Some of the most destructive tornadoes throughout history have occurred in what is known as Dixie Alley within the Southeast U.S. Previous studies for Florida defined a tornado event as ≥4 tornadoes within a 24-h period during December–May (avoiding tropical cyclone related events), while a null event was defined as a period when the NOAA Storm Prediction Center had tornado outlook probabilities ≥5% over any part of the respective study area, but <4 tornadoes occurred in 24 h. This study presents a multiscale composite analysis (2002–2019) of 33 Alabama tornado and 65 null events; 46 Mississippi tornado and 92 null events; 24 Louisiana tornado and 98 null events; and 21 Georgia tornado and 32 null events. Like the Florida cases, tornado events were primarily experienced across the northern and central parts of each state. Using archived frontal analyses from the NOAA Weather Prediction Center, tornado events were associated with stationary boundaries more than any other synoptic or mesoscale feature (e.g., cold front; warm front). Tornado events also occurred more frequently during La Niña and a positive Arctic Oscillation (AO) pattern. A composite synoptic analysis showed jet-streak divergence, amplified anomalous midtropospheric troughing, and large positive precipitable water and potential temperature anomalies from Louisiana through Georgia during tornado events. However, Louisiana tornado events featured more 850-hPa southerly winds compared to the classic southwesterly winds seen across its Dixie Alley counterparts.
We identified 147 contiguous United States warm-season (May, June, July, and August) derechos between 1995 and 2024. This results in an average of approximately five derechos each warm season. Using the compiled database, trends in derecho occurrence over the last 30 y were evaluated. The frequency of derecho-producing convection has increased over the past 30 y, most notably during the last 10 y, although the degree to which inconsistencies in storm reporting over the decades and multi-decadal climate variability contribute to this trend is unclear. The number of high-end derechos occurring every year has also increased with time. Spatiotemporal distributions and derecho series are identified, and distributions of severe wind reports across derecho events are presented for the 1995–2024 period. Spatial trends in derecho-path locations emulate the (warm) seasonal shift of mesoscale convective systems and elevated mixed layers across the US, with a poleward shift as the warm season progresses. In addition to the derecho-trend analysis, we also look at associations between annual and monthly derechos with seasonal and sub-seasonal climate-forcing mechanisms. The most notable association is Niño-4 sea-surface temperature anomalies with warming in the central Pacific.
Kosiba and Wurman (KW) investigated the vertical variations of windspeeds observed in tornadoes by mobile Doppler radars over the last 30 y. By comparing windspeeds observed at the lowest altitudes, including some within 15 m of the surface, KW conclude that the most extreme instantaneous winds in tornadoes occur very near the ground. Importantly, KW distinguish between this finding and recent studies using large-eddy simulations (LES) by Nolan et al. (N17) that show maximum azimuthally and temporally averaged windspeeds occurring at 30–70 m AGL. Here we show that, when the N17 model output is processed in a manner similar to KW, the same result is found: the most extreme instantaneous winds occur below 20 m AGL. This result strengthens the claims of KW, since LES produce the same result as their observational analysis.
From the spring of 2016 through the spring of 2019, three EF2-rated tornadoes were sampled with in-situ tornado probes as part of the Pressure Acoustics Recordings Inside Tornadoes Experiment. The probe meteorological instrumentation resolved the temperature, humidity, and pressure-deficit characteristics near or inside all three tornadoes. Probe recorded pressure and visual observations during each tornado event show unique attributes, including multiple strong pressure irregularities before and just after the first EF2 tornado, a rotating bowl-shaped cloud feature located some distance ahead of the second EF2 tornado, independent of the tornado core and immediate inflow layer. Pressure measurements of the bowl feature reveal a brief pressure perturbation lasting roughly 4 s as it passed near or directly over the research team before the documentation of a 41 hPa pressure deficit. During the third EF2 near in-situ deployment, pressure trace observations also reveal pressure fluctuations ahead of the tornado core lasting some 65 s before the documentation of a 13 hPa pressure deficit. These pressure measurements and video observations are consistent in location with recent ultra-high-resolution simulations of the Streamwise Vorticity Current (SVC), or more precisely, the related Vertical Vorticity Sheet (VVS) and Pressure Deficit Lobe (PDL) regions. This paper describes the measured pressure deficit traces during three EF2 tornadoes near and in situ, augmented with brief, one-of-a-kind, near and in-situ video observations of a unique bowl feature, tornado inflow, corner-flow, and tornado core regions. The recorded data and instrumentation were analyzed; results are presented and discussed.
