1 The Oakfield Tornado of 18 July 1996 Andrew S. Thut Atmospheric and Oceanic Sciences, University of Wisconsin Madison ABSTRACT The Oakfield, Wisconsin tornado of 18 July 1996 is examined using datasets provided by GARP, GOES-8 infrared satellite imagery and WSR-88D radar data. The analysis provides an insightful description of the meteorological setting and evolution that led to the severe weather on this day. Strong thunderstorms and a tornado were spawned by a loaded gun sounding which was indicative of warm dry air over moist air near the surface. The storms strength can also be attributed to the convergence of winds along a cold front and a sharp temperature gradient which extended throughout Wisconsin. Convergence and the position of the upper level jet created conditions for the capping inversion to be broken causing a release in conditional instability. This case also displays how a Miller Diagram, a conceptual model as well as other data sets can be used together to provide a valuable and precise overview of a rapidly evolving meteorological event. I. Introduction On 18 July 1996, Oakfield, Wisconsin, in the northeastern portion of the state, was affected by an F5 tornado. In addition to the tornado, strong thunderstorms produced heavy rains, lightning and gusty winds in the region. The original report from the National Weather Service categorized the tornado to be either an F3 or F4, with winds varying in strength between 158 and 260 miles per hour. However, after an inspection of the damage from the tornado in the Oakfield area, the tornado was issued the status of an F5 which was indicative of winds stronger than 260 miles per hour. The strength and assigned status of the tornado bring to attention the significance of this storm. F5 tornadoes are very rare and only tend to occur every other year in the United States. It is expected that converging winds along the cold front and a significant temperature gradient were the primary reasons for the development of this powerful storm. The tornado was responsible for over $40 million in damage as 47 of 320 homes were destroyed. 56 additional homes and businesses sustained significant damage. The tornado demolished the Friday Canning Company and caused empty cans to be sprawled up to 50 miles from the business. Additionally, the tornado also affected agriculture as crops, livestock and farm equipment were destroyed. Despite the severity of the tornado, there were no reported deaths and only 17 injuries from the incident. A tornado watch was issued well in advance of the tornado occurrence. The tornado remained in existence for over 20 minutes and traveled nearly 20 miles before dissipating. The tornado developed in Fond du Lac County and moved southeastward across Wisconsin till it reached Oakfield at 0015 UTC (Achtor et al.). II. Data The severe weather event over Wisconsin on 18 July 1996 are examined using WSR-88D radar data, GOES-8 visible satellite imagery, ETA model analysis and2 surface data in GARP, as well as soundings from the University of Wyoming. The combined use of these data sets provides an insightful description of the meteorological setting and evolution that led to the Oakfield tornado. III. Morning Synoptic Situation At 1200 UTC on 18 July 1996, a low pressure system was centered over northern Minnesota with a warm front extending off to the east through central Wisconsin and Michigan. A cold front associated with this system extends from northern Minnesota to the southern Plains. Anti-cyclonic rotation associated with high pressure over the Southeast is advecting Gulf moisture towards the Midwest. More significantly perhaps was the moisture provided by the corn due to evapotranspiration. This moisture is providing humid conditions with high dew points. Cooler and drier air is present to the west of the cold front. Analysis of the 1200 UTC sounding at Minneapolis (Figure 1) reveals a capping inversion that would result in convective initiation later in the day. Figure 1: Minneapolis Sounding, 7.18.96, 12Z This is a typical loaded gun sounding (University). A temperature inversion exists near 850-mb; the cap is represented by the warm layer above it, wherein the parcel would be cooler than the surrounding air and thus would accelerate upward. Also at this time, the environment between 850 and 550-mb was conditionally unstable and was indicative of an elevated well mixed boundary layer. Relatively weak wind shear exists at levels above 850-mb. The most significant shear occurs below 850-mb where the winds shift from the southeast to the west-southwest. The convective available potential energy (CAPE) is nearly 2000 which designates a good condition for the formation of a supercell. Also, the lifted index is in a range where severe thunderstorms are possible. A relatively strong mid-level jet extended through the Dakotas and Minnesota at the 500-mb level. Diverging winds at the 500-mb level suggest that the surface air is being lifted upwards due to convergence at the surface associated with a cold front. Divergence and a strong jet continues to be the case at the 250-mb level. The most significant divergence was present in the northern portion of the state at 12Z. The divergence acts to bring surface moisture upwards and aid in breaking the inversion. IV. Afternoon and Evening Synoptic Situation As the day progressed, there was little change in the strength of the low pressure system which shifted towards the southeast and was now present over northeastern Wisconsin.3 Figure 2: Surface Oservations, Sea Level Pressure (mb), Streamlines, 7.19.96, 00Z The high pressure system located in the Southeast slightly moved to the east while weakening by 3-mb The anti-cyclonic flow around the high continued to feed moisture into the Midwest. Figure 2 exhibits the surface observations, pressure contours as well as the frontal structure and streamlines. The low pressure system is centered over northeastern Wisconsin and a warm front extends off to the east-northeast. A cold front extends from northeastern Wisconsin to southwestern Iowa. Also notable is that the surface winds to the south of the cold front are generally from the south-southeast. The orientation of the flow at the surface, as shown by the streamlines, reiterates that moisture is being advected from the Gulf through the cornbelt