Tornadic Supercell Research Articles

Effective Storm-Relative Helicity and Bulk Shear in Supercell Thunderstorm Environments

Abstract A sample of 1185 Rapid Update Cycle (RUC) model analysis (0 h) proximity soundings, within 40 km and 30 min of radar-identified discrete storms, was categorized by several storm types: significantly tornadic supercells (F2 or greater damage), weakly tornadic supercells (F0–F1 damage), nontornadic supercells, elevated right-moving supercells, storms with marginal supercell characteristics, and nonsupercells. These proximity soundings served as the basis for calculations of storm-relative helicity and bulk shear intended to apply across a broad spectrum of thunderstorm types. An effective storm inflow layer was defined in terms of minimum constraints on lifted parcel CAPE and convective inhibition (CIN). Sixteen CAPE and CIN constraint combinations were examined, and the smallest CAPE (25 and 100 J kg−1) and largest CIN (−250 J kg−1) constraints provided the greatest probability of detecting an effective inflow layer within an 835-supercell subset of the proximity soundings. Effective storm-relative helicity (ESRH) calculations were based on the upper and lower bounds of the effective inflow layer. By confining the SRH calculation to the effective inflow layer, ESRH values can be compared consistently across a wide range of storm environments, including storms rooted above the ground. Similarly, the effective bulk shear (EBS) was defined in terms of the vertical shear through a percentage of the “storm depth,” as defined by the vertical distance from the effective inflow base to the equilibrium level associated with the most unstable parcel (maximum θe value) in the lowest 300 hPa. ESRH and EBS discriminate strongly between various storm types, and between supercells and nonsupercells, respectively.

Thermodynamic Analysis of Supercell Rear-Flank Downdrafts from Project ANSWERS

Abstract Data collected during Project Analysis of the Near-Surface Wind and Environment along the Rear-flank of Supercells (ANSWERS) provided an opportunity to test recently published associations between rear-flank downdraft (RFD) thermodynamic characteristics and supercell tornadic activity on a set of 10 events from the northern plains. On average, RFDs associated with tornadic supercells had surface equivalent potential temperature and virtual potential temperature values only slightly lower than storm inflow values. RFDs associated with nontornadic supercells had mean group equivalent potential temperature and virtual potential temperature values that were colder relative to storm inflow values than their respective tornadic counterparts. Additionally, the analysis revealed that RFDs associated with tornadic supercells had higher CAPE and lower convective inhibition than the RFDs of nontornadic supercells, on average. The results of this study provide further support for the general concept that a thermodynamic delineation generally exists between the RFDs of tornadic and nontornadic supercells.

Infrared Thermal Imagery of Cloud Base in Tornadic Supercells

AbstractDuring the spring seasons of 2003 and 2004, an infrared thermal camera was deployed in and around supercell thunderstorms in an attempt to retrieve the temperature at the cloud base of a mesocyclone prior to tornadogenesis. The motivation for this exercise was to obtain temperature information that might indicate the thermal structure, timing, and extent of the rear-flank downdraft (RFD) and possibly elucidate its relationship to tornadogenesis.An atmospheric transmissivity study was conducted to account for the effects of atmospheric transmission on the measured temperatures, and to determine an ideal range of distances from which infrared images of a wall cloud or a tornado could be safely captured while still retrieving accurate cloud temperatures. This range was found to be 1.5–3 km.Two case days are highlighted in which the infrared camera was deployed within 1.5–3 km of a tornado; the visible and infrared images are shown side by side for comparison. On the single occasion on which the tornadogenesis phase was captured, the infrared images show no strong horizontal temperature gradients. From the infrared images taken of tornadoes, it can be inferred that the infrared signal from the tornado consisted primarily of infrared emissions from lofted dust particles or cloud droplets, and that the infrared signal from the tornado condensation funnel was easily obscured by infrared emissions from lofted dust particles or intervening precipitation curtains.The deployment of the infrared camera near supercell thunderstorms and the analysis of the resulting images proved challenging. It is concluded that the infrared camera is a useful tool for measuring cloud-base temperature gradients provided that distance and viewing angle constraints are met and that the cloud base is unobscured by rain or other intervening infrared emission sources. When these restrictions were met, the infrared camera successfully retrieved horizontal temperature gradients along the cloud base and vertical temperature gradients (close to the moist adiabatic lapse rate) along the tornado funnel.

