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Harold E Brooks - One of the best experts on this subject based on the ideXlab platform.
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spatial temporal clustering of Tornadoes
EGU General Assembly Conference Abstracts, 2017Co-Authors: Bruce D Malamud, Donald L Turcotte, Harold E BrooksAbstract:Abstract. The standard measure of the intensity of a tornado is the Enhanced Fujita scale, which is based qualitatively on the damage caused by a tornado. An alternative measure of tornado intensity is the tornado path length, L. Here we examine the spatial–temporal clustering of severe Tornadoes, which we define as having path lengths L ≥ 10 km. Of particular concern are tornado outbreaks, when a large number of severe Tornadoes occur in a day in a restricted region. We apply a spatial–temporal clustering analysis developed for earthquakes. We take all pairs of severe Tornadoes in observed and modelled outbreaks, and for each pair plot the spatial lag (distance between touchdown points) against the temporal lag (time between touchdown points). We apply our spatial–temporal lag methodology to the intense tornado outbreaks in the central United States on 26 and 27 April 2011, which resulted in over 300 fatalities and produced 109 severe (L ≥ 10 km) Tornadoes. The patterns of spatial–temporal lag correlations that we obtain for the 2 days are strikingly different. On 26 April 2011, there were 45 severe Tornadoes and our clustering analysis is dominated by a complex sequence of linear features. We associate the linear patterns with the Tornadoes generated in either a single cell thunderstorm or a closely spaced cluster of single cell thunderstorms moving at a near-constant velocity. Our study of a derecho tornado outbreak of six severe Tornadoes on 4 April 2011 along with modelled outbreak scenarios confirms this association. On 27 April 2011, there were 64 severe Tornadoes and our clustering analysis is predominantly random with virtually no embedded linear patterns. We associate this pattern with a large number of interacting supercell thunderstorms generating Tornadoes randomly in space and time. In order to better understand these associations, we also applied our approach to the Great Plains tornado outbreak of 3 May 1999. Careful studies by others have associated individual Tornadoes with specified supercell thunderstorms. Our analysis of the 3 May 1999 tornado outbreak directly associated linear features in the largely random spatial–temporal analysis with several supercell thunderstorms, which we then confirmed using model scenarios of synthetic tornado outbreaks. We suggest that it may be possible to develop a semi-automated modelling of tornado touchdowns to match the type of observations made on the 3 May 1999 outbreak.
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convective modes for significant severe thunderstorms in the contiguous united states part i storm classification and climatology
Weather and Forecasting, 2012Co-Authors: Bryan T Smith, Richard L Thompson, Jeremy S Grams, Chris Broyles, Harold E BrooksAbstract:AbstractRadar-based convective modes were assigned to a sample of Tornadoes and significant severe thunderstorms reported in the contiguous United States (CONUS) during 2003–11. The significant hail (≥2-in. diameter), significant wind (≥65-kt thunderstorm gusts), and Tornadoes were filtered by the maximum event magnitude per hour on a 40-km Rapid Update Cycle model horizontal grid. The filtering process produced 22 901 tornado and significant severe thunderstorm events, representing 78.5% of all such reports in the CONUS during the sample period. The convective mode scheme presented herein begins with three radar-based storm categories: 1) discrete cells, 2) clusters of cells, and 3) quasi-linear convective systems (QLCSs). Volumetric radar data were examined for right-moving supercell (RM) and left-moving supercell characteristics within the three radar reflectivity designations. Additional categories included storms with marginal supercell characteristics and linear hybrids with a mix of supercell and Q...
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the Tornadoes of spring 2011 in the usa an historical perspective
Weather, 2012Co-Authors: Charles A. Doswell, Gregory W Carbin, Harold E BrooksAbstract:Tornado activity during spring of 2011 in the USA was of historic proportions; a series of major outbreaks of Tornadoes in April and May produced hundreds of fatalities and thousands of millions of $US in damage. These events are considered in terms of the history of Tornadoes in the USA and are seen to approach the worst ever recorded. With all the current concern for the effects of global climate change, this raises a number of questions about why such devastation occurred during this spring's severe weather season. This paper attempts to provide scientifically justifiable answers to those questions.
