As a fitting celebration of Earth Day, the MC3E field campaign officially began at 6:30 AM local time with the successful launch of radiosondes from all five boundary facilities and the Central Facility. Our daily morning briefing was focused on the possibility of aircraft operations as the ER-2 ferried from NASA’s Dryden Flight Research Center in California to Offutt Air Force Base in Nebraska. The forecast called for a SW-NE line of thunderstorms to form in the late afternoon just east of an Oklahoma City-Tulsa line. These storms would be too far to the south and east to be sampled by the MC3E radar systems but still looked like an excellent opportunity to get some good observations on the ER-2 ferry. The forecast proved to be excellent as an intense line of thunderstorms began forming at about 4:00 PM. The ER-2 began sampling along the line just before 5:00 PM and was soon joined by the UND Citation, where a couple stacked legs were flown in the anvil of the convective line. Coincident observations were also taken by the University of Oklahoma’s PRIME radar at Norman, OK. This line was very intense with frequent lightning. A tornado warning was issued in the region near our Morris, OK, where sounding staff were sent to their storm shelter in lieu of the 6:30 PM launch.  The lines of storms offered some impressive visual displays from the SGP site and on the drive back to Ponca City along I-35. The anvils covered most of the horizon, often with penetrating convective turrets and underlying mammatus. News reports later showed this same line of storms moving through the St. Louis area, giving rise to more tornadoes and causing significant damage to the airport terminal.