A multiscale observational case study of an isolated tornadic supercell

This work deals with the analysis of an isolated tornadic supercell thunderstorm that took place on the plain of Friuli Venezia Giulia (Italy) causing several damage. The analysis is performed using different kind of data, looking in the available information for possible signatures that could be helpful in the forecasting and nowcasting procedures. The analysis reveals that rather than a single cause for the tornadic storm development, there is an “interaction” of several effects. In particular, a favorable environment seems to be produced by the interplay between synoptic and mesoscale environments with a fundamental role played by orography. Radar data, in particular Doppler measurements, are fundamental to the recognition and interpretation of the events but in this particular case they could give little useful information for the forecasting and nowcasting procedures. The most robust signals of an environment prone to the onset of severe deep moist convection come from the thermal gradients at the ground and by the vertical profile analysis. The reason why the event took place in that specific place and not in others is still unknown.

The 19 April 1996 Illinois Tornado Outbreak. Part II: Cell Mergers and Associated Tornado Incidence

Abstract In the 19 April 1996 Illinois tornado outbreak, cell mergers played a very important role in the convective evolution. With a large number of cells forming within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells as described in Part I of this study. Twenty-six mergers were documented and analyzed. Storm-rotation-induced differential cell propagation accounted for 58% of these 26 cell mergers while differing cell speeds prompted 27% of the mergers. Cell merger characterizations were utilized to describe the cell reflectivity coalescence morphology including aspects of new cell development, development along the periphery of an existing cell, or an upward pulse in the cell intensity of a dominant cell. In cases where the merging cells were of similar intensity, a rapidly developing cellular pulse “bridging” the two echoes was often observed. When the relationship between short-term cell intensity changes and cell mergers was examined, it was found that the maximum reflectivity tendency showed a bias toward higher reflectivity for the product storm. Depending upon the radar elevation angle utilized, 27%–44% of mergers were associated with an increase in peak reflectivity while 40%–58% of the storms realized little or no increase. With respect to short-term cell rotation changes, the merger signal was marked. Depending upon the length of the evaluation window, in 44%–60% of the mergers, there was evidence of a merger-associated increase in cell rotation. When considering the association between tornado occurrence and cell mergers, a striking 54% of the tornadoes occurred within 15 min before or after a cell merger. This high percentage is strongly suggestive of a physical relationship between storm mergers and tornadogenesis. A discussion is presented of potential merger scenarios and favorable ambient environmental conditions that may have been conducive to tornadogenesis in this event. Suggestions are presented to raise the awareness level of forecasters to key aspects of cell evolution and interaction in nowcasting severe convection.

Developing Tools for Nowcasting Storm Severity

Abstract A conceptual model is presented for developing a new tool for nowcasting severe thunderstorms using existing operational data. Selected output from two operational, automated, weather detection and forecasting systems have been combined together within a fuzzy logic–based, data fusion system to test the concept and produce 15-min nowcasts of severe weather. The NCAR Auto-Nowcast System provides information and nowcasts on the evolving boundary layer and storm initiation, growth, and decay. The National Severe Storms Laboratory Warning Decision Support System (WDSS) identifies severe weather attributes within storms and provides storm-centric and specific detections of strong winds, mesocyclones, tornadoes, and probabilities of hail and severe hail. A modified version of the Auto-Nowcast System is employed as the engine for combining the Auto-Nowcast gridded output with the object-based WDSS output. Severe thunderstorm nowcasts are compared with available spotter reports for a multicellular, hail-producing squall-line event and a tornadic supercell event. Proof of concept is demonstrated and the results are encouraging as some skill is observed with the 15-min nowcasts. Many challenges still exist in producing a robust tool and these challenges are discussed.