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relationship between sounding derived parameters and the strength of Tornadoes in europe and the usa from reanalysis data
Atmospheric Research, 2011Co-Authors: S Grunwald, Harold E BrooksAbstract:Abstract Proximity soundings from reanalysis data for tornado events in Europe for the years 1958 to 1999 and in the US for the years 1991 to 1999 have been used for generating distributions of parameter combinations important for severe convection. They include parcel updraft velocity (WMAX) and deep-layer shear (DLS), lifting condensation level (LCL) and deep-layer shear (DLS), and LCL and shallow-layer shear (LLS) for weak and significant Tornadoes. We investigate how well they discriminate between weak and significant Tornadoes. For Europe, these distributions have been generated for unrated, F0 and F1 Tornadoes as well to discover if the unrated Tornadoes can be associated with the weak Tornadoes. The pattern of parameter combination distributions for unrated Tornadoes in Europe strongly resembles the pattern of F0 Tornadoes. Thus, the unrated Tornadoes are likely to consist of mostly F0 Tornadoes. Consequently, the unrated Tornadoes have been included into the weak Tornadoes and distributions of parameter combinations have been generated for these. In Europe, none of the three combinations can discriminate well between weak and significant Tornadoes, but all can discriminate if the unrated Tornadoes are included with the weak Tornadoes (unrated/weak). In the US, the combinations of LCL and either of the shear parameters discriminate well between weak and significant Tornadoes, with significant Tornadoes occurring at lower LCL and higher shear values than the weak ones. In Europe, the shear shows the same behavior, but the LCL behaves differently, with significant Tornadoes occurring at higher LCL than the unrated/weak ones. The combination of WMAX and DLS is a good discriminator between unrated/weak and significant Tornadoes in Europe, but not in the US, with significant Tornadoes occurring at a higher WMAX and DLS than the unrated/weak Tornadoes.
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Tornadoes from squall lines and bow echoes part i climatological distribution
Weather and Forecasting, 2005Co-Authors: Robert J Trapp, Sarah A Tessendorf, Elaine S Godfrey, Harold E BrooksAbstract:The primary objective of this study was to estimate the percentage of U.S. Tornadoes that are spawned annually by squall lines and bow echoes, or quasi-linear convective systems (QLCSs). This was achieved by examining radar reflectivity images for every tornado event recorded during 1998–2000 in the contiguous United States. Based on these images, the type of storm associated with each tornado was classified as cell, QLCS, or other. Of the 3828 Tornadoes in the database, 79% were produced by cells, 18% were produced by QLCSs, and the remaining 3% were produced by other storm types, primarily rainbands of landfallen tropical cyclones. Geographically, these percentages as well as those based on tornado days exhibited wide variations. For example, 50% of the tornado days in Indiana were associated with QLCSs. In an examination of other tornado attributes, statistically more weak (F1) and fewer strong (F2–F3) Tornadoes were associated with QLCSs than with cells. QLCS Tornadoes were more probable during the winter months than were cells. And finally, QLCS Tornadoes displayed a comparatively higher and statistically significant tendency to occur during the late night/early morning hours. Further analysis revealed a disproportional decrease in F0–F1 events during this time of day, which led the authors to propose that many (perhaps as many as 12% of the total) weak QLCSs Tornadoes were not reported.
Howard B. Bluestein - One of the best experts on this subject based on the ideXlab platform.
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doppler radar observations of anticyclonic Tornadoes in cyclonically rotating right moving supercells
Monthly Weather Review, 2016Co-Authors: Howard B. Bluestein, Jeffrey C. Snyder, Michael M French, Jana B HouserAbstract:AbstractSupercells dominated by mesocyclones, which tend to propagate to the right of the tropospheric pressure-weighted mean wind, on rare occasions produce anticyclonic Tornadoes at the trailing end of the rear-flank gust front. More frequently, mesoanticyclones are found at this location, most of which do not spawn any Tornadoes. In this paper, four cases are discussed in which the formation of anticyclonic Tornadoes was documented in the plains by mobile or fixed-site Doppler radars. These brief case studies include the analysis of Doppler radar data for Tornadoes at the following dates and locations: 1) 24 April 2006, near El Reno, Oklahoma; 2) 23 May 2008, near Ellis, Kansas; 3) 18 March 2012, near Willow, Oklahoma; and 4) 31 May 2013, near El Reno, Oklahoma. Three of these Tornadoes were also documented photographically. In all of these cases, a strong mesocyclone (i.e., vortex signature characterized by azimuthal shear in excess of ~5 × 10−3 s−1 or a 20 m s−1 change in Doppler velocity over 5 km) ...
