The 13 April 2006 Severe Thunderstorm and Large Hail Event in Iowa & Wisconsin:A Case Study David M. DeMeuse Undergraduate University of Wisconsin – Department of Atmospheric & Oceanic Science Mesoscale Meteorology – AOS 453 ABSTRACT One of the most spectacular severe weather events witnessed is large-hail events. These ice stones can cause millions of dollars in damage in a very short time and turn an otherwise green pasture into a looking like a January-esque snow event. The oddity of white precipitation falling during the warm spring and summer months coupled with the loud, ominous sounds of hail hitting roofs can leave lasting impressions on many people. Such was case in the late evening hours in southwest Wisconsin. After a devastating tornado outbreak in eastern Iowa, a supercell thunderstorm had migrated from Iowa and as it moved across the central counties of Wisconsin, dozens of reports of hail and high-wind events were filed. Hail in excess of softball-size were reported in Iowa and Jefferson (Wisconsin) counties with the largest being a 4.25” ice rock near Lake Mills, Wisconsin. Radar analysis of hail has grown tremendously with the addition of WSR-88D radar and certain signatures become apparent on reflectivities when hail is present in severe thunderstorms. Three-body scatter spikes (TBSS) and flare echoes are two of the ways meteorologists can detect large hail before it reaches the ground. In this is case study, careful examination will attempt to explain the mesoscale dynamics of the initiation of these supercell thunderstorms in Iowa, their advancement eastward, and hail production/observations as they moved through southern Wisconsin. Focus shall remain on the largest severe thunderstorm in southern Wisconsin (~0140Z – 400Z) with only slight reference to the tornadic event in Iowa. __________________________________________ I. Introduction. During the late evening, early night hours of 13 April 2006, several supercell storms were pushing through eastern Iowa and into southwest Wisconsin. Earlier, the severe storms began around 2300Z 13 April 2006 in Central Iowa with numerous hail reports. As the storms strengthened as they roared eastward, several tornado reports occurred. Such was the case in Iowa City (Figure 1) where 21 people were injured and the University of Iowa sufferedsignificant damage to dormitories and other university buildings. This mesoscale convective complex began to reform and the multicellular pattern began to take shape. Before the main complex moved southeast into northern Illinois, one cell broke left and took aim on southern Wisconsin. The very nature of supercells being attracted to certain, localized regions (hence, their otherwise erratic movements) could have many possible reasons. These include but are not limited to moisture convergences, the ability to follow any mean winds, strong deep-layer vertical wind shear with weaker mid-level storm-relative winds, and a stronger cold pool. The complexities arise by the very nature of supercells. The internal dynamics of the storms themselves create mesoscale circulations that can drive these storms to move in unpredictable ways. However, initiation of such storms under the right atmospheric conditions are well known. It is the hypothesis of the researcher that the supercells developed from these classically understood dynamics and the extraordinary hail event was the result of shear from the surface to mid-levels to the upper-air, the remnants of the already intense updraft, and crossing the warm frontal boundary. II. Data. The National Weather Service (NWS), the National Severe Storms Laboratory (NSSL), and the Storm Prediction Center (SPC) contributed to the comprehensive damage and storm reports. In addition, the National Climate Data Center served to provide radar data. The Cooperative Institute for Meteorological Satellite Studies (CIMSS) and Space Science and Engineering Center (SSEC) supplied an extensive array of products. Data analysis was synthesized by graphical-user interface software programs such as Integrated Data Viewer (IDV), General Meteorological Package (GEMPAK), GEMPAK Analysis and Rendering Program (GARP), Java NEXRAD Viewer, GrLevel2, and GrLevel2Analysis. Satellite retrievals were taken from GOES-8 archives using the infrared spectrums as evidence in this investigation. Radar data was provided by the NCDC with scans at multiple elevation angles using both WSR-88D’s base and composite reflectives. Convective Outlooks, and Forecast and Mesoscale Discussions were provided by the SPC’s archives as well as previous case study synopsizes provided by the NWS and CIMSS. Model results and forecasts were taken from the Rapid Update Cycle (RUC) and North American Model (NAM, or ETA) model set. The date and times were as of 14 April 2006 from approximately 0000Z to 0400Z 14 April 2006. Upper air and surface plots were provided by Unisys Weather Image and Map Archive. Regional analysis for the mesoscale investigation includes the Midwest; namely, eastern Iowa, southwest Wisconsin, and northern Illinois. The synoptic overview spans the entire contiguous United States focusing on the states in the upper Mississippi Valley. III. Synoptic Overview. A surface plot of CONUS is shown in Figure 2 0000Z 14 April 2006. The stormsFigure 2: Surface Data Plot 0000Z 14 April 2006. Courtesy: Unisys Data. in eastern Iowa had already formed into a mesoscale convective complex just below a warm frontal boundary stretching from southern Manitoba southeastward into northwestern Indiana. Connected to this warm front was a surface anticyclone situated north of North Dakota and an associated cold front draped across the Dakotas and into Montana. The warm moist air (Figure 3) over the Mid-Mississippi Valley is being advected north to northwestward by the lower level wind field. Dewpoints across Iowa ranged from the upper 50s C to mid 60s throughout the better part of the day. Coupled with converging surface winds and strong