The 19 April 1996 Illinois Tornado Outbreak. Part I: Cell Evolution and Supercell Isolation

Abstract In this study of the 19 April 1996 Illinois tornado outbreak, 109 cells were tracked using radar data to understand the transition of the cell configuration from a considerable number of initial cells to a small subset of supercells after several hours of evolution. Of these 109 cells, 85 developed along three synoptic boundaries (dryline, warm front, and dryline–warm front occlusion) between 1940 and 2230 UTC. A large majority of these 85 cells formed in a 1-h period between 2040 and 2140 UTC. With a considerable number of cells initiating within a short time period, the early stages of cell organization were marked by cell merger interactions and cell attrition that led to a pattern of isolated tornadic supercells. Cell-type initiation analysis revealed that storms that would become supercells were initiated, on average, 17 min before nonsupercell storms. Cyclonic supercells, with mean storm life spans of 214 min, had much longer lives than nonsupercell storms. Anticyclonic supercells resulting from storm splits were the second longest lived at 166 min. In comparison, the largest nonsupercell category, those cells that dissipated in relative isolation, only had 35-min life spans. Supercell isolation resulted from storm mergers due to differential cell propagation and the frequent attrition of cells that formed along a common boundary. The varying rotational properties of individual cells enhanced the probability for numerous mergers while fostering a scenario where, after a few hours, the supercells became increasingly isolated. Suggestions are presented to raise the awareness level of forecasters to key aspects of cell evolution and interaction in nowcasting severe convection. In Part II of this study, storm interactions are examined in the context of merger morphology, merged cell intensity changes, and the association between storm mergers and tornadogenesis.

A virtual tornadic thunderstorm enabling students to construct knowledge about storm dynamics through data collection and analysis

Abstract. A visually realistic tornadic supercell thunderstorm has been constructed in a fully immersive virtual reality environment to allow students to better understand the complex small-scale dynamics present in such a storm through data probing. Less-immersive versions have been created that run on PCs, facilitating broader dissemination. The activity has been tested in introductory meteorology classes over the last four years. An exercise involving the virtual storm was first used by a subset of students from a large introductory meteorology course in spring 2002. Surveys were used at that time to evaluate the impact of this activity as a constructivist learning tool. More recently, data probe capabilities were added to the virtual storm activity enabling students to take measurements of temperature, wind, pressure, relative humidity, and vertical velocity at any point within the 3-D volume of the virtual world, and see the data plotted via a graphical user interface. Similar surveys applied to groups of students in 2003 and 2004 suggest that the addition of data probing improved the understanding of storm-scale features, but the improved understanding may not be statistically significant when evaluated using quizzes reflecting short-term retention. The use of the activity was revised in 2005 to first have students pose scientific questions about these storms and think about a scientific strategy to answer their questions before exploring the storm. Once again, scores on quizzes for students who used the virtual storm activity were slightly better than those of students who were exposed to only a typical lecture, but differences were not statistically significant.

The 29 June 2000 Supercell Observed during STEPS. Part II: Lightning and Charge Structure

