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Edwin Kessler, Ariel E. Cohen, Stephen F. Corfidi, and Sarah J. Corfidi

The following essay is excerpted from conversations with Edwin Kessler, founding director of NOAA's National Severe Storms Laboratory in Norman, Oklahoma, conducted by Ariel Cohen and Stephen and Sarah Corfidi of the National Weather Service's Storm Prediction Center. Cohen and the Corfidis based the discussion in part on Kessler's speech at the 50th anniversary of the School of Meteorology at the University of Oklahoma in October 2010, and Cohen converted this discussion to a Q&A format. The conversations range over many subjects, including accomplishments of NSSL, Kessler's role as its manager, and the many people who were crucial to the lab's growth. The entire conversation is published by BAMS online, but here we've