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rapid scan polarimetric doppler radar observations of tornadogenesis and tornado dissipation in a tornadic supercell the el reno oklahoma storm of 24 may 2011
Monthly Weather Review, 2015Co-Authors: Jana Lesak Houser, Howard B. Bluestein, Jeffrey C. SnyderAbstract:AbstractOn 24 May 2011, a mobile, rapid-scan, X-band, polarimetric, Doppler radar (RaXPol) collected data on a supercell as it produced two Tornadoes near El Reno, Oklahoma. The first tornado, rated an EF-3, was documented from intensification to decay, and the genesis and intensification of a second tornado that was rated an EF-5 was subsequently also documented.The objective of this study is to examine the spatiotemporal evolution of the rotation associated with the Tornadoes (i) as the first tornado weakened to subtornadic intensity and (ii) as the second tornado formed and intensified. It is found that weakening did not occur monotonically. The transition from tornadic to subtornadic intensity over the depth of the radar volume (~4 km) occurred in less than 30 s, but this behavior is contingent upon the threshold for Doppler shear used to define the tornado. Similarly, the onset of a tornadic-strength Doppler velocity couplet occurred within a 30-s period over all elevations.Additionally, the evolutio...
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severe convective storms and Tornadoes observations and dynamics
2013Co-Authors: Howard B. BluesteinAbstract:Basic equations.- Observing systems and the analysis and interpretation of their data.- Ordinary-cell convective storms.- Supercells.- Mesoscale convective systems (MCSs).- Tornadoes.- Forecasting.- Areas of future research.- Theory.
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The Second Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX2
Bulletin of the American Meteorological Society, 2012Co-Authors: Joshua Wurman, David C. Dowell, Yvette Richardson, Paul Markowski, Erik N. Rasmussen, Donald W. Burgess, Louis J. Wicker, Howard B. BluesteinAbstract:The second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), which had its field phases in May and June of 2009 and 2010, was designed to explore i) the physical processes of tornadogenesis, maintenance, and demise; ii) the relationships among Tornadoes, tornadic storms, and the larger-scale environment; iii) numerical weather prediction and forecasting of supercell thunderstorms and Tornadoes; and iv) the wind field near the ground in Tornadoes. VORTEX2 is by far the largest and most ambitious observational and modeling study of Tornadoes and tornadic storms ever undertaken. It employed 13 mobile mesonet–instrumented vehicles, 11 ground-based mobile radars (several of which had dual-polarization capability and two of which were phased-array rapid scan), a mobile Doppler lidar, four mobile balloon sounding systems, 42 deployable in situ observational weather stations, an unmanned aerial system, video and photogrammetric teams, damage survey teams, deployable disdrometers, and othe...
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mobile x band polarimetric doppler radar observations of the 4 may 2007 greensburg kansas tornadic supercell
Monthly Weather Review, 2012Co-Authors: Robin L Tanamachi, Howard B. Bluestein, Jana B Houser, Stephen J Frasier, Kery HardwickAbstract:AbstractOn 4 May 2007, a supercell produced an EF-5 tornado that severely damaged the town of Greensburg, Kansas. Volumetric data were collected in the “Greensburg storm” by the University of Massachusetts X-band, mobile, polarimetric Doppler radar (UMass X-Pol) for 70 min; 10 Tornadoes were detected. This mobile Doppler radar dataset is one of only a few documenting an EF-5 tornado and the supercell’s transition from short-track, cyclic tornado production (mode 1) to long-track tornado production (mode 2). Using bootstrap confidence intervals, it is determined that the mode-2 Tornadoes moved in the same direction as the supercell vault. In contrast, the mode-1 Tornadoes moved to the left with respect to the vault.From polarimetric data collected in this storm, the authors infer the presence of large, oblate drops (high ZDR, high ρhv) in the forward flank and surrounding some of the Tornadoes. The authors speculate that the weak-echo column (WEC) in the Greensburg tornado, which extended above 10 km AGL, ...
Charles A. Doswell - One of the best experts on this subject based on the ideXlab platform.