Abstract This second part of a two-part study examines the lightning and charge structure evolution of the 29 June 2000 tornadic supercell observed during the Severe Thunderstorm Electrification and Precipitation Study (STEPS). Data from the National Lightning Detection Network and the New Mexico Tech Lightning Mapping Array (LMA) are used to quantify the total and cloud-to-ground (CG) flash rates. Additionally, the LMA data are used to infer gross charge structure and to determine the origin locations and charge regions involved in the CG flashes. The total flash rate reached nearly 300 min−1 and was well correlated with radar-inferred updraft and graupel echo volumes. Intracloud flashes accounted for 95%–100% of the total lightning activity during any given minute. Nearly 90% of the CG flashes delivered a positive charge to ground (+CGs). The charge structure during the first 20 min of this storm consisted of a midlevel negative charge overlying lower positive charge with no evidence of an upper positive charge. The charge structure in the later (severe) phase was more complex but maintained what could be roughly described as an inverted tripole, dominated by a deep midlevel (5–9 km MSL) region of positive charge. The storm produced only two CG flashes (both positive) in the first 2 h of lightning activity, both of which occurred during a brief surge in updraft and hail production. Frequent +CG flashes began nearly coincident with dramatic increases in storm updraft, hail production, total flash rate, and the formation of an F1 tornado. The +CG flashes tended to cluster in or just downwind of the heaviest precipitation, which usually contained hail. The +CG flashes all originated between 5 and 9 km MSL, centered at 6.8 km (−10°C), and tapped LMA-inferred positive charge both in the precipitation core and (more often) in weaker reflectivity extending downwind. All but one of the −CG flashes originated from >9 km MSL and tended to strike near the precipitation core.

A Multiscale Examination of the 31 May 1998 Mechanicville, New York, Tornado

Abstract On 31 May 1998, an F3 tornado struck Mechanicville, New York, injuring 68 people and causing $71 million in damage. The tornado was part of a widespread, severe weather outbreak across the northeast United States. The synoptic conditions that caused the outbreak and the mesoscale and storm-scale environments that produced the tornado are discussed. The coupling of two strong upper-level jets and a very strong low-level jet, in association with an unseasonably strong surface cyclone, created a synoptic-scale environment favorable for severe weather. As the result of these jet interactions, a very warm, moist air mass was transported into the Northeast with an associated increase in the wind shear in the lower troposphere. A terrain-channeled low-level southerly flow up the Hudson Valley may have created a mesoscale environment that was especially favorable for tornadic supercell development by increasing storm-relative helicity in the low levels of the atmosphere and by transporting warm, moist air northward up the valley, leading to increased instability. A broken line of locally severe thunderstorms moved eastward across New York several hours prior to the tornado. The storm that produced the Mechanicville tornado developed over central New York ahead of this line of storms. As the line of storms moved east, it intensified into a solid line and bowed forward down the Mohawk Valley of New York. These storms were moving faster than the isolated supercell to the east and overtook the supercell where the eastern end of the Mohawk Valley opens into the Hudson Valley. Based on limited observational evidence and the results of simulations of idealized quasi-linear convective systems reported elsewhere in the literature, it is hypothesized that backed low-level flow ahead of a bookend vortex at the northern end of the bowing line of storms over the Mohawk Valley may have contributed to the tornadogenesis process as the squall line overtook and interacted with the intensifying supercell.

Lightning activity related to satellite and radar observations of a mesoscale convective system over Texas on 7–8 April 2002

A multi-sensor study of the leading-line, trailing-stratiform (LLTS) mesoscale convective system (MCS) that developed over Texas in the afternoon of 7 April 2002 is presented. The analysis relies mainly on operationally available data sources such as GOES East satellite imagery, WSR-88D radar data and NLDN cloud-to-ground flash data. In addition, total lightning information in three dimensions from the LDAR II network in the Dallas–Ft. Worth region is used. GOES East satellite imagery revealed several ring-like cloud top structures with a diameter of about 100 km during MCS formation. The Throckmorton tornadic supercell, which had formed just ahead of the developing linear MCS, was characterized by a high CG+ percentage below a V-shaped cloud top overshoot north of the tornado swath. There were indications of the presence of a tilted electrical dipole in this storm. Also this supercell had low average CG− first stroke currents and flash multiplicities. Interestingly, especially the average CG+ flash multiplicity in the Throckmorton storm showed oscillations with an estimated period of about 15 min. Later on, in the mature LLTS MCS, the radar versus lightning activity comparison revealed two dominant discharge regions at the back of the convective leading edge and a gentle descent of the upper intracloud lightning region into the trailing stratiform region, apparently coupled to hydrometeor sedimentation. There was evidence for an inverted dipole in the stratiform region of the LLTS MCS, and CG+ flashes from the stratiform region had high first return stroke peak currents.

