Significant Tornado Research Articles

Impacts of Sampling and Storm-Motion Estimates on RUC/RAP-Based Discriminations of Nontornadic and Tornadic Supercell Environments

Abstract This study explores reasons for differences in discriminations of nontornadic and tornadic supercell environments between a recent study of field project (FP) radiosonde observations and RUC/RAP-based studies. Two differences are explored: 1) differences in the relative skill between near-ground and deeper-layer storm-relative helicity (SRH) and 2) differences in skill for storm-relative winds (SRWs) seen in observed soundings that are not seen in RUC/RAP-based analyses. Results show that RUC/RAP-derived near-ground SRH continues to show larger skill than deeper-layer SRH for springtime, afternoon/evening cases over the plains (the “FP” domain), although 0–1-km SRH becomes more skillful than 0–500-m SRH. The skill of kinematic variables decreases over the FP domain, as the skill of mixed-layer convective available potential energy (MLCAPE) and the percent of the low-level horizontal vorticity that is streamwise increase for significant tornadoes. Large skill is found for mean ground-relative winds (GRWs) over all layers tested, but the skill of SRWs using Bunkers motion is relatively small. The field project dataset is shown to be biased toward particularly high-end nontornadic supercells, with more tornado-favorable mixed-layer lifted condensation levels (MLLCLs), lapse rates, and low-level shear/SRH compared to the nontornadic cases in the RUC/RAP dataset over the FP domain. The skill of deeper-layer SRH, GRWs, SRWs, and MLCAPE is unusually large in the field project sample, which highlights variables that may increase the likelihood of tornadoes when other variables that relate to the supercell tornado production (low-level shear/SRH and MLLCLs) are already in a tornado-favorable range. The skill of deeper-layer kinematic variables is particularly evident when observed storm motions are used instead of Bunkers motion. Significance Statement Researchers continue to explore reasons why some storms produce tornadoes and others do not. Some recent studies show conflicting results, including the best layers to analyze the environmental wind shear and the ability of wind speeds relative to the storm to distinguish nontornadic from tornadic storms. One study is rooted in observations taken for field projects that targeted tornadic storms and the others in model analyses. Restricting model analyses to a spatiotemporal region that emulates field projects does not explain the disagreement but reveals how the skill of some other variables changes. The tendency for field projects to target particularly strong storms and the use of actual storm motions versus estimates of storm motions both contribute to the conflicting results.

Environmental Conditions Associated with Long-Track Tornadoes

Abstract Reanalysis proximity vertical profile attributes associated with long-track tornadoes [LTTs; pathlength ≥48 km (30 mi)] and short-track tornadoes [STTs; pathlengths <48 km (30 mi)] for a total of 48 212 tornadoes with pathlengths ≥0.16 km (0.1 mi) from 1979–2022 in the United States were examined. Both longer- and shorter-track tornadoes were associated with vast ranges of mixed-layer convective available potential energy, together with relatively low mixed-layer lifted condensation level heights and minimal convective inhibition. A large range of 500–9000-m wind speeds and bulk wind differences, 500–3000-m streamwise vorticities, storm-relative helicities, and storm-relative wind speeds were found for STTs. In stark contrast, LTTs only occurred when these kinematic attributes were larger in amplitude through the troposphere, supporting previously documented associations between observed longer-track tornado pathlengths and faster-propagating parent storms. A novel parameter, heavily weighted by kinematic parameters and lightly weighted by thermodynamic parameters, outperformed the significant tornado parameter in differentiating environments that were more supportive of both LTTs as well as tornadoes rated <EF5. The high correlation values R2 = 0.79 between tornado pathlength and Bunkers’ approximate tornado duration (pathlength / VBunkers) calls for improved understanding of mesocyclone periodicities, which impact tornado longevity, to improve tornado pathlength diagnoses and forecasts. Pragmatically, diagnosing LTT environments using vertical profile attributes, perhaps, is not so much a problem of determining when there might be higher expectations for LTTs, but rather a problem of when there might be lower expectations for LTTs, e.g., weaker kinematic attributes in the lower troposphere.