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Tornadoes in the Central United States and the “Clash of Air Masses”
Bulletin of the American Meteorological Society, 2014Co-Authors: David M. Schultz, Yvette Richardson, Paul Markowski, Charles A. DoswellAbstract:After tornado outbreaks or individual violent Tornadoes occur in the central United States, media stories often attribute the location, number, or intensity of Tornadoes to the “clash of air masses” between warm tropical air and cold polar air. This article argues that such a characterization of tornadogenesis is oversimplified, outdated, and incorrect. Airmass boundaries and associated temperature gradients can be important in tornadogenesis, but not in the ways envisioned on the synoptic scale with the clash-of-air-masses conceptual model. In fact, excessively strong horizontal temperature gradients (either on the synoptic scale or associated with a storm's own cool outflow) may be detrimental to tornadogenesis. Where adjacent air masses are relevant is through their vertical distribution that produces the requisite instability for the convective storm, but that instability is not directly related to the formation of Tornadoes. Therefore, this article recommends that a greater effort be made to communic...
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the Tornadoes of spring 2011 in the usa an historical perspective
Weather, 2012Co-Authors: Charles A. Doswell, Gregory W Carbin, Harold E BrooksAbstract:Tornado activity during spring of 2011 in the USA was of historic proportions; a series of major outbreaks of Tornadoes in April and May produced hundreds of fatalities and thousands of millions of $US in damage. These events are considered in terms of the history of Tornadoes in the USA and are seen to approach the worst ever recorded. With all the current concern for the effects of global climate change, this raises a number of questions about why such devastation occurred during this spring's severe weather season. This paper attempts to provide scientifically justifiable answers to those questions.
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the Tornadoes of 3 may 1999 event verification in central oklahoma and related issues
Weather and Forecasting, 2002Co-Authors: Douglas A Speheger, Charles A. Doswell, Gregory J StumpfAbstract:Abstract The tornado events of 3 May 1999 within the county warning area of the Norman, Oklahoma, office of the National Weather Service are reviewed, emphasizing the challenges associated with obtaining accurate information about the existence, timing, location, and intensity of individual Tornadoes. Accurate documentation of tornado and other hazardous weather events is critical to research, is needed for operational assessments, and is important for developing hazard mitigation strategies. The situation following this major event was unusual because of the high concentration of meteorologists in the area, relative to most parts of the United States. As a result of this relative abundance of resources, it is likely that these Tornadoes were reasonably well documented. Despite this unique situation in central Oklahoma, it is argued that this event also provides evidence of a national need for a rapid-response scientific and engineering survey team to provide documentation of major hazardous weather event...
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some aspects of the international climatology of Tornadoes by damage classification
Atmospheric Research, 2001Co-Authors: Harold E Brooks, Charles A. DoswellAbstract:Reports of Tornadoes, broken down by damage, from seven countries have been examined. In particular, the long-term relatively high-quality dataset from the US is used to develop distribu- tions which indicate that the number of Tornadoes decreases log-linearly with increasing F-scale. Two distinct distributions, one apparently associated with supercell tornadogenesis processes and the other with non-supercell processes, are found in both the US data and in other countries. The similarity of the distribution in the US prior to the 1950s, when an official, organized collection effort began, and the French record, suggests that only 15% of French Tornadoes are being reported currently. In addition, we can use the simple statistical distributions to estimate the return . period of violent Tornadoes in France approximately one every 5-10 years and the UK .approximately one every 250-300 years . Published by Elsevier Science B.V.
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climatological risk of strong and violent Tornadoes in the united states
80th AMS Annual Meeting, 2000Co-Authors: Peggy R Concannon, E Brooks, Charles A. DoswellAbstract:Knowledge of tornado climatology is valuable to a variety of groups, especially weather forecasters, emergency management officials, and the public. It is crucial for such individuals to understand the threat posed by Tornadoes in the United States, particularly the threat of strong and violent Tornadoes. In this study, the term strong refers to those Tornadoes producing F2 or F3 damage while violent refers to those producing F4 or greater damage. Significant refers to any F2 or greater tornado. Even though only about 10% of Tornadoes are significant, these Tornadoes are responsible for the majority of deaths caused by Tornadoes in the country, with violent Tornadoes claiming 67% of the total. Furthermore, with the aftermath of such events, the US suffers millions of dollars in damage costs—an important consideration for the insurance industry. Due to this destructive potential toward life and property, we chose to consider these Tornadoes only in our study, using data from 1921-1995. Furthermore, the significant tornado dataset is likely to be more reliable than that for any tornado since they are more likely to be observed. Typically, they are larger and thus more visible, with longer path lengths, and they cause the most damage. We attempted to estimate the daily climatological probability of an F2 or more damaging tornado occurring near any location in the US.
