supercell will long be remembered for its prolific hail production. The storm produced golf ball to baseball size hail during its track east through the highly populated Interstate-70 corridor from Kansas City through Columbia and St. Louis, resulting in hail damage of historic proportions. Even to the south of the largest hail, dime size hail caused considerable damage as it was propelled by rear flank downdraft winds of 60 to 70 mph. Insurance claims to date for the storm are at 1.4 billion dollars. This figure does not include commercial losses at the Ford Motor Company Plant in St. Louis County where nearly 400 SUVs were totaled, or at Lambert St. Louis International Airport where tens of aircraft were damaged. These losses make this HP supercell one of the costliest hailstorms in U.S. History rivaling the Mayfest Storm of May 1995 in Dallas.

This study will review the role of a pre-existing thunderstorm outflow boundary along with environment shear and instability profiles in the development and evolution of the long-lived storm. Doppler radar observations from the National Weather Service offices in St. Louis (KLSX) and Pleasant Hill (KEAX) will be presented. These observations included continually evolving storm structure, cyclic mesocyclone production, and multiple co-existent rotating updrafts within a larger updraft current.