Cell Mergers, Boundary Interactions, and Convective Systems in Cases of Significant Tornadoes and Hail

Abstract Though discrete supercells are usually emphasized in severe weather forecasting, hazard production is often preceded by their interaction with external features. Past studies have examined the impacts of cell mergers, boundaries, other supercells, convective systems, etc., but usually in isolation. Here, we investigate 230 significant tornadoes, 246 significant hail events, and 191 null cases across the United States using WSR-88D data. We find that in over 90% of cases, supercells that produced significant hazards were accompanied by external features. These features varied between hazards; for example, hailstorms were more frequently near boundaries than tornadic storms. That said, the positions of these features with respect to the storm (and storm-relative inflow) distinguished between hazard potential and type. For example, tornadic storms were predominantly on the more-unstable side of a boundary, while non-tornadic storms and hailstorms were on the less-unstable side. Similarly, tornadic storms had more cells in their rear flanks than forward flanks, while hailstorms had more cells in their forward flanks than rear flanks. Although these conditions were observed regardless of the background environment, they were affected by certain variables in the vertical profile, especially in tornadic cases. Namely, when storm-relative inflow was stronger and lifting condensation level (LCL) was lower, tornadic storms were accompanied by more rear-flank cells that were closer to the storm, more directly opposite the storm-relative inflow, for a longer period of time. We propose that these interactions likely modulate hazard potential, in ways that are not accounted for in traditional environmental parameter-based forecasting.

Climatological Aspects of Notable Tornado Events in Chile

Abstract Tornadoes in Chile seem to develop in what are called “high-shear, low-CAPE” (HSLC) environments. An analysis of convective parameters from the ERA5 reanalysis during 16 notable tornadoes in Chile showed that several increased markedly before the time of the reports. The significant tornado parameter (STP) was able to discriminate the timing and location of the tornadoes, even though it was not created with that goal. We established thresholds for the severe hazards in environments with reduced buoyancy (SHERBE) parameter (≥1) and the STP (≤−0.3) to further identify days favorable for tornado activity in Chile. The SHERBE and STP parameters were then used to conduct a climatological analysis from 1959 to 2021 of the seasonal, interannual, and latitudinal variations of the environments that might favor tornadoes. Both parameters were found to have a strong annual cycle. The largest magnitudes of STP were found to be generally confined to south-central Chile, in agreement with the (sparse) tornado record. The probability of a day with both SHERBE and STP values beyond their thresholds was greatest between May and August, which aligns with the months with the most tornado reports. The number of days with both SHERBE and STP beyond their respective thresholds was found to fluctuate interannually. This result warrants further study given the known interannual variability of synoptic and mesoscale weather in Chile. The results of this study extend our understanding of tornado environments in Chile and provide insight into their spatiotemporal variability.

The Tornado Archive: Compiling and Visualizing a Worldwide, Digitized Tornado Database

Abstract Given inconsistencies in reporting methods and general lack of documentation, the creation of a unified tornado database across the world has been an elusive target for severe weather climatology purposes and historical interest. Previous online tornado documentation has also often been inconsistent or is now defunct. Many individual countries or continents maintain tornado information through either government-sponsored or independent organizations. The Tornado Archive was developed to create a first-of-its-kind digitized synthesis of worldwide tornado documentation, using the most complete sources of information available for regions known to be tornadically active. Spatial and temporal trends in tornado occurrence and reporting can be visualized through an interactive user interface with a variety of filtering methods and environmental reanalysis datasets, such as the fifth major global reanalysis produced by ECMWF (ERA5). The additional data introduced using Thomas Grazulis’ Significant Tornadoes may be beneficial for tornado climatology studies over the United States. The Tornado Archive is also intended to be a collaborative exercise, with clear data attribution and open avenues for augmentation, and the creation of a common data model to store the tornado information will assist in maintaining and updating the database. In this work, we document the methods necessary for creating the Tornado Archive database, provide broader climatological analysis of spatiotemporal patterns in tornado occurrence, and outline potential use cases for the data. We also highlight its key limitations and emphasize the need for further international standardization of tornado documentation.