Yvette Richardson - One of the best experts on this subject based on the ideXlab platform.
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near storm environments of outbreak and isolated Tornadoes
Weather and Forecasting, 2018Co-Authors: Alexandra K Andersonfrey, Yvette Richardson, Andrew R Dean, Richard L Thompson, Bryan T SmithAbstract:AbstractBetween 2003 and 2015, there were 5343 outbreak Tornadoes and 9389 isolated Tornadoes reported in the continental United States. Here, the near-storm environmental parameter-space distribut...
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Comparison of the Tornadic and Nontornadic Supercells Intercepted by VORTEX2 on 10 June 2010
Monthly Weather Review, 2016Co-Authors: Alicia Klees, Joshua Wurman, Yvette Richardson, Paul Markowski, Christopher C. Weiss, Karen KosibaAbstract:AbstractOn 10 June 2010, the second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2) armada collected a rare set of observations of a nontornadic and a tornadic supercell evolving in close proximity to each other. The storms and their environments were analyzed using single- and dual-Doppler radar, mobile mesonet, deployable surface mesonet, and mobile sounding data, with the goal of understanding why one supercell produced no Tornadoes while the other produced at least two. Outflow temperature deficits were similar for the two storms, both within the normal range for weakly tornadic supercells but somewhat cold relative to significantly tornadic supercells. The storms formed in a complex environment, with slightly higher storm-relative helicity near the tornadic supercell. The environment evolved significantly in time, with large thermodynamic changes and increases in storm-relative helicity, leading to conditions much more favorable for tornadogenesis. After a few hours, a new s...
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Tornadoes in the Central United States and the “Clash of Air Masses”
Bulletin of the American Meteorological Society, 2014Co-Authors: David M. Schultz, Yvette Richardson, Paul Markowski, Charles A. DoswellAbstract:After tornado outbreaks or individual violent Tornadoes occur in the central United States, media stories often attribute the location, number, or intensity of Tornadoes to the “clash of air masses” between warm tropical air and cold polar air. This article argues that such a characterization of tornadogenesis is oversimplified, outdated, and incorrect. Airmass boundaries and associated temperature gradients can be important in tornadogenesis, but not in the ways envisioned on the synoptic scale with the clash-of-air-masses conceptual model. In fact, excessively strong horizontal temperature gradients (either on the synoptic scale or associated with a storm's own cool outflow) may be detrimental to tornadogenesis. Where adjacent air masses are relevant is through their vertical distribution that produces the requisite instability for the convective storm, but that instability is not directly related to the formation of Tornadoes. Therefore, this article recommends that a greater effort be made to communic...
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The Second Verification of the Origins of Rotation in Tornadoes Experiment: VORTEX2
Bulletin of the American Meteorological Society, 2012Co-Authors: Joshua Wurman, David C. Dowell, Yvette Richardson, Paul Markowski, Erik N. Rasmussen, Donald W. Burgess, Louis J. Wicker, Howard B. BluesteinAbstract:The second Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX2), which had its field phases in May and June of 2009 and 2010, was designed to explore i) the physical processes of tornadogenesis, maintenance, and demise; ii) the relationships among Tornadoes, tornadic storms, and the larger-scale environment; iii) numerical weather prediction and forecasting of supercell thunderstorms and Tornadoes; and iv) the wind field near the ground in Tornadoes. VORTEX2 is by far the largest and most ambitious observational and modeling study of Tornadoes and tornadic storms ever undertaken. It employed 13 mobile mesonet–instrumented vehicles, 11 ground-based mobile radars (several of which had dual-polarization capability and two of which were phased-array rapid scan), a mobile Doppler lidar, four mobile balloon sounding systems, 42 deployable in situ observational weather stations, an unmanned aerial system, video and photogrammetric teams, damage survey teams, deployable disdrometers, and othe...
