1 The Benson, MN Tornado of June 11, 2001 Lynn Kjernes, University of Wisconsin-Madison ABSTRACT _______________________________________________________ A severe weather outbreak over the three days of June 11-13, 2001 helped set a record number of tornadoes for the annual total of 32. Throughout the morning of the 11th, surface conditions became favorable for a severe weather event. Recent rainfall increased soil moisture across Iowa, Illinois, and Missouri readily evaporated in the clear conditions prior to the 11thhelping to raise surface moisture levels. The peak of the agricultural season added the process of transpiration to increase the moisture content also. As the moisture advected into central Minnesota it collided with a surface warm front and upper level short wave to produce a synoptic lifting mechanism. The increasing instability of atmosphere allowed for a strong F2 tornado to break out in Benson, MN as with a string of smaller tornadoes in the three days period as the low pressure system moved across the central part of the state. _______________________________________________________ Introduction 2001 was a record year for tornado outbreaks across Minnesota. Between the three days of June 11-13, 2006 there were 32 tornadoes reported, accounting for nearly half of the annual reported tornadoes witnessed in Minnesota that year. On June 11, 2001 around 3:00pm a strong F2 tornado ripped through Benson, MN causing over ten million dollars in damage. The eight mile long path damaged 71 structures, leaving four homes and seven businesses condemned. No deaths occurred possibly due to the public knowledge of the threat for severe weather from the SPC, but seven people were injured during the storm. Several days prior to the 11th, the National Storm Prediction Center recognized the potential threat for severe weather in Iowa, Minnesota and Wisconsin. As the 11th approached the potential for severe weather was only increased, and warnings were issued. After a week of quiet weather a surface warm pressure center moved toward Minnesota, situating a warm front through the central part of the state. The location of the front and moist, warm air advection from the Corn Belt States provided ample instability in the atmosphere over central Minnesota. As soon as a lifting mechanism was initiated the instability of the atmosphere caused the storms to become severe rather than convective rainshowers. The Benson tornado was the largest tornado spawned of the nine touchdowns in Swift County alone. Data The University of Wyoming site provided observational soundings and was used to determine the actual conditions across Minnesota throughout2 the day of the 11th of June 2001. Since observations were only documented on the 0z and 12z hours (with the exception of 18z run on this particular day with weather occurring) the Eta model run is used to evaluates times in-between missing data at times right before and after the tornado. The largest tornado is documented to have occurred at approximately 3:07 pm in Benson. Most data evaluated is from 20z on the 11th, just before the tornado touched down. Nearly all data was processed using the program of GARP. Hand analysis of potential temperature and equivalent potential temperature on cross sections across the warm front and moisture gradient provides knowledge of location of convective plumes and where the tornado likely formed. A conceptual model of the dynamics of a tornado and a miller diagram of the Midwest area plainly illustrate the most important features contributing to the severe weather outbreak. Radar functions of base reflectivities and velocity data are used from outside sources since GARP data was unavailable to determine radar signatures of severe weather and forecasting ability. The exact location of the tornado can be observed with the radar and satellite data as well. Following the radar returns will show the series of tornadoes associated with the outbreak. Mesoscale Analysis The large synoptic flow patterns across the Midwest were favorable to support a severe weather outbreak. The National Storm Prediction Center evaluated the risk up to two days prior to the string of tornadoes across Minnesota and Wisconsin from the 11-13. Figure 1 shows the large scale flow of the atmosphere at several different levels. The 850mb temperature and height fields show the warm air advection from the south/southwest into Minnesota at 12z on the 11th, about eight hours prior to the outbreak. The low pressure in the Dakota’s have a warm front associated with it which stretches into Minnesota. At upper levels, the 300mb polar jet is north of Minnesota involved with a shortwave. Throughout the next 12 hours the jet marches eastward. The right exit region of the shortwave jet is positioned over central Minnesota provided a slight synoptic scale lifting mechanism for later in the day. The lifted index values and relative humidity shown in the lower left corner of Figure 1 show the air over Minnesota near saturation and LI values near -3, and only decreasing as the time of the storm approaches.3 Fig. 1 four panel plot valid 12z on June 11, 2001 courtesy of Unisys A zonally oriented warm front associated with a surface low pressure region in South Dakota stretched across the middle of Minnesota and into Wisconsin. During the early morning of the 11th, the southern part of Minnesota experienced clear skies. Surface temperatures rose into the mid-nineties due to the intense solar radiation and strengthened the warm frontal boundary already present, while the cloud covered northern part of the state only reached into the mid-sixties; this is seen in the surface observations in Figure 2 showing the surface temperature gradient and moisture gradient off to the west of Minnesota approaching. This stark contrast in temperatures and dewpoints create yet another surface boundary. Fig 2 Surface temps and dewpoint valid 11 June 2001 20z Throughout the day, surface winds from the south increased as the low pressure region and pressure gradient moved closer to Minnesota. The increase in surface wind intensity brought forth the hot, moist air from the Iowa, Missouri, and Illinois. Since winds along the Gulf coast were4 northerly, due to a tropical storm system, it has been suggested by Cheresnick and Basara, 2005, that the high moisture content in the Central Plains State’s is a combination of both