Multiple studies have investigated the occurrence of severe convective winds and have increased our understanding of the forces driving severe winds and their spatial and temporal patterns. Some of the data used in studies have come from airport stations maintained by the National Weather Service. Their standardization across the United States makes them ideal for research, but they are limited in their distribution. This study aims to create a climatology of severe convective winds in West Texas using a mesoscale network (“mesonet”). Like their ASOS (Automated Surface Observing System) counterparts, these stations are standardized and well maintained. This study provides a 15-year climatology of severe convective wind gusts measured by the West Texas Mesonet (WTM). After extracting and manually verifying the measured gusts from over 30 WTM stations, both spatial and temporal distributions are presented. While temporal patterns in the gust distribution generally matched previous research, the high spatial resolution of the mesonet elucidated differences across a regional escarpment known as the Caprock. Comparison with regional AWOS / ASOS stations also revealed potential effects of a larger urban area. In addition to gust data, thermodynamic characteristics and rainfall accumulations associated with each gust were also investigated. In doing so, a substantial contribution from dry thunderstorm outflow winds and heat bursts to the production of regional severe wind was documented.
Two ≥3-in (7.6-cm) hailstorms affected Norman, Oklahoma in 2021: one on 28–29 April and the other on 10–11 October. Wind-driven hail associated with the April storm was estimated to have caused over two million dollars in damage just to county-owned buildings, with estimates of hundreds of millions in damage when including privately owned properties. The October storm caused further damage in approximately the same locations in Norman as the April storm. That these storms affected Norman in a single year was unusual, as only three other prior dates had ≥3-in (7.6-cm) hail reports in Norman from 1955–2021. Radar presentations, surface and upper-air observations, and environmental parameters are presented and intercompared. The April storm was situated behind a surface boundary, and was mostly sub-severe until it rapidly intensified upon approach to the boundary, while the October storm evolved near a surface boundary and had a history of tornadoes. A cursory comparison of model-derived and observed proximity soundings for the two 2021 hailstorms with those for the previous six ≥3-in (7.6-cm) hailstorm dates for Cleveland County (in which Norman is located), from 1955–2021, showed that the two 2021 storms had lower environmental buoyancy, but larger deep shear. This study supports both: 1) the idea that large shear or buoyancy can compensate for lack of the other for the production of significant hail, and 2) the association of long hodographs with large-hail events.
A 40-year climatology (1979–2018) is presented for long-track tornadoes in the United States using the Storm Prediction Center tornado database. Path length (PL) stratification thresholds are defined using characteristics of supercell storm evolution, and are categorized as: all (ALL), PL >0 mi or 0 km; short (S), PL <30 mi or 48.3 km; long (L), [30, 60) mi or [48.3, 96.6) km; very long (V), [60, 90) mi or [96.6, 144.8) km; extremely long (X), PL ≥90 mi or 144.8 km; and long-track sum (LVX), PL ≥30 mi or 48.3 km. Results show LVX tornadoes: a) made up <1% of all tornadoes; b) occurred east of the Rocky Mountains; c) were generally wider; d) caused disproportionate numbers of deaths and injuries; e) typically had damage ratings ≥F/EF2 (peak F/EF3); f) occurred more often with more deaths and injuries in the Southeast than in the Midwest or Great Plains; g) had larger area scale and Destruction Potential Index; h) occurred mostly in April and May versus May and June for S tornadoes; i) occurred primarily from midafternoon to early evening; j) had a peak formation hour at 1600 local solar time (also true for ALL S, V and X), though L tornadoes had a peak and secondary peak at 1800 and 1500, respectively; k) were less common during nighttime than S tornadoes; l) were more frequent during nighttime in the Southeast than nighttime in the Midwest or Great Plains; and m) occurred more often with tornado outbreaks.
An unusual tornado event involving two large and violent EF4 tornadoes occurring simultaneously impacted the Pilger, NE area on 16 June 2014, resulting in two deaths. A privately funded scientific field campaign successfully obtained in-situ video observations inside the western-most tornado prior to it striking the town of Pilger. The results resolve fine details of the tornado core wind field, which are presented and discussed, including several important or unique observations not previously documented within existing in-situ tornado video research. These include documentation of many sub-vortices (as many as nine concurrently at one point) evolving and dissolving on the order of seconds or fractions of seconds while rotating about a concentrical axis. Visual observation of a single but separate vortex that was likely a sub-vortex on the outside rim of the tornado core and observations confirm tornadic damage well outside the visible parent tornado vortex. This study also adds to the small number of tornado cases documented using in-situ observations as a reference for further research.