Polarimetric Tornado Detection

Abstract Polarimetric radars are shown to be capable of tornado detection through the recognition of tornadic debris signatures that are characterized by the anomalously low cross-correlation coefficient ρhv and differential reflectivity ZDR. This capability is demonstrated for three significant tornadic storms that struck the Oklahoma City, Oklahoma, metropolitan area. The first tornadic debris signature, based on the measurements with the National Severe Storms Laboratory’s Cimarron polarimetric radar, was reported for a storm on 3 May 1999. Similar signatures were identified for two significant tornadic events during the Joint Polarization Experiment (JPOLE) in May 2003. The data from these storms were collected with a polarimetric prototype of the Next-Generation Weather Radar (NEXRAD). In addition to a small-scale debris signature, larger-scale polarimetric signatures that might be relevant to tornadogenesis were persistently observed in tornadic supercells. The latter signatures are likely associated with lofted light debris (leaves, grass, dust, etc.) in the inflow region and intense size sorting of hydrometeors in the presence of strong wind shear and circulation.

Observations of a Severe, Left-Moving Supercell on 4 May 2003

Abstract A case study of a left-moving supercell with a rapid motion is presented to (i) elucidate differences in anvil orientations between left- and right-moving supercells and (ii) highlight the interaction of the left mover with a tornadic right mover. It is shown how anvil orientations, as viewed from satellite, may be used to assist in the identification of thunderstorms with differing motions and how this applies to splitting supercells. Additionally, the movement of the left mover into the forward flank of the right mover may have temporarily affected its tornadic circulation, as tornadoes occurred both before and after the merger, despite the structure of the right mover being interrupted during the merging process. Given the dearth of literature on thunderstorm mergers in general, and how mergers affect tornadic supercells in particular, this is an area that demands further research.

Wind and Temperature Retrievals in the 17 May 1981 Arcadia, Oklahoma, Supercell: Ensemble Kalman Filter Experiments

The feasibility of using an ensemble Kalman filter (EnKF) to retrieve the wind and temperature fields in an isolated convective storm has been tested by applying the technique to observations of the 17 May 1981 Arcadia, Oklahoma, tornadic supercell. Radial-velocity and reflectivity observations from a single radar were assimilated into a nonhydrostatic, anelastic numerical model initialized with an idealized (horizontally homogeneous) base state. The assimilation results were compared to observations from another Doppler radar, the results of dual-Doppler wind syntheses, and in situ measurements from an instrumented tower. Observation errors make it more difficult to assess EnKF performance than in previous storm-scale EnKF experiments that employed synthetic observations and a perfect model; nevertheless, the comparisons in this case indicate that the locations of the main updraft and mesocyclone in the Arcadia storm were determined rather accurately, especially at midlevels. The magnitudes of vertical velocity and vertical vorticity in these features are similar to those in the dual-Doppler analyses, except that the low-level updraft is stronger in the EnKF analyses than in the dual-Doppler analyses. Several assimilation-scheme parameters are adjustable, including the method of initializing the ensemble, the inflation factor applied to perturbations, the magnitude of the assumed observation-error variance, and the degree of localization of the filter. In the Arcadia storm experiments, in which observations of a mature storm were assimilated over a relatively short (47 min) period, the results depended most on the ensemble-initialization method. In the data assimilation experiments, too much northerly storm-relative outflow along the south side of the low-level cold pool eventually developed during the assimilation period. Assimilation of Doppler observations did little to correct temperature errors near the surface in the cold pool. Both observational limitations (poor spatial resolution in the radar data near the ground) and model errors (coarse resolution and uncertainties in the parameterizations of moist processes) probably contributed to poor low-level temperature analyses in these experiments.

