| On May 21, 2004, twenty-one tornadoes swept across Iowa and Nebraska between 2:28 and 10:13 PM CDT. The twenty-one tornadoes occurred within 150km of a warm front, with twenty being within 75 km of the front. Eight of the tornadoes in Iowa were the result of elevated convection, and occurred on the cold side of a warm front, which oscillated across the region throughout the day. The four tornadoes in Nebraska were not the result of elevated convection. In Iowa, convective available potential energy (CAPE) values were indicating a moderately unstable environment, low convective inhibition (CIN) values were indicating the downward buoyancy was easily overcome, and helicity values were indicating strong to violent tornadoes. In Nebraska, CAPE values were indicating a very unstable to extremely unstable environment, CIN values were indicating low to high downward buoyancy, and helicity values were indicating weak tornado development. |