Previous work examining dual-polarization (dual-pol) radar signatures of supercells has shown that differential reflectivity (ZDR) column area, hailfall area and specific differential phase (KDP)–ZDR separation angles may differ between tornadic and nontornadic storms. However, these signatures often can be difficult to quantify quickly enough to enable their use in forecasting operations, and little work has been done examining how these characteristics vary with environmental parameters in large samples of observed storms. This paper introduces the Supercell Polarimetric Observation Research Kit (SPORK) as an update to our automated ZDR arc-detection algorithm. This update adds the capability to identify ZDR column and inferred hailfall signatures in supercells automatically, and quickly quantify their characteristics. Dual-pol metrics calculated by SPORK are compared to manually calculated dual-pol metrics from previous work. SPORK is run on a large sample of supercells to examine whether SPORK-calculated dual-pol metrics exhibit the same differences between tornadic and nontornadic supercells seen in manual analyses. Storm-mean dual-pol metrics obtained from SPORK also are used to evaluate how supercell dual-pol metrics vary with different environmental parameters. Results from SPORK support previous findings that tornadic supercells have larger ZDR column areas, smaller hailfall areas, and larger KDP–ZDR separation angles than nontornadic storms. Additionally, ZDR columns tend to be larger and deeper in more conditionally unstable environments. Hailfall areal extents are larger in environments with lower environmental 0ºC levels, higher LCLs and LFCs, and less SRH. Separation angles are larger in environments with larger low-level shear vectors, SRH and lower lifted condensation levels (LCLs) and levels of free convection (LFCs). However, none of these correlations exceed r = 0.52. Overall, our results indicate that SPORK can quantify supercell dual-pol signatures accurately enough to detect potentially useful differences between dual-pol signatures of pretornadic and nontornadic supercells, and provide a first look at how dual-pol signatures vary with environmental characteristics in a large sample of supercells.
The tornado that struck Joplin, MO on 22 May 2011 resulted in the first triple-digit death toll from a single tornado in the United States since the 1950s. This paper documents the meteorological setting for this exceptional event, as no published studies have done so yet. Synoptic-scale surface and upper air maps, a brief radar overview, environmental parameters via the SPC mesoanalysis and soundings, and mesoscale surface analysis are used to examine the background setting that led to the deadly tornado. Several other tornado days in the same general area with synoptic patterns similar to the Joplin tornado day also are examined from the standpoint of environmental parameters for comparison. The results show the Joplin tornado case to be an example of very supportive ingredients developing from a favorable evolution of synoptic-scale features; similar patterns have produced several deadly tornadoes in recent years. A couple of mesoscale boundaries also may have contributed to increased tornado potential in the Joplin area. Informal comments published elsewhere have described the environment for this event as “unfavorable” for violent tornadoes. The findings here contradict and refute that characterization, and show that the background environment for the Joplin tornado was actually quite favorable for supporting strong or violent tornadoes.
This study is the first of two papers concerning the dynamics of a heavy snowfall event in the Snake River Plain of eastern Idaho on 26 November 2005. Heavy snowfall occurred along the southern perimeter of the wide (~100 km) valley and in three to four mesoscale bands aligned across the valley and parallel with the post cold-frontal northwesterly flow. This event was driven by two main topographic forcing mechanisms. First, widespread precipitation resulted from a low-level barrier jet (oriented down a tight horizontal pressure gradient) ascending the Snake River Valley and spilling over the southern edge of the valley. Second, the mesoscale snow bands in the valley resulted from boundary layer convergence forced by flow through the upstream barrier, the Idaho Central Mountains, in three narrow valleys aligned with the 700 hPa wind. This case study illustrates how the unique topography of eastern Idaho contributed to near-blizzard conditions in a post-frontal regime.
In an 11-y period (2000–2010) 126 tornadoes affected central Oklahoma within a 111-km (60-nm) radius of the Twin Lakes, Oklahoma (KTLX) WSR-88D. The tornadoes resulted in 265 injuries and 3 deaths. This study used archived WSR-88D data to obtain information about storm characteristics such as mode, width, height, and measures of the mesocyclone, at the time of initial tornado formation. The radar data provided information about the supercell spectrum and highlighted differences between tornado-producing supercells and tornado-producing quasi-linear convective systems (QLCSs), especially with respect to midlevel rotational velocity. Warning lead-time information also was obtained and compared with the radar characteristics. No specific radar attribute was strongly correlated with lead time, likely due to the multitude of variables involved in the tornado warning process. A strong correlation did exist between lead time and storm mode. Applying these findings in an operational environment similar to that found in central Oklahoma may enhance tornado warning performance.
