The Influence of Cell Mergers on Supercell Characteristics and Tornado Evolution on 27–28 April 2011 (Published 2023)
Authors: Anthony W. Lyza and Matthew D. Flournoy
Journal/Volume/Issue: Monthly Weather Review, Early Online Release
Abstract: Numerous questions remain regarding the influence of environmental inhomogeneities on supercell evolution. Motivated by this topic, this study associates cell merger occurrence with supercell evolution and tornado production during the prolific 27–28 April 2011 outbreak in the Southeast U.S. This event included 29 discrete supercells that produced 102 tornadoes and featured 300 cell mergers. Cell-merger frequency increased for supercells that initiated farthest east, possibly owing to changes in overall convective coverage over time. There is some signal for stronger mesocyclones to be associated with more mergers in the primary supercell’s forward flank. There is also a slight tendency for supercells that encounter more cell mergers to produce tornadoes more quickly, especially for those that formed away from a significant zonal boundary. However, there is a slight tendency for supercells spawning the longest-lived tornadoes (especially those with durations over 60 min) to be associated with fewer cell mergers during the 15-min window preceding tornadogenesis. Of particular importance, a significant inverse relationship exists between pre-merger mesocyclone strength and the subsequent change in mesocyclone strength during the merger (i.e., weaker mesocyclones tended to strengthen as a result of the merger, and vice versa). These findings highlight the influence that cell mergers can have on supercell evolution and tornado production—even within an incredibly volatile environment—and motivate future work exploring the physical processes involved and ways to translate these findings into experimental techniques or guidance for operational forecasters.
Observed Characteristics of the Tornadic Supercells of 27-28 April 2011 in the Southeast United States (Published 2022)
Authors: Anthony W. Lyza, Matthew D. Flournoy, and Erik N. Rasmussen
Journal/Volume/Issue: Monthly Weather Review, 150 (11)
Abstract: A historic outbreak of tornadoes impacted a large swath of the eastern United States on 26-28 April 2011. The most severe series of tornadoes was associated with numerous classic supercell thunderstorms that developed across the Southeast during the afternoon and evening of 27 April and continued into the predawn of 28 April. This study documents characteristics of these storms with respect to tornado production and mesocyclone strength during different periods of each storm’s lifecycle. The supercells initiated in four quasi-distinct spatiotemporal regions, with each cluster exhibiting slightly different evolutionary traits and tornado production. These included differences in the mean times between convection initiation and the time of first tornadogenesis for each supercell, as well as variations in overall and significant tornado production. This suggests that mesoscale environmental differences, such as proximity to a mesoscale boundary, and/or storm-scale events strongly influenced the variety of supercell evolutionary paths that were observed during this event, even in the presence of a synoptic-scale background environment extremely favorable for supercell and tornado production. The azimuthal shear products from the Multi-Year Reanalysis of Remotely Sensed Storms database perform well in discriminating between mesocyclones associated with ongoing weak, strong, and violent tornadoes during the event. Furthermore, mean azimuthal shear values during pretornadic (e.g., within 30 min of tornadogenesis) and tornadic phases are significantly larger than those during nontornadic phases. This warrants further study of azimuthal shear characteristics in different environments and its potential usefulness in aiding real-time forecasting efforts.
Damage Analysis and Close-Range Radar Observations of the 13 April 2019 Greenwood Springs, Mississippi, Tornado during VORTEX-SE Meso18-19 (Published July 2022)
Authors: Anthony W. Lyza, Barrett T. Goudeau, and Kevin R. Knupp
Journal/Volume/Issue: Monthly Weather Review, 150 (7)
Abstract: A tornado outbreak occurred across the Southeast United States on 13–14 April 2019, during the Verification of the Origins of Rotation in Tornadoes Experiment–Southeast (VORTEX-SE) Meso18-19 experiment. Among the most noteworthy events was a pair of large tornadoes in Monroe County, Mississippi, near the Columbus Air Force Base (GWX) Weather Surveillance Radar–1988 Doppler (WSR-88D). The second tornado, near the Greenwood Springs community, formed within the “no data” region near the radar and passed about 900 m to its east, rapidly strengthening into an intense tornado. This tornado produced forest devastation and electrical infrastructure damage up to at least EF4 intensity. The maximum radial velocity from GWX was 81.5 m s−1 (182 mph) in a resolution volume centered at 56 m (183 ft) above radar level. This paper presents a damage survey of the Greenwood Springs tornado and compares this assessment to the GWX data. A displacement of the maximum forest damage from the maximum radial velocity, despite the radar beam sampling <100 m ARL, is documented, as well as other likely effects of debris loading by the tornado on the observed radar signatures. The radar observations are placed into context with past mobile radar studies to illustrate the unique nature of this dataset. The relationship between radar data and damage observations, the implications for tornado structure in rough terrain and land cover, and the use of forest damage and radar data in tornado intensity estimation are discussed.