ERA5‐Based Significant Tornado Environments in Canada Between 1980 and 2020

AbstractThis study uses ERA5 close‐proximity soundings and associated convective parameters to characterize significant tornadic storm (F/EF2+) environments between 1980 and 2020 in parts of Canada. It is shown that ERA5 convective parameters are suitable to represent observed parameters, based on radiosonde comparisons. Results indicate that the eastern Canadian Prairies have nearly double the lifted condensation level with higher level of free convection compared to eastern Canada (southern Ontario/Quebec). Eastern Canada has more a humid boundary layer and free troposphere that can lead to warmer cold pools, favoring tornadogenesis. Central Canada (Manitoba) has the largest mixed‐layer (ML) convective available potential energy (CAPE) mainly due to a combination of regional differences in low level moisture and steeper mid‐level lapse rates in western Canada. Central continental U.S. and Canadian regions appear to have the highest (most negative) convective inhibition, leading to more explosive initiation. Mean bulk wind shear and storm relative helicity (SRH) increases from west to east, with eastern regions being significantly larger. The supercell composite and significant tornado parameters are generally less than U.S. magnitudes, particularly in western Canada, and would require recalibration for more practical use in Canada. Overall, western Canada significant tornadic storms are associated with more low‐level looping hodographs and dominated by thermodynamic influences compared to larger wind influences in eastern regions. This is likely due to more spring, late summer, and autumn events that typically have well‐developed synoptic systems (stronger wind shear) with overall less buoyant energy in eastern regions.

A Comparison between the Only Two Documented Tornado Outbreak Events in China: Tropical Cyclone versus Extratropical Cyclone Environments

Abstract This study documents the features of tornadoes, their parent storms, and the environments of the only two documented tornado outbreak events in China. The two events were associated with Tropical Cyclone (TC) Yagi on 12 August 2018 with 11 tornadoes and with an extratropical cyclone (EC) on 11 July 2021 (EC 711) with 13 tornadoes. Most tornadoes in TC Yagi were spawned from discrete minisupercells, while a majority of tornadoes in EC 711 were produced from supercells imbedded in QLCSs or cloud clusters. In both events, the high-tornado-density area was better collocated with the K index rather than MLCAPE, and with entraining rather than non-entraining parameters possibly due to their sensitivity to midlevel moisture. EC 711 had a larger displacement between maximum entraining CAPE and vertical wind shear than TC Yagi, with the maximum entraining CAPE better collocated with the high-tornado-density area than vertical wind shear. Relative to TC Yagi, EC 711 had stronger entraining CAPE, 0–1-km storm relative helicity, 0–6-km vertical wind shear, and composite parameters such as an entraining significant tornado parameter, which caused its generally stronger tornado vortex signatures (TVSs) and mesocyclones with a larger diameter and longer life span. No significant differences were found in the composite parameter of these two events from U.S. statistics. Although obvious dry air intrusions were observed in both events, no apparent impact was observed on the potential of tornado outbreak in EC 711. In TC Yagi, however, the dry air intrusion may have helped tornado outbreak due to cloudiness erosion and thus the increase in surface temperature and low-level lapse rate.