This study analyzes a long-lived thunderstorm with supercell characteristics that took place in the northeastern Iberian Peninsula on 5 July 2012. Severe weather features identified in Doppler radar and total lightning data have been used to infer the severity of this large-hail-bearing storm that substantially damaged local agriculture. Key elements identified in the radar product analysis were: relatively short development time, a long mature phase lasting >2 h, and high and sustained values for most of the radar parameters (reflectivity, vertically integrated liquid, echo tops), which showed an evolution from multicell to supercell structure. Nevertheless, the most significant patterns were the vertical lifting of the cell core, the three-body scatter spike, the bounded weak-echo region, and the anticyclonic rotation, observed in the Doppler velocity fields. Key features identified in the lightning analysis were: 1) the total lightning “jump” as an early sign for severity, 2) the low negative cloud-to-ground (CG) flash rate and 3) the low intensities in negative CG strokes and the regular rate of positive CG as indicators of complexity in the electrical structure. Finally, data strongly suggest the worst damage occurred when the thunderstorm was in its supercell stage. This case study presents one of the first documented supercells in the region.
The scientific documentation and investigation of tropical cyclone (TC) tornadoes has spanned portions of ten decades, but has been missing a documentary overview of topical knowledge accumulated to any given point in that time span. This review article summarizes the evolution of TC tornado-related literature from the perspectives of crucial historic tornadoes, climatology, distribution patterns, applied research into their environments, remote and environmental observations, forecasting practices, and numerical simulations at various scales. Discussion of the future of TC tornado research and prediction includes several testable hypotheses, along with potentially beneficial tools soon to be available to operational forecasters.
Wildland fire behavior is highly dependent upon local meteorological conditions. While topography and the state of available fuels also influence fire behavior and spread, near-surface atmospheric conditions in proximity to wildland fires are the most dynamic determining variables for wildfire evolution. Recent episodes of drought across the southern High Plains have contributed to unprecedented wildfire activity in the region’s grasslands, including within the Texas Tech University West Texas Mesonet (WTM) domain. The juxtaposition of this meso-network with the occurrence of numerous wind-driven wildfires has provided a unique dataset of proximity meteorological observations useful in analyzing fire start environments. This study presents statistical analyses of WTM 2-m relative humidity and 6-m wind speed, parameters utilized in local Red Flag fire weather warning criteria, along with 2-m temperature in temporal and spatial proximity to 99 wind-driven grassland wildfire starts which occurred between January 2006 and May 2010. Since the state of vegetative fuels also influences fire behavior, but is dependent upon local weather, the proximity observations are used to calculate fine dead-fuel moisture and to examine pre-conditioning potential per the preceding humidity recoveries for each documented fire start. A comparison of the meteorological observations to local Red Flag warning criteria, which was met or exceeded for critical values of relative humidity and wind speed in 64% of the surveyed fire starts, also is included. Furthermore, seasonal and diurnal tendencies for local wind-driven wildfire activity are noted.
Motorists traveling on Interstate highways are likely to have an increased vulnerability to weather hazards due to their unfamiliarity with nearby towns, limited methods to receive short-term weather information, and a general deficiency of a suitable shelter. To assess the threat, a database of 678 tornadoes, crossing primary and auxiliary Interstates across portions of the central and southeastern contiguous United States, was compiled for the period of 1990 to 2008. Approximately 17% of Interstate-crossing tornadoes impacted vehicles. Factors such as time of the day, EF-Scale rating, and travel density were examined to assess potential association with the probability of a tornado impact. This paper discusses current warning and preparedness activities in the operational meteorological community and state transportation departments, and recommends future actions and new technology to mitigate the loss of life and property from tornadoes that cross Interstate highways.
