A. J. Harrington 1 Residual Outflow Boundary Impacts on CAPE versus Shear Contributions to Tornadic Supercells: the F-5 Barneveld, Wisconsin Tornado Revisited Alex Harrington M.S. Research Assistant, University of Wisconsin – Madison, Cooperative Institute for Meteorological Satellite Studies (CIMSS) ABSTRACT The impact remnant outflow boundary over the Upper Midwest from early morning convection on maximized afternoon and evening CAPE values is examined through the use of Geostationary Operational Environmental Satellite (GOES) data and available surface observations. Satellite imagery and surface analyses conveyed that a residuum deformation structure provided the focus for further convective development over southern and eastern Wisconsin. Thus, the prevalence of widespread cloud debris over southern Wisconsin prohibited excessive boundary layer instability from reaching high values during the window of maximum diurnal diabatic heating. The combination of reduced CAPE values and ambient high shear profiles is investigated for priming Barneveld, Wisconsin with low Bulk Richardson Numbers (BRN) conducive for a nocturnal tornadic supercell. Furthermore, it is hypothesized that the presence of the deformation zone itself led to an augmentation of streamwise vorticity, enhancing low-level rotation such that the F-5 Barneveld tornado could materialize. _______________________________________ I. Introduction While late night tornadoes are not uncommon, violent tornadoes; which make up approximately three percent of all reported tornadoes, are a rare phenomenon. At 0550Z 08 June 1984, an F-5 tornado destroyed the small, southwestern Wisconsin community of Barneveld; killing nine, only ten percent of Barneveld was left habitable. Fig. 1 (top) showed the devastation in the wake of the tornado. Noteworthy was the fact that debris penetrated the reinforced steel of the still-standing water tower in the background (Harrington, 2006). The supercell which later spawned the tornado was one of many tornadoes which formed in the Upper Midwest on 07-08 June, 1984. Fig. 1 (bottom) depicted the tornado tracks and storm reports from the severe weather outbreak (SPC, 2006). Evident was an extensive, though episodic, tornado track from near Topeka, Kansas, to sixty miles north of Madison, Wisconsin. The existence of severe weather reports between confirmed tornado events was evidence of the pulse nature of a long-lived supercell that lasted over four hours (Bunkers et. al., 2006). The supercell produced a violent F-4 tornado over south-central and southeastern Iowa; then, after considerable weakening, the cell rapidly regenerated a hundred miles to the northeast over Lafayette and Iowa counties of southwestern Wisconsin. Thirty minutes after its intensification, the supercell fostered a quarter-mile wide F-5 tornado that descended on Barneveld. The synoptic environment was ideal for convection, including severe multicellular and supercellular storms throughout the 07-08 severe weather outbreak. A Mesoscale Convective Complex (MCC) over southeastern Iowa, northern Illinois, and southern and central Wisconsin, pushed to the northeast during the late morning hours of 07 June. 17Z Geostationary Operational Environmental Satellite (GOES) imagery conveyed a remnant boundary induced by the effects of a southward propagating gust front from the morning MCC, extending from southern Iowa to the Wisconsin/Illinois border. A 12Z mesoscale analysis confirmed the presence of the southward extent of the density current through a surface temperature discontinuity. North of the boundary, widespread cloud debris prevailed; and,A. J. Harrington 2 by 19Z, it served as a focusing mechanism for thunderstorm development across southern and eastern Wisconsin. While the physics of outflow boundaries remains poorly understood, meteorological consensus regards them as integral features to convective initiation (Browning, 1997). Cloud debris associated with the deformation zone and convective activity precluded Convective Available Potential Energy (CAPE) values across southern Wisconsin from reaching maximized values of 5000 J/kg experienced south of the boundary. Thus, the loss of diabatic heating following sunset limited already reduced CAPE values over southern Wisconsin to the 1500-2000 J/kg range by 06Z 08 June. Sufficiently strong 0-6km shear profiles, coupled with modest CAPE values, fostered an environment primed for strong tornadic development. Figure 1. Top: Ninety percent of Barneveld was destroyed after the F-5 08 June 1984 tornado (Barneveld Public Library, 1985). Bottom: 07-08 June 1984 archived SPC tornado tracks and storm reports. Thus, Bulk Richardson Numbers (BRN) of 10-20 were experienced over Lafayette and Iowa counties at the time of the supercell intensification. Furthermore, the imprint of the outflow boundary on the local environment resulted in a fortification in the streamwise vorticity such that low-level rotation of the mesocyclone ensued; likely leading to the formation of the F-5 Barneveld tornado. II. Data GOES data primarily served the investigation and placement of the outflow boundary produced by a morning MCC over the Upper Midwest. GOES data was collected from Space Science and Engineering Center GOES archive. 1984 satellite data was devoid of the multiple bands and projection capabilities of present date. GOES Band-1 Visible imagery was only available at 20Z 07 June and after 00Z 08 June. Hence, the lost of solar flux prohibited the use of the more beneficial visible band. Thus, GOES Band-8 thermal IR was accepted for discerning the existence of the deformation zone. National Weather Service (NWS) surface analyses archived by the University of Wisconsin – Madison (UW-Madison) Atmospheric and Oceanic Sciences (AOS) department were also advantageous in assessment of the vestige boundary. Although the quality of preservation was not pristine; it was however, crucial in discovery of a temperature discontinuity and