A Climatology of Cell Mergers with Supercells and Their Association with Mesocyclone Evolution (Published February 2022)
Authors: Matthew D. Flournoy, Anthony W. Lyza, Martin A. Satrio, Madeline R. Diedrichsen, Michael C. Coniglio, and Sean Waugh
Journal/Volume/Issue: Monthly Weather Review, 150 (2)
Abstract: In this study, we present a climatology of observed cell mergers along the paths of 342 discrete, right-moving supercells and their association with temporal changes in low-level mesocyclone strength (measured using azimuthal shear). Nearly one-half of the examined supercells experience at least one cell merger. The frequency of cell merger occurrence varies somewhat by geographical region and the time of day. No general relationship exists between cell merger occurrence and temporal changes in low-level azimuthal shear; this corroborates prior studies in showing that the outcome of a merger is probably sensitive to storm-scale and environmental details not captured in this study. Interestingly, we find a significant inverse relationship between premerger azimuthal shear and the subsequent temporal evolution of azimuthal shear. In other words, stronger low-level mesocyclones are more likely to weaken after cell mergers and weaker low-level mesocyclones are more likely to strengthen. We also show that shorter-duration cell merger “events” (comprising multiple individual mergers) are more likely to be associated with a steady or weakening low-level mesocyclone whereas longer-duration cell merger events (3–4 individual mergers) are more likely to be associated with a strengthening low-level mesocyclone. These findings suggest what physical processes may influence the outcome of a merger in different scenarios and that the impact of these processes on low-level mesocyclone strength may change depending on storm maturity. We establish a baseline understanding of the supercell–cell merger climatology and highlight areas for future research in how to better anticipate the outcomes of cell mergers.
doi: https://doi.org/10.1175/MWR-D-21-0204.1
Observed Near-Storm Environment Variations across the Southern Cumberland Plateau System in Northeastern Alabama (Published April 2020)
Authors: Anthony W. Lyza, Todd A. Murphy, Barrett T. Goudeau, Preston T. Pangle, Kevin R. Knupp, and Ryan A. Wade
Journal/Volume/Issue: Monthly Weather Review, 148 (4)
Abstract: The Sand Mountain and Lookout Mountain Plateaus in northeastern Alabama have been established as a regional relative maximum in tornadogenesis reports within the southeastern United States. Investigation of long-term surface datasets has revealed (i) stronger and more backed winds atop Sand Mountain than over the Tennessee Valley, and (ii) measured cloud-base heights are lower to the surface atop Sand Mountain than over the Tennessee Valley. These observations suggest that low-level wind shear and lifting condensation level (LCL) height changes may lead to conditions more favorable for tornadogenesis atop the plateaus than over the Tennessee Valley. However, prior to fall 2016, no intensive observations had been made to further investigate low-level flow or thermodynamic changes in the topography of northeastern Alabama. This paper provides detailed analysis of observations gathered during VORTEX-SE field campaign cases from fall 2016 through spring 2019. These observations indicate that downslope winds form along the northwest edge of Sand Mountain in at least some severe storm environments in northeastern Alabama. Wind profiles gathered across northeastern Alabama indicate that low-level helicity changes can be substantial over small distances across different areas of the topographic system. LCL height changes often scale to changes in land elevation, which can be on the order of 200–300 m across northeastern Alabama.