Tornadoes in Southeast South America: Mesoscale to Planetary-Scale Environments

Abstract A multiscale analysis of the environment supporting tornadoes in southeast South America (SESA) was conducted based on a self-constructed database of 74 reports. Composites of environmental and convective parameters from ERA5 were generated relative to tornado events. The distribution of the reported tornadoes maximizes over the Argentine plains, while events are rare close to the Andes and south of Sierras de Córdoba. Events are relatively common in all seasons except in winter. Proximity environment evolution shows enhanced instability, deep-layer vertical wind shear, storm-relative helicity, reduced convective inhibition, and a lowered lifting condensation level before or during the development of tornadic storms in SESA. No consistent signal in low-level wind shear is seen during tornado occurrence. However, a curved hodograph with counterclockwise rotation is present. The Significant Tornado Parameter (STP) is also maximized prior to tornadogenesis, most strongly associated with enhanced CAPE. Differences in the convective environment between tornadoes in SESA and the U.S. Great Plains are discussed. On the synoptic scale, tornado events are associated with a strong anomalous trough crossing the southern Andes that triggers lee cyclogenesis, subsequently enhancing the South American low-level jet (SALLJ) that increases moisture advection to support deep convection. This synoptic trough also enhances vertical shear that, along with enhanced instability, sustains organized convection capable of producing tornadic storms. At planetary scales, the tornadic environment is modulated by Rossby wave trains that appear to be forced by convection near northern Australia. Madden–Julian oscillation phase 3 preferentially occurs 1–2 weeks ahead of tornado occurrence. Significance Statement The main goal of this study is to describe what atmospheric conditions (from local to global scales) are present prior to and during tornadic storms impacting southeast South America (SESA). Increasing potential for deep convection, wind shear, and potential for rotating updrafts, as well as reducing convective inhibition and cloud-base height, are predominant a few hours before and during the events in connection to low-level northerly winds enhancing moisture transport to the region. Remote convective activity near northern Australia appears to influence large-scale atmospheric circulation that subsequently triggers convective storms supporting tornadogenesis 1–2 weeks later in SESA. Our findings highlight the importance of accounting for atmospheric processes occurring at different scales to understand and predict tornado occurrences.

Evaluation of tornadic environments and their trends and projected changes in Japan

Tornadoes are responsible for several high-impact weather disasters in Japan. However, little is known about how these events have changed over the last several decades or how they may change in future climates. This study examines environmental conditions associated with tornados in Japan using pseudo-soundings from the high-resolution fifth-generation ECMWF reanalysis. We first determine appropriate discriminators of F2+ tornadoes using thermodynamic (convective available potential energy, convective inhibition, lifting condensation level, and the K-index), kinematic (bulk wind difference and storm-relative helicity), and multivariate tornado parameters (energy helicity index, K-helicity index, and the significant tornado parameter), and confirm that F2+ tornadoes occur in environments with higher instability and helicity, but are better distinguished using multivariate parameters. Recent trends indicate that F2+ environments have increased significantly in some regions over the last four decades. We also examined future changes for each parameter using a large ensemble 2-K warming experiment. Robust increases in strong tornado environments are depicted in many regions in Japan, particularly on the Sea of Japan side and the Kanto region. This indicates that despite projected decreases in bulk wind difference and higher convective inhibition, significant increases in atmospheric instability compensate, leading to more days with F2+ tornado potential.

Environmental and Radar-Derived Predictors of Tornado Intensity within Ongoing Convective Storms

Analyses of Doppler radar data and environmental parameters for 300 tornado cases are used to propose an alternative framework for tornado intensity prediction during pretornadic stages of ongoing storms, conditional on tornadogenesis. This framework is founded on the robust relationship (R² = 0.69) between pretornadic mesocyclone width and the EF rating of the subsequent tornado. In contrast, the linear relationship between pretornadic mesocyclone intensity and EF scale is much weaker (R² = 0.29). Environmental information for each case was additionally used to explore relationships between environmental parameters and tornado intensity. Such relationships depend in part on how the tornado-intensity categories are distributed [i.e., nonsignificant (EF0–1) versus significant (EF2–5), or weak (EF0–1) versus strong (EF2–3) versus violent (EF4–5)]. Low-level shear parameters discriminate the environments of significant tornadoes from nonsignificant tornadoes, but not the environments of violent tornadoes from strong tornadoes. The converse is true for thermodynamic parameters. Operational implementation of this framework for thepurposes of impact-based warnings will require real-time, automated quantification of mesocyclone width in addition to intensity and other attributes. The information gained from the pretornadic analysis demonstrated in this study would allow an operational forecaster to be aware of—and communicate—information about potential tornado intensity in warning text to the public before a tornado develops to better protect life and property. Currently, these relationships are being utilized in machine learning models for binary prediction of non-significant versus significant tornado intensity where skill is being demonstrated.

