WISCONSIN TORNADO OUTBREAK OF 18 AUGUST 2005 AN EXAMINATION OF THE VIOLA WISCONSIN TORNADO Charles D Koch University of Wisconsin Madison AOS 453 Mesoscale Meteorology Professor Greg Tripoli 13 May 2007 ABSTRACT On the afternoon of 18 August 2005 the largest single day tornado outbreak in Wisconsin history unfolded to the surprise of many people This tornado outbreak spawned 27 total tornadoes including an F2 that struck the village of Viola Wisconsin and an F3 that hit the city of Stoughton Wisconsin This case focuses on the former of the above mentioned tornadoes The tornado that struck Viola Wisconsin touched the lives of many and miraculously nobody was injured or killed despite the millions of dollars of damage done I INTRODUCTION At around 2030Z on 18 August 2005 a squall line associated with a low pressure system and cold front produced an isolated supercell that would later move into the village of Viola Wisconsin less than 45 minutes later This tornado officially rated an F2 by the National Weather Service created 3 6 million dollars in total damage and permanently scarred the rural community by destroying thousands of trees and damaging hundreds of homes and structures Although there was no advanced warning no injuries or loss of life were reported This tornado was the first significant tornado that was produced that day by this system In all 27 tornadoes were reported including an F3 tornado in Dane County that took one life This case looks to examine the mechanisms that were in place that created the first tornadoes in Vernon and Richland Counties in southwest Wisconsin II DATA AND METHODS Various software suites and programs were used to analyze various data for this case GEMPAK was utilized to analyze upper air data from the RUC and ETA models GARP was also used to analyze upper air data in addition to surface observations level II and III radar data and satellite data from the NOAA GOES East imager The visible water vapor and 10 7 infrared channel were selected for use from the GOES East imager for their significance in this case The software program IDV Integrated Data Viewer was also used to examine level II NEXRAD reflectivity and velocity data from the NWS WSR 88D radar in La Crosse Wisconsin KARX and Davenport Iowa KDVN as well as satellite imagery from GOES East In addition to analysis from software programs the National Weather Service National Climatic Data Center and Storm Prediction Center archives were all accessed for surface analysis storm reports and watch and warning products III SYNOPTIC OVERVIEW Surface analysis at 0900Z 18 August 2005 indicated a developing surface low pressure center over eastern South Dakota with an associated trough extending through the Panhandle region of Texas and Oklahoma At 12Z the low pressure center had moved into northwestern Iowa and a surface warm front was established stretching from the low pressure center of 1003 millibars across the state and extending into portions of north central Illinois The low pressure center continued to move northeastward into southern Minnesota at 15Z and a cold front was beginning to develop over western Iowa and eastern Nebraska By 18Z the low pressure center had moved into southeastern Minnesota and the frontal structure became more defined The warm sector of the storm located in east central Iowa was defined by temperatures in the mid 80 s with dewpoints in the low to mid 70 s The low continued to move to the east northeast and deepened to 1002 millibars by 21Z Figure 1 The low was centered near La Crosse Wisconsin and the warm front had moved through southwest and south central Wisconsin Figure 1 North Central surface analysis from the National Weather Service for 18 August 2005 Times shown 12Z upper left 15Z upper right 18Z lower left and 21Z lower right Analysis shows developing frontal low pressure system and evolution bringing warm temperatures in the mid 80 s and dewpoints in the mid 70 s The cold front at this time extended from the low pressure center through central Iowa where it became a stationary front extending through central Kansas South to southwest winds were observed ahead of the storm in the warm sector while northwesterly winds were observed behind the cold front The southerly winds ahead of the system provided for surface moisture transport in to southwest Wisconsin as some dewpoint observations over eastern Iowa were in the upper 70 s after the passing of the surface warm front Figure 2 The wind shift appeared to be the greatest change associated with the cold front as temperatures ranged in the low 80 s over Iowa with a slight dewpoint drop as well The 850 millibar analysis showed an area of strong temperature advection over Iowa stretching into southwest Wisconsin at 12Z This advection was at the base of a geopotential minimum extending from the prairie provinces of Canada southeastward into northern Iowa By 18Z the geopotential field had a local minimum associated with the surface low pressure over southern Minnesota Winds at this level were also out of the southwest and much stronger than at the surface indicating the presence of vertical speed shear and weak directional A B Figure 2 GARP surface observations taken at A 0700Z and B 1900Z 18 August 2005 showing the increase in surface dewpoint temperatures F after passage of MCC and warm front due to residual high theta e and moisture transport from central Iowa A B C Figure 3 850 RUC model analysis showing geopotental contours and filled contours of temperature from 18 August 2005 at A 12Z B 18Z and C 21Z Analysis indicates WAA ahead of the low pressure at 12Z and a geopotential minimum over SW Wisconsin at 21Z shear over portions of south central Wisconsin where surface winds were out of the south to southeast At 21Z the geopotential minimum had pressed into far western Wisconsin with the associated warm air advection occurring farther to the east over southeast Wisconsin as well as over Lake Michigan Figure 3 A shortwave trough was identified at 700 millibars and was echoed in the 500 millibar analysis The shortwave was positioned over eastern South Dakota at 09Z and was associated with a strong vorticity maximum The vorticity maximum and shortwave progressed to the east without losing intensity and was positioned over central Minnesota at 15Z The shortwave and vorticity maximum were positioned slightly to the west of the surface low indicating a slight westward tilt with height of the system indicating