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low level winds in Tornadoes and potential catastrophic tornado impacts in urban areas
Bulletin of the American Meteorological Society, 2007Co-Authors: Joshua Wurman, Curtis R Alexander, Paul Robinson, Yvette RichardsonAbstract:Using an axisymmetric model of tornado structure tightly constrained by high-resolution wind field measurements collected by Doppler on Wheels (DOW) mobile radars, the potential impacts of intense Tornadoes crossing densely populated urban areas are evaluated. DOW radar measurements combined with in situ low-level wind measurements permit the quantification of low-level tornadic winds that would impact structures. Axisymmetric modeled wind fields from actual and hypothetical Tornadoes are simulated to impact high-density residential and commercial districts of several major cities. U.S. census block data, satellite imagery, and other sources are used to characterize and count the number of structures impacted by intense winds, up to 132 m s−1, and estimate the level and cost of resulting damage. Census data are used to estimate residential occupancy and human casualties. Results indicate that a large and intense tornado crossing through residential portions of Chicago, Illinois, could result in tragic con...
Pieter Groenemeijer - One of the best experts on this subject based on the ideXlab platform.
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Tornadoes in europe an underestimated threat
Bulletin of the American Meteorological Society, 2017Co-Authors: Bogdan Antonescu, David M. Schultz, Alois M Holzer, Pieter GroenemeijerAbstract:AbstractThe social and economic impact of Tornadoes in Europe is analyzed using tornado reports from the European Severe Weather Database between 1950 and 2015. Despite what is often assumed by the general public and even by meteorologists and researchers, Tornadoes do occur in Europe and they are associated with injuries, fatalities, and damages, although their reported frequencies and intensities are lower compared with the United States. Currently, the threat of Tornadoes to Europe is underestimated. Few European meteorological services have developed and maintained tornado databases and even fewer have issued tornado warnings. This article summarizes our current understanding of the tornado threat to Europe by showing the changes in tornado injuries and fatalities since the 1950s and by estimating for the first time the damages associated with European Tornadoes. To increase awareness of Tornadoes and their threat to Europe, we propose a strategy that includes 1) collaboration between meteorological s...
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a climatology of Tornadoes in europe results from the european severe weather database
Monthly Weather Review, 2014Co-Authors: Pieter Groenemeijer, Thilo KuhneAbstract:AbstractA climatology of Tornadoes (over land and water) is presented, based on the European Severe Weather Database (ESWD), which contains reports of 9529 Tornadoes. With the exception of a few small countries, Tornadoes have been reported from all regions of Europe. The highest density of tornado reports is in western and central Europe. ESWD tornado reports increased strongly from 1995 to 2006 as a result of increased data collection efforts, followed by a decrease that likely has a meteorological nature. There is strong underreporting in the Mediterranean region and eastern Europe. The daily cycle of Tornadoes over land (sea) peaks between 1500 and 1600 (0900 and 1000) local time. The Mediterranean annual maximum is in autumn and winter, while regions farther north have a maximum in summer. In total, 822 tornado fatalities have been recorded in the ESWD, which include 10 Tornadoes with more than 20 fatalities. The average annual number of tornado fatalities in Europe is estimated to be between 10 and ...
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sounding derived parameters associated with large hail and Tornadoes in the netherlands
Atmospheric Research, 2007Co-Authors: Pieter Groenemeijer, A J Van DeldenAbstract:A study is presented focusing on the potential value of parameters derived from radiosonde data or data from numerical atmospheric models for the forecasting of severe weather associated with convective storms. Parameters have been derived from soundings in the proximity of large hail, Tornadoes (including Tornadoes over water: waterspouts) and thunderstorms in the Netherlands. 66,365 radiosonde soundings from six stations in and around the Netherlands between 1 Dec. 1975 to 31 Aug. 2003 were classified as being associated or not associated with these weather phenomena using observational data from voluntary observers, the Dutch National Meteorological Institute (KNMI) and lightning data from the U.K. Met. Office. It was found that instability as measured by the Lifted Index or CAPE and 0–6 km wind shear independently have considerable skill in distinguishing environments of large hail and of non-hail-producing thunderstorms. It was also found that CAPE released below 3 km above ground level is on average high near waterspouts and weak Tornadoes that mostly occur with low shear in the lowest 1 km above the Earth's surface. On the other hand, low-level shear is strong in environments of stronger (F1 and F2) Tornadoes and increases with increasing F-scale. This is consistent with the notion that stretching of pre-existing vertical vorticity is the most important mechanism for the formation of weak Tornadoes while the tilting of vorticity is more important with stronger Tornadoes. The presented results may assist forecasters to assess the likelihood of severe hail or Tornadoes.