Long term temporal trends in synoptic-scale weather conditions favoring significant tornado occurrence over the central United States.

We perform a statistical climatological study of the synoptic- to meso-scale weather conditions favoring significant tornado occurrence to empirically investigate the existence of long term temporal trends. To identify environments that favor tornadoes, we apply an empirical orthogonal function (EOF) analysis to temperature, relative humidity, and winds from the Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2) dataset. We consider MERRA-2 data and tornado data from 1980 to 2017 over four adjacent study regions that span the Central, Midwestern, and Southeastern United States. To identify which EOFs are related to significant tornado occurrence, we fit two separate groups of logistic regression models. The first group (LEOF models) estimates the probability of occurrence of a significant tornado day (EF2-EF5) within each region. The second group (IEOF models) classifies the intensity of tornadic days either as strong (EF3-EF5) or weak (EF1-EF2). When compared to approaches using proxies such as convective available potential energy, our EOF approach is advantageous for two main reasons: first, the EOF approach allows for the discovery of important synoptic- to mesoscale variables previously not considered in the tornado science literature; second, proxy-based analyses may not capture important aspects of three-dimensional atmospheric conditions represented by the EOFs. Indeed, one of our main novel findings is the importance of a stratospheric forcing mode on occurrence of significant tornadoes. Other important novel findings are the existence of long-term temporal trends in the stratospheric forcing mode, in a dry line mode, and in an ageostrophic circulation mode related to the jet stream configuration. A relative risk analysis also indicates that changes in stratospheric forcings are partially or completely offsetting increased tornado risk associated with the dry line mode, except in the eastern Midwest region where tornado risk is increasing.

Climatology of Significant Tornadoes within China and Comparison of Tornado Environments between the United States and China

Abstract A climatology of significant tornadoes [SIGTOR, tornadoes rated (E)F2+ on the (enhanced) Fujita scale] within China and in three subregions, including northern, central, and southern China, is first presented for the period 1980–2016. In total, 129 SIGTOR are recorded in China, with an average of 3.5 per year. The tornado inflow environments of the south-central and southeast regions of the United States (USC and USSE) are compared with those of China and its subregions based on sounding-derived parameters including shear, storm-relative helicity, convective available potential energy (CAPE), lifting condensation level (LCL), etc. Soundings are extracted from the ERA5 reanalysis dataset. The results confirm that the SIGTOR in USSE are characterized by high shear, low CAPE, and low LCL whereas those in USC are characterized by moderate-to-high shear, high CAPE, and high LCL. The thermodynamic conditions of tornadic cases are favorable for China, with moderate-to-high CAPE and low-to-moderate LCL, but their kinematic conditions are much less favorable than in the United States, a fact that is believed to be primarily responsible for the lower tornado frequency and intensity in China. The high CAPE in China is due mostly to high humidity. For three subregions in China, the central China cases account for 60% of total samples, and its environmental features are similar to those of China. The average shear with northern China cases is stronger than that with the other two subregions, and the midlayer is relatively dry. The southern China SIGTOR have the most conducive humidity conditions, but the CAPE and shear there are the lowest. The northern, central, and southern China environments can be considered as representative of midlatitude, subtropical, and tropical regions. Significance Statement We document the climatological characteristics of significant tornadoes (SIGTOR) within China and compare the inflow environments of SIGTOR in China and its subregions with those in the U.S. central and southeastern regions. The availability of hourly high-resolution ERA5 data makes the environments based on extracted proximity soundings much more accurate than possible with earlier reanalyses. The environmental characteristics show systematic differences in the tornado environments of different regions of China and the United States and suggest different roles played by thermodynamic and kinematic conditions for tornado formation. Overall, the environmental differences are consistent with the resulting frequency and intensities of SIGTOR. The findings are helpful toward improving tornado forecasting and warning or even understanding of potential impacts of climate change on SIGTOR, especially in China, where such studies are rarer.