Doppler Radar and Lightning Network Observations of a Severe Outbreak of Tropical Cyclone Tornadoes

Data from a single WSR-88D Doppler radar and the National Lightning Detection Network are used to examine in detail the characteristics of the convective storms that produced a severe tornado outbreak within Tropical Storm Beryl's remnants on 16 August 1994. Comparison of the radar data with reports of tornadoes suggests that only 13 cells produced the 29 tornadoes that were documented in Georgia and the Carolinas on that date. Six of these cells spawned multiple tornadoes, and the radar data confirm the presence of miniature supercells. One of the cells was identifiable on radar for 11 hours, spawning tornadoes over a time period spanning approximately 6.5 hours. Several other tornadic cells also exhibited great longevity, with cell lifetimes greater than ever previously documented in a landfalling tropical cyclone tornado event, and comparable to those found in major midlatitude tornadic supercell outbreaks. Time-height analyses of the three strongest tornadic supercells are presented in order to document storm kinematic structure and to show how these storms appear at different ranges from a WSR-88D radar. In addition, cloud-to-ground (CG) lightning data are examined for the outbreak, the most intense tropical cyclone tornado event studied thus far. Although the tornadic cells were responsible for most of Beryl's CG lightning, flash rates were only weak to moderate, even in the most intense supercells, and in all the tornadic storms the lightning flashes were almost entirely negative in polarity. A few of the single-tornado storms produced no detectable CG lightning at all. In the stronger cells, there is some evidence that CG lightning rates decreased during tornadogenesis, as has been documented before in some midlatitude tornadic storms. A number of the storms spawned tornadoes just after producing their final CG lightning flashes. Surprisingly, both peak currents and positive flash percentages were larger in Beryl s nontornadic storms than in the tornadic ones. Despite some intriguing patterns, the CG lightning behavior in this outbreak remains mostly inconsistent and ambiguous, and offers only secondary value for warning guidance. The present findings argue in favor of the implementation of observing systems capable of continuous monitoring of total lightning activity in storms.

A Variational Technique for Dealiasing Doppler Radial Velocity Data

Velocity folding, or aliasing, is one of most significant impediments to the use of radial winds from Doppler weather radar. In this note, a variational algorithm is developed in which dealiasing is performed using wind gradient information. The key to the proposed method is that, by operating on gradients of velocity rather than on the velocity itself, aliasing ambiguities are readily identified and eliminated. The viability of the method is demonstrated by applying it to Weather Surveillance Radar-1988 Doppler (WSR-88D) observations from a winter-weather event and a tornadic supercell storm.

The 3 November Tornadic Event during Sydney 2000: Storm Evolution and the Role of Low-Level Boundaries

Several severe thunderstorms, including a tornadic supercell, developed on the afternoon of 3 November 2000, during the Sydney 2000 Forecast Demonstration Project. Severe weather included three tornadoes, damaging wind gusts, hail to 7-cm diameter, and heavy rain causing flash flooding. A unique dataset was collected including data from two Doppler radars, a surface mesonet, enhanced upper-air profiling, storm photography, and a storm damage survey. Synoptic-scale forcing was weak and mesoscale factors were central to the development of severe weather. In particular, low-level boundaries such as gust fronts and the sea-breeze front played critical roles in the initiation and enhancement of storms, the motion of storms, and the generation of rotation at low levels. The complex and often subtle boundary interactions that led to the development of the tornadic supercell in this case highlight the need for advanced detection and prediction tools to improve the warning capacity for such events.

Characteristics of Vertical Wind Profiles near Supercells Obtained from the Rapid Update Cycle

Abstract Over 400 vertical wind profiles in close proximity to nontornadic and tornadic supercell thunderstorms are examined. The profiles were obtained from the Rapid Update Cycle (RUC) model/analysis system. Ground-relative wind speeds throughout the lower and middle troposphere are larger, on average, in tornadic supercell environments than in nontornadic supercell environments. The average vertical profiles of storm-relative wind speed, vertical wind shear, hodograph curvature, crosswise and streamwise vorticity, and storm-relative helicity are generally similar above 1 km in the tornadic and nontornadic supercell environments, with differences that are either not statistically significant or not what most would regard as meteorologically significant. On the other hand, considerable differences are found in these average vertical profiles within 1 km of the ground, with environments associated with significantly tornadic supercells (those producing tornadoes of at least F2 intensity) having substantia...

Close Proximity Soundings within Supercell Environments Obtained from the Rapid Update Cycle

A sample of 413 soundings in close proximity to tornadic and nontornadic supercells is examined. The soundings were obtained from hourly analyses generated by the 40-km Rapid Update Cycle-2 (RUC-2) analysis and forecast system. A comparison of 149 observed soundings and collocated RUC-2 soundings in regional supercell environments reveals that the RUC-2 model analyses were reasonably accurate through much of the troposphere. The largest error tendencies were in temperatures and mixing ratios near the surface, primarily in 1-h forecast soundings immediately prior to the standard rawinsonde launches around 1200 and 0000 UTC. Overall, the RUC-2 analysis soundings appear to be a reasonable proxy for observed soundings in supercell environments. Thermodynamic and vertical wind shear parameters derived from RUC-2 proximity soundings are evaluated for the following supercell and storm subsets: significantly tornadic supercells (54 soundings), weakly tornadic supercells (144 soundings), nontornadic supercells (215 soundings), and discrete nonsupercell storms (75 soundings). Findings presented herein are then compared to results from previous and ongoing proximity soundings studies. Most significantly, proximity soundings presented here reinforce the findings of previous studies in that vertical shear and moisture within 1 km of the ground can discriminate between nontornadic supercells and supercells producing tornadoes with F2 or greater damage. Parameters that combine measures of buoyancy, vertical shear, and low-level moisture show the strongest ability to discriminate between supercell classes.

Evolution of Cloud-to-Ground Lightning and Storm Structure in the Spencer, South Dakota, Tornadic Supercell of 30 May 1998

Abstract On 30 May 1998, a tornado devastated the town of Spencer, South Dakota. The Spencer tornado (rated F4 on the Fujita tornado intensity scale) was the third and most intense of five tornadoes produced by a single supercell storm during an approximate 1-h period. The supercell produced over 76% positive cloud-to-ground (CG) lightning and a peak positive CG flash rate of 18 flashes min−1 (5-min average) during a 2-h period surrounding the tornado damage. Earlier studies have reported anomalous positive CG lightning activity in some supercell storms producing violent tornadoes. However, what makes the CG lightning activity in this tornadic storm unique is the magnitude and timing of the positive ground flashes relative to the F4 tornado. In previous studies, supercells dominated by positive CG lightning produced their most violent tornado after they attained their maximum positive ground flash rate, whenever the rate exceeded 1.5 flashes min−1. Further, tornadogenesis often occurred during a lull in CG lightning activity and sometimes during a reversal from positive to negative polarity. Contrary to these findings, the positive CG lightning flash rate and percentage of positive CG lightning in the Spencer supercell increased dramatically while the storm was producing F4 damage on Spencer. These results have important implications for the use of CG lightning in the nowcasting of tornadoes and for the understanding of cloud electrification in these unique storms. In order to further explore these issues, the authors present detailed analyses of storm evolution and structure using Sioux Falls, South Dakota, (KFSD) Weather Surveillance Radar-1988 Doppler (WSR-88D) radar reflectivity and Doppler velocity and National Lightning Detection Network (NLDN) CG lightning data. The analyses suggest that a merger between the Spencer supercell and a squall line on its rear flank may have provided the impetus for both the F4 tornadic damage and the dramatic increase in positive CG lightning during the tornado, possibly explaining the difference in timing compared to past studies.