doi: https://doi.org/10.1175/MWR-D-19-0190.1
A Multi-Platform Reanalysis of the Kankakee Valley Tornado Cluster on 30 June 2014 (Published August 2019)
Authors: Anthony W. Lyza, Richard Castro, Eric Lenning, Matthew T. Friedlein, Brett S. Borchardt, Adam W. Clayton, and Kevin R. Knupp
Journal/Volume/Issue: E-Journal of Severe Storms Meteorology, 14 (3)
Abstract: Two derecho-producing quasi-linear convective systems moved across northeast Illinois and northwest Indiana on the evening of 30 June 2014. Both produced damage across a large area, including a concentrated cluster in the Kankakee River Valley associated with two long-lived and adjacent mesovortices in the second derecho. Post-event surveys of this region revealed a widespread, complex damage pattern and initially documented 14 tornadoes of EF1 rating on the Enhanced Fujita (EF) Scale, along with a swath of damaging nontornadic winds. Later, a number of uncertainties from the original surveys were examined in conjunction with high-resolution satellite imagery not available immediately after the event. Inconsistencies noted during this process led to an intensive reanalysis of the entire cluster using radar data, the original ground and aerial surveys, and the newly available satellite imagery. This new analysis concluded that at least 18 tornadoes impacted the Kankakee Valley, most with a path orientation different from the initial results. This paper details the reassessment process from initial motivation to final results, addresses potential sources of error, and discusses operational considerations stemming from such a prolifically tornadic event.
doi: https://doi.org/10.55599/ejssm.v14i3.73
A Background Investigation of Tornado Activity across the Southern Cumberland Plateau Terrain System of Northeastern Alabama (Published December 2018)
Authors: Anthony W. Lyza and Kevin R. Knupp
Journal/Volume/Issue: Monthly Weather Review, 146 (12)
Abstract: The effects of terrain on tornadoes are poorly understood. Efforts to understand terrain effects on tornadoes have been limited in scope, typically examining a small number of cases with limited observations or idealized numerical simulations. This study evaluates an apparent tornado activity maximum across the Sand Mountain and Lookout Mountain plateaus of northeastern Alabama. These plateaus, separated by the narrow Wills Valley, span ~5000 km2 and were impacted by 79 tornadoes from 1992 to 2016. This area represents a relative regional statistical maximum in tornadogenesis, with a particular tendency for tornadogenesis on the northwestern side of Sand Mountain. This exploratory paper investigates storm behavior and possible physical explanations for this density of tornadogenesis events and tornadoes. Long-term surface observation datasets indicate that surface winds tend to be stronger and more backed atop Sand Mountain than over the adjacent Tennessee Valley, potentially indicative of changes in the low-level wind profile supportive to storm rotation. The surface data additionally indicate potentially lower lifting condensation levels over the plateaus versus the adjacent valleys, an attribute previously shown to be favorable for tornadogenesis. Rapid Update Cycle and Rapid Refresh model output indicate that Froude numbers for the plateaus in tornadic environments are likely supportive of enhanced low-level flow over the plateaus, which further indicates the potential for favorable wind profile changes for tornado production. Examples of tornadic storms rapidly acquiring increased low-level rotation while reaching the plateaus of northeast Alabama are presented. The use of this background to inform the VORTEX-SE 2017 field campaign is discussed.
doi: https://doi.org/10.1175/MWR-D-18-0300.1
A Significant Tornado in a Heterogeneous Environment During VORTEX-SE (Published July 2018)
Authors: Timothy A. Coleman, Anthony W. Lyza, Kevin R. Knupp, Kevin Laws, and Wes Wyatt
Journal/Volume/Issue: E-Journal of Severe Storms Meteorology, 13 (2)
Abstract: On 1 March 2016, an EF2 tornado occurred near Birmingham, AL, and was examined as part of VORTEX-SE. The boundary-layer environment near the tornadic supercell was heterogeneous in space and unsteady in time, with what typically would be considered an excellent proximity sounding. In this case, however, the proximity sounding severely underestimated the CAPE. SPC mesoanalyses substantially underestimated the CAPE and wind shear as well. Tornadogenesis occurred near a weak, frontogenetical thermal boundary, where evaporation from antecedent light showers had also increased dewpoint values. A local maximum in surface dewpoint (and instability), and a local maximum in helicity both existed near the region of frontogenesis. As a QLCS moved into the region of higher CAPE air, part of it became supercellular. Tornadogenesis occurred near the local maximum in surface dewpoint.