The Future of Supercells in the United States

Abstract A supercell is a distinct type of intense, long-lived thunderstorm that is defined by its quasi-steady, rotating updraft. Supercells are responsible for most damaging hail and deadly tornadoes, causing billions of dollars in losses and hundreds of casualties annually. This research uses high-resolution, convection-permitting climate simulations across 15-yr epochs that span the twenty-first century to assess how supercells may change across the United States. Specifically, the study explores how late-twentieth-century supercell populations compare with their late-twenty-first-century counterparts for two—intermediate and pessimistic—anthropogenic climate change trajectories. An algorithm identifies, segments, and tracks supercells in the simulation output using updraft helicity, which measures the magnitude of corkscrew flow through a storm’s updraft and is a common proxy for supercells. Results reveal that supercells will be more frequent and intense in future climates, with robust spatiotemporal shifts in their populations. Supercells are projected to become more numerous in regions of the eastern United States, while decreasing in frequency in portions of the Great Plains. Supercell risk is expected to escalate outside of the traditional severe storm season, with supercells and their perils likely to increase in late winter and early spring months under both emissions scenarios. Conversely, the latter part of the severe storm season may be curtailed, with supercells expected to decrease midsummer through early fall. These results suggest the potential for more significant tornadoes, hail, and extreme rainfall that, when combined with an increasingly vulnerable society, may produce disastrous consequences.

Analysis of significant tornado events in Central Europe: synoptic situation and convective development

Analysis of significant tornado events in Central Europe: synoptic situation and convective development

Observed Characteristics of the Tornadic Supercells of 27–28 April 2011 in the Southeast United States

Abstract An 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 life cycle. 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 pre-tornadic (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. Significance Statement This study documents the prolific supercell tornado outbreak that occurred in the southeastern United States on 27–28 April 2011. We associate tornado families with their parent supercells and use a radar-derived database to quantify changes in mesocyclone strength. We show that a variety of supercell evolutionary paths occurred during the event that were somewhat distinct based on where and when each supercell initiated. We also find significant differences between supercell intensity, characterized using azimuthal shear as a measure of mesocyclone strength, during nontornadic periods as opposed to the 30-min window prior to tornadogenesis. These findings are relevant for both researchers and operational forecasters and motivate future work to better understand relationships and processes influencing supercells and their background environments.

Reconstruction of Violent Tornado Environments in Europe: High‐Resolution Dynamical Downscaling of ERA5

AbstractApproximately 1–3 violent tornadoes hit Europe each decade. We use the ERA5 reanalysis and WRF model to reconstruct environments for 12 cases between 1957 and 2021. Violent tornadoes in Europe occur in a variety of synoptic and mesoscale patterns, but they share environmental similarities with significant tornadoes from the United States. Downscaling simulations to 3 km grid‐spacing showed the added value of improved resolution in representing local convective environments. In 8 out of 12 simulations, the model indicated updraft helicity (UH) tracks in a favorable convective environment in spatial (+/−50 km) and temporal (+/−3 hr) proximity to the tornado report. Tornadoes were accompanied by a mean 0–6 km wind shear of 24.3 m s−1 and mean convective available potential energy of 1678 J kg−1. The combination of UH tracks with convective environments offers promising results for operational forecasting in Europe, and should be explored in future studies.

