The Tornado Outbreak of 17 November 2013: Operational Considerations for a High Impact Event and Anticipating Tornado Development within a QLCS

The severe weather outbreak that occurred on 17 November 2013 produced fifteen tornadoes in the County Warning Area (CWA) of the National Weather Service in Northern Indiana (IWX). While the damage resulting from these tornadoes were all rated between EF0 and EF2, it does rank as one of the largest tornado outbreaks in the number confirmed for a single day. Additionally, the twenty-eight tornadoes that occurred in the state of Indiana this day was the largest outbreak for November in recorded history for Indiana, and the third largest outbreak in state history for any month. A classic low CAPE/high shear environment was in place on this day as a vigorous negatively tilted shortwave trough tracked east-northeast toward the Illinois/Wisconsin border by late afternoon. An anomalously strong upper/mid-level jet (120kt at 500mb) undercut this upper wave with deep diffluent flow observed locally with the left exit region of the jet. This coupled with a 60-70kt low-level jet supported strong cyclogenesis as the surface low deepened and tracked from eastern Iowa to Lake Superior. Convection developed along the prefrontal trough in advance of the attendant surface cold front across western Illinois by mid-morning and rapidly organized eastward into Indiana during the afternoon. The mesoscale environment was clearly supportive of severe thunderstorms with deep layer effective shear more than sufficient to support rotating updrafts. Line normal 0-3km bulk shear near 50kts was also supportive of mesovortex tornado formation in bowing segments. Slight backing of surface winds was noted which yielded 0-1km SRH values over 500m2/s2 and in combination with low LCL heights (<1000m) were significant factors in increasing the tornado potential. Discrete supercells developed from northern through central Illinois during the late morning hours. These storms exhibited strong rotational couplets and classic supercell radar signatures, quickly producing several large and violent tornadoes in central Illinois. As these storms raced northeast toward the Indiana state line at 60kts, they began to merge with other storms and formed the backbone of a larger scale quasi-linear convective system (QLCS). Numerous small-scale circulations and embedded supercell structures were noted within this line as it entered into the western edge of the IWX CWA. This presentation will examine some of the unique challenges that are faced in the warning decision making process involving tornado development that is commonly embedded within a QLCS. The application of research into the operational setting as well as experience gained from recent QLCS events in formulating a conceptual model of storm structure will also be examined. Finally, the incorporation of social media and the effectiveness of impact-based warnings (IBW) in communicating the severity of this event will be discussed.