The 14 July 2015 Tennessee Valley Tornado Event: Challenges in QLCS Tornado Forecasting and Identification

A significant quasi-linear convective system (QLCS) tornado event occurred across the Tennessee Valley region between 2230 UTC 14 July and 0030 UTC 15 July 2015. This QLCS produced at least 6 tornadoes as well as significant non-tornadic wind damage. The 14 July case presented a myriad of both challenges and opportunities in QLCS tornado detection, warning, and verification.

1) The event was atypical for July in the Tennessee Valley and now consists a plurality of all recorded July tornadoes since 1950 in the Huntsville (HUN) county warning area (CWA). Prior to 14 July 2015, July had the third-fewest number of tornadoes of any calendar month since 1950 (13), with no more than 3 tornadoes recorded on an individual July day.

2) Several of the tornadoes were longer-lived than any other recorded July tornadoes in the HUN CWA. The longest track length recorded was at least 18.3 km (11.4 mi), with other path lengths of 14.7 km (9.1 mi) and 7.9 km (4.9 mi) observed. The previous record path length for a tornado in the HUN CWA in July was approximately 6.4 km (4.0 mi).

3) This deviation from climatology, in addition to the environment not being a “typical” Alabama QLCS tornado environment (which often trend toward higher shear and lower convective available potential energy (CAPE), as opposed to low shear and high CAPE), led to an expectation of damaging winds and perhaps a few brief tornadoes. The environment did not immediately suggest the potential for wider, long-lived circulations that were observed, nor the prolific number of observed circulations across a rather concentrated area.

4) Though Weather Surveillance Radar-88 Doppler (WSR-88D) coverage is sufficient across most of the Tennessee Valley, detection of many of the circulations during the 14 July event was still challenging. The University of Alabama in Huntsville (UAH) Advanced Radar for Meteorological and Operational Research (ARMOR) proved to be a significant asset in post-event analysis, particularly in the discovery of tornado tracks. However, power and/or communications line issues due to the severity of the storms prevented ARMOR from providing timely updates during the event. The WSR-88D radars did resolve some of the signatures, but the intensity of the circulations did not warrant enough certainty to issue tornado warnings.

In this presentation, we offer a brief meteorological overview of the 14 July 2015 tornado event in the Tennessee Valley, including placement of the event in a historical context. We review the environment ahead of the tornadic storms and elicit environmental clues that can aid in the forecasting of similar events in the future. We compare radar signatures from WSR-88D radars at Hytop, Alabama (KHTX) and Columbus, Mississippi (KGWX) to signatures from ARMOR to demonstrate the differences in detection of tornadic circulation during and after this severe weather event. We conclude with a discussion of the QLCS tornado detection, warning, and confirmation paradigm. In particular, we discuss how the observations and challenges from the 14 July 2015 outbreak can help shape forecasting, warning decision making, documentation, post-event communication, and scientific research of QLCS tornado events in the future, particularly in the context of the upcoming VORTEX-SE field campaign.