This case study describes a severe-storm event over Florida, Georgia, and South Carolina on 25 December 2006, with a particular focus on an F2 tornado that struck Daytona Beach, FL and caused over $50 million in damages. The severe weather occurred over a 12-h period and was associated with a deep upper-level trough and surface front moving through the southeastern U.S. Morning soundings over Florida showed low to moderate CAPE and strong vertical wind shear, consistent with seasonal composite tornadic soundings for the region. A quasi-linear convective system moved onshore near Tampa during the midmorning hours, the northern half of which accelerated and produced bow echoes that resulted in two tornadoes and nontornadic wind damage over Pasco, Sumter, and Lake Counties between 1620 and 1725 UTC. This portion of the line then moved into Volusia County and spawned F2 tornadoes in Deland and Daytona Beach after 1800 UTC. Data from the Melbourne National Weather Service Forecast Office’s Weather Surveillance Radar-1988D (WSR-88D), Daytona Beach International Airport (DAB) Automated Surface Observing System (ASOS), and DAB Low Level Wind Shear Alert System (LLWAS) were integrated to analyze conditions at the east end of the DAB runway 7L/25R complex, where the tornado first appeared. The LLWAS is normally used by air traffic control personnel for monitoring airport wind-shear conditions. The 10-s LLWAS wind data filled critical temporal and spatial gaps in the WSR-88D and ASOS data, and captured evidence of strong winds and cyclonic curvature nearly coincident with the locations of the radar-identified velocity couplet and tornado itself.
All tornado reports across the contiguous United States from 2003-2011 were filtered for the maximum damage rating on an hourly grid with 40-km horizontal spacing. Convective mode was assigned to each grid-hour tornado event via manual examination of full volumetric WSR-88D data, and supercell-related environmental parameters accompanied each grid-hour tornado event from the hourly objective analyses calculated and archived at the Storm Prediction Center. Only tornado events associated with right-moving supercells (RM) or quasi-linear convective systems (QLCS) were considered in this work, which resulted in a sample of 8837 tornado grid-hour events. Spatial distributions of supercell-related parameters were constructed for the RM and QLCS tornado events. Sample sizes were increased by accumulating tornado events within a 120-km neighborhood to each 40-km grid box. All neighborhoods with ≥10 events were retained for percentile rank distributions of the supercell-related parameters, and then smoothed using a Gaussian kernel with a 120-km influence radius. Regional variations in buoyancy and lifting condensation level (LCL) are apparent-RM tornadoes are more common with greater buoyancy and higher LCL heights across the Great Plains compared to the Mississippi Valley region. QLCS tornadoes tend to be focused across the Ohio and Mississippi Valleys, in environments with weaker buoyancy and lower LCL heights. Vertical wind shear parameters are typically well within the parameter space associated with tornadic RM for both the RM and QLCS tornado events. The significant tornado parameter shows improved discrimination between weak and significant RM tornadoes, compared to individual kinematic or thermodynamic parameters.
A rare combination of anomalously cold air and rapidly falling snow produced a massive travel disruption in parts of the southeastern U.S. on 28 January 2014. Storm-total snowfalls were generally only 5 cm in the Birmingham, AL area, but most people were at work or school when the snow began. Due to icy roadways, thousands of people were forced to sleep in offices and schools, while many vehicles were abandoned on freeways and surface streets. This event was a unique forecasting challenge in many aspects, especially for the southeastern U.S. In the 24 h leading up to the event, numerical model quantitative precipitation forecasting (QPF) was focused south of the main population centers where the main disruptions eventually occurred. A layer of very dry air was present up to 800 hPa, but saturation deficits were low due to the cold air. Radar data a few hours before the event provided some of the only tangible evidence that significant snow may occur. However, forecasters did not anticipate the havoc that would ensue from the relatively light snow accumulations. In this paper, we examine the event, including the synoptic setup, vertical profiles of temperature and moisture, soil temperatures, numerical models, and radar data. On the early morning of 28 January, when school superintendents and business owners had to make decisions on whether to open, no warnings nor advisories were in effect for the majority of the Birmingham area, and the media indicated there would be no travel problems in Birmingham.
Different types of land cover are associated with different surface roughness, which produce variations in the frictional force on the wind. Therefore, mean wind profiles at low levels often differ markedly over short distances where there is a gradient in surface roughness. Horizontal gradients in surface roughness may produce vertical vorticity, circulation, and horizontal divergence. The effect of roughness on vertical wind shear and storm-relative helicity is also qualitatively important and may lead to large gradients in helicity over short distances. Recent studies also suggest an important role of friction in tornadogenesis. We show conceptually and theoretically how gradients in surface roughness produce quasi-ambient convergence and vertical vorticity, and gradients in vertical shear and storm-relative helicity. We then present observational data and numerical simulations that demonstrate the effects of surface roughness on the kinematics and shear of boundary-layer flow, for future work examining the importance of these processes for tornadogenesis.
