The Parkersburg, IA EF-5 Tornado: Destruction Amidst the RainSarah A. MonetteUniversity of Wisconsin - Madison Department of Atmospheric and Oceanic Science1225 W. Dayton Street, Madison, WI 53706, USAAOS 453 Final Case StudyAbstractOn May 25, 2008, at 5:00PM local time, the city of Parkersburg, IA, experienced a tornado which produced peak wind gusts of 92 meters per second, or 205 miles per hour, based on damage assessment of homes and other buildings. (Marshall et al., 2008) Therefore, this tornado was classified as an EF-5, the first one to record a 5 on the Fujita, or Enhanced Fujita, Scale in Iowa since 1976. Unfortunately, 8 people lost their lives in what is now known as the Parkersburg Tornado. The focus of this case study is to investigate the synoptic and mesoscale parameters which combined to produced a supercell capable of generating the EF-5 Parkersburg tornado. Aided by synoptic scale lifting mechanisms including upper level divergence, at both 300 mb and 500 mb, and a surface warm front combined with moisture flux from the mesoscale low level jet, the environment in Parkersburg featured thermodynamic and wind shear instability favorable for the development of a significant tornado. This instability includes a conditionally unstable lapse rate, large ratio of CAPE to CIN, veering wind with height, as well as positive surface relative helicity. In addition to this instability, the intensity of the tornado was further enhanced by interactions with gravity waves just prior to tornadic development. This combination of mesoscale phenomenon allowed an EF-5 tornado to be produced from what storm chasers described as a heavy precipitating supercell, an event rarely seen, which will be discussed with the assistance of a Miller diagram. IntroductionMay 25, 2008, will be a day most Iowans will potentially remember for the rest of their lives. In a state where 85% of the tornadoes recorded between 1980 and 2008 were at the lower end of the Enhanced Fujita Scale (EF0, EF1), the tornado that touched down at 4:48 PM local time on this day would be one for the record books. Around 5:00 PM local time, 22Z, the tornado reached peak intensity around Parkersburg, IA, with estimated winds at 92, meters per second (m/s), 205 miles per hour (mph), based on damage assessment, earning an EF-5 rating (above 200 mph) on the Enhance Fujita Scale. This was the first tornado to record a 5 on the Fujita (or Enhanced Fujita) Scale since a F5 tornado struck Boone and Stony counties on June 13th, 1976. However, unlike the F5 tornado in 1976, the tornado that struck Parkersburg and its surrounding communities killed 8 people, cutting a 43 mile long path from Aplington through Parkersburg to the Balckhawk/Buchanan county line. Parkersburg is located approximately 100 miles west of Dubuque, IA, and 150 miles northwest of Davenport, IA. (Cogil, et al., 2008; Parkersburg - New Hartford - Dunkerton EF5 Tornado of May 25, 2008, 2008; Service Assessment: EF5 Tornado in Parkersburg and New Hartford, Iowa, 2009) For a better visual, please see the surface analysis in Figure 1 for the exact location of Parkersburg, IA. 1Figure 1: Location of Parkersburg in Iowa. Tornadoes are generally form through two different mechanisms. The first mechanism creates a shear line tornado, which will not be discussed in this paper. Almost all violent, like the one which struck Parkersburg, are produced by supercell storms. Supercell thunderstorms, conceptualized in Figure 10, are unique when compared to airmass thunderstorms. A supercell features a rotating mesocyclone. This rotating mesocyclone acts as a dynamic wall surrounding the updraft. Therefore, entrainment of dry air, as well as precipitation loading, the two processes which suppress the updraft, are reduced. Precipitation loading is defined as droplets accumulating mass,weighing down the updraft and reducing upward vertical acceleration. Without these two processes interfering with the updraft, the supercell is able to persist for hours as long as the right environment is available. Supercells can be grouped into three different types: low precipitating, classic, and heavy precipitating. According to Miller (2006), the classic supercell is responsible for producing most of the violent tornadoes. However, according to storm chaser reports, and the radar image in Figure 11, the tornado which struck Parkersburg, IA, was rain-wrapped, indicating formation from a heavy precipitating supercell. It is hypothesized that the supercell which spawned the tornado that struck Parkersburg was initially initiated by synoptic scale forcings, such as ascending vertical motions in the form of upper level divergence and surface fronts, combined with a mesoscale environment ideal for the production of a strong and violent tornado. In addition, since the supercell was heavy precipitating, external forces, such as gravity waves, also enhanced the tornado. Therefore, this case study will first investigate the synoptic environment around the time of the tornado, 22Z. Next, the mesoscale environment, including the role of a low level jet for moisture flux as well as severe index parameters such as instability and helicity derived from the 22Z sounding at Parkersburg, will be analyzed to understand how the advantageously the environment in Parkersburg was for the produced of a violent tornado. In addition, gravity waves will be investigated as further mechanisms for producing an EF5 tornado from a heavy precipitating supercell. The characteristics of a heavy precipitating supercell, and the mechanisms which allow them to produce violent tornadoes, will be examined. Finally, a Miller Diagram of the synoptic features at 22Z will be analyzed to describe the favorable synoptic environment for heavy precipitating supercells. II. DataDate used for this case study was obtained from the ETA mathematical coordinate system model run at initialized at 12Z on 25 May 2008. Figures were then generated utilizing the General Meteorology Package (GEMPAK). Radar data from the NEXRCOMP, the National Weather Service NEXRAD Composition of the United States at 1 kilometer (km) resolution, was used to generate radar images using the Global Atmosphere Research Program (GARP) for analysis. In addition, figures were obtained from various scientific sources. Synoptic figures were obtained from Service Assessment: