On April 10-11, 2001, a significant severe weather outbreak produced 28 tornadoes across Iowa which was part of a region wide 2-day tornado outbreak that spawned 79 tornadoes across Nebraska, Iowa, Kansas, Missouri, Oklahoma, and Texas. The 28 tornadoes in Iowa became an all-time state record for tornadoes in a single day. The majority of these tornadoes were toward the weaker end of the Fujita scale. However, an F2 struck Agency, Iowa in Wapello County that caused two deaths and three injuries. Another tornado, a long-lived tornado, tracked from northern Missouri to Madison County and caused extensive damage in Ringgold County. In addition to the widespread tornadoes, large hail was also reported ranging up to golf ball size at Parkersburg and there were scattered reports of 70 to 85 mph wind gusts with the worst straight-line wind damage in Black Hawk and Franklin counties.
A very strong low pressure system moved from central Kansas into southeast Nebraska on April 11, 2001. A warm front extended east along the Missouri/Iowa border during the late morning and surged northward throughout the afternoon (see the 3 hand drawn surface analysis below). A dryline oriented northwest-southeast moved from southwest Iowa into central to south-central Iowa in the afternoon. Storms erupted along this dryline in the afternoon, but there were also storms that focused along the warm front. As you can see from Figure 1 below, the radar operator had a very busy day identifying the rotation within each storm that were rapidly moved north-northeast through the afternoon hours. There were 12 tornadoes that occurred within the NWS Des Moines county warning area.
Figure 1: Operator identified low-level mesocyclone tracks on 11 April 2001 between 1900-2130 UTC (2:00 PM CDT to 4:30 PM CDT). The dots indicate location of the rotation center every 5 minutes and the dash indicates that a “cell” was still identifiable on radar, but no velocity couplet was found. “FO” or “F1″ to the right of the tracks indicate location of tornadoes and Fujita scale intensity. The “M” to the left of track indicates WSR-88D mesocyclone detection and the “MA” has a base above 5 km. From Figure 1 via Karl A. Jungbluth; “The Tornado Warning Process During a Fast-Moving Low-Topped Event: 11 April 2011 in Iowa.”
F3 damage to an apparently well built home. The destruction included the roof being completely gone and several interior and exterior walls destroyed. Location of the home was along highway 2 one mile east of Mount Ayr, Iowa. NWS Des Moines survey.
F2 damage along J55 three miles north of the Missouri border, or seven miles south of Mounty Ayr, Iowa. A partial underground home severely damaged. Roof completely off and destroyed. Interior walls and main wall facing south remained intact limiting damage to F2. NWS Des Moines survey.
Blog post by Kenny Podrazik – NWS Des Moines
Reference: Karl A. Jungbluth; “The Tornado Warning Process During a Fast-Moving Low-Topped Event: 11 April 2011 in Iowa.