Short-Term Prediction of a Nocturnal Significant Tornado Outbreak Using a Convection-Allowing Ensemble

Abstract A multiscale analysis of the significant nocturnal tornado outbreak in Tennessee on 2–3 March 2020 is presented. This outbreak included several significant tornadoes and resulted in the second most fatalities (25) and most injuries (309) of all nocturnal tornado events in Tennessee in 1950–2020. The two deadliest tornadoes struck Nashville (EF3 intensity) and Cookeville (EF4) resulting in 5 and 19 fatalities, respectively. The supercell responsible for the tornado outbreak initiated at 0330 UTC 3 March within a region of warm frontogenesis in western Tennessee. Throughout its life cycle, the supercell was located in a region of convective available potential energy near 1000 J kg−1 and 0–1-km storm-relative helicity over 350 m2 s−2. Retrospective 3-h forecasts from the experimental Warn-on-Forecast System (WoFS) convection-allowing ensemble initialized after the parent supercell initiated indicated a high probability, high severity scenario for tornadoes across Tennessee and into Nashville through 0700 UTC. Earlier WoFS forecasts indicated a low probability, high severity scenario owing to uncertainty in the initiation of supercells. The presence of these supercells was sensitive to the upstream thermodynamic conditions and warm frontogenesis regions that were inherited from the lateral boundary conditions. In all, this study highlights the potential of the WoFS ensemble to contribute useful probabilistic severe weather information to the short-term forecast process during a nocturnal significant tornado outbreak.

Storm-Scale Predictability and Analysis of the 13 April 2020 Central Savannah River Area Tornado Outbreak

Abstract An early morning tornado outbreak occurred on 13 April 2020 in the Central Savannah River Area. Multiple significant tornadoes were reported, resulting in fatalities and injuries. While the operational tornado warnings had positive lead times, the convective mode (quasi-linear convective system) increased the warning decision complexity. The timing of the event [0500–0600 local time (LT)] also made NWS-to-public communication difficult. The experimental NSSL Warn-on-Forecast System (WoFS) was run retrospectively for this case. The WoFS consists of 3–6-h ensemble forecasts initialized every 30 min, and the goals of the system are to bridge the gap between severe weather watches and warnings and to increase warning lead times. Multiple WoFS forecasts were initialized leading up to the first tornado report; those initialized prior to tornado warning issuance have high ensemble probabilities of low-level rotation in the appropriate areas based on subsequent tornado reports. This case highlights another example of the usefulness of WoFS before its eventual transition to operations. Using the WoFS forecasts, kinematic and thermodynamic storm–environment relationships are analyzed using ensemble sensitivity analysis (ESA). The analyses suggest variations in the mesoscale environmental vertical wind profile are not as influential on mesovortex intensity as variations in the thermodynamic environment. Surface observations recorded prior to the tornado outbreak reveal subtle temperature and moisture gradients that may be the impetus for mesovortex intensification and tornadogenesis.

The Effects of Tornadoes on Housing Prices in Moore, Oklahoma

This paper examines the effects of four significant tornadoes on housing prices in Moore, Oklahoma. We use a hedonic difference-in-difference approach by considering transactions made preceding and following major tornadic events in Moore (occurring in 1999, 2003, 2010, and 2013). Our dataset spans nearly 30,000 housing transactions between 1990 and 2020. The length of tornado impacts is evaluated using a set of time indicators for the years leading up to (and after) the tornadoes. We find a 2-5% decrease in housing prices during the first year after a tornado for houses in the destructive path. However, no such impacts exist in years 2-7. We also employ three different specifications (OLS, spatial lag, and spatial error) to find the most appropriate model for considering the potential spatial processes at work. The results are largely similar across specifications.

Are Significant Tornadoes Occurring Later in the Year in Southern Ontario?

AbstractUsing a long‐term data set (1875–2019), we investigate whether the month having peak tornado frequency changes over time in southern Ontario, Canada. Using only F/EF2+ tornadoes and 10‐year rolling periods having modes at 5‐year intervals, we find a statistically significant (99% confidence interval) moderate positive trend. However, we identify no statistically significant trend (positive or negative) using the same methods with tornado data from neighboring US states Michigan and New York. Though we do not investigate causes here, the study is the first to identify a robust long‐term trend in the frequency of significant tornadoes for any region of Canada. Such a shift also has implications for public safety, with the occurrence of such tornadoes later in the year having the potential for greater impacts.