doi: https://doi.org/10.55599/ejssm.v13i2.70
Spatio-Temporal November and March Snowfall Trends in the Lake Michigan Region (Published March 2018)
Authors: Craig A. Clark, Bharath Ganesh-Babu, Travis J. Elless, Anthony W. Lyza, Dana M. Koning, Alexander R. Carne, Holly A. Boney, Amanda M. Sink, and Justin M. Barrick
Journal/Volume/Issue: International Journal of Climatology, 38 (3)
Abstract: This study has investigated the spatio-temporal variability of November and March Lake Michigan snowfall for the period 1950–2013. Snowfall characteristics were assessed using time series analysis, geographic information systems, and visualization. Results indicate significant temporal decreases of November and March snowfall, peak inter-annual variability within the lake-effect belt, modest concurrent and lagged sensitivity to teleconnection indices, and strong dependence of snowfall on temperature. The decreased snowfall is in contrast to December–February snowfall and associated with a decreasing fraction of November and March precipitation days occurring as snow days, rather than changes in precipitation frequency. The decreasing fraction of precipitation days occurs as snowfall is consistent with synoptic-scale disturbances producing rain rather than snow and lake-effect rain falling in lieu of snow.
doi: https://doi.org/10.1002/joc.5498
Analysis of Mesovortex Characteristics, Behavior, and Interactions during the Second 30 June-1 July 2014 Midwestern Derecho Event (Published August 2017)
Authors: Anthony W. Lyza, Adam W. Clayton, Kevin R. Knupp, Eric Lenning, Matthew T. Friedlein, Richard L. Castro, and Evan S. Bentley
Journal/Volume/Issue: E-Journal of Severe Storms Meteorology, 12 (2)
Abstract: A pair of intense, derecho-producing quasi-linear convective systems (QLCSs) impacted northern Illinois and northern Indiana during the evening hours of 30 June through the predawn hours of 1 July 2014. The second QLCS trailed the first one by only 250 km and approximately 3 h, yet produced 29 confirmed tornadoes and numerous areas of nontornadic wind damage estimated to be caused by 30‒40 m s‒1 flow. Much of the damage from the second QLCS was associated with a series of 38 mesovortices, with up to 15 mesovortices ongoing simultaneously. Many complex behaviors were documented in the mesovortices, including: a binary (Fujiwhara) interaction, the splitting of a large mesovortex in two followed by prolific tornado production, cyclic mesovortexgenesis in the remains of a large mesovortex, and a satellite interaction of three small mesovortices around a larger parent mesovortex. A detailed radar analysis indicates no definitive differences between tornadic and nontornadic mesovortices. All observed mesovortices were cyclonic, indicating that either the vertical tilting of streamwise vorticity, generation of vortices via the release of horizontal shearing instability, or both were involved in mesovortex genesis. This paper examines the environment ahead of the second QLCS, the characteristics of the mesovortices produced, and the aforementioned complex interactions. It also discusses implications for mesovortex genesis and dynamics as well as operational considerations.
doi: https://doi.org/10.55599/ejssm.v12i2.67
Spatiotemporal Snowfall Variability in the Lake Michigan Region: How is Warming Affecting Wintertime Snowfall? (Published August 2016)
Authors: Craig A. Clark, Travis J. Elless, Anthony W. Lyza, Bharath Ganesh-Babu, Dana M. Koning, Alexander R. Carne, Holly A. Boney, Amanda M. Sink, Sarah K. Mustered, and Justin M. Barrick
Journal/Volume/Issue: Journal of Applied Meteorology and Climatology, 55 (8)
Abstract: This study has investigated the spatiotemporal structure and changes in Lake Michigan snowfall for the period 1950–2013. With data quality caveats acknowledged, a larger envelope of stations was included than in previous studies to explore the data using time series analysis, principal component analysis, and geographic information systems. Results indicate warming in recent decades, a near-dearth of serial correlation, midwinter dependence on teleconnection patterns, strong sensitivity of snowfall to temperature, peak snowfall variability and dependence on temperature within the lake-effect belt, an increasing fraction of seasonal snowfall occurring from December to February, and temporal behavior consistent with the previously reported trend reversal in snowfall.
Dr. Anthony W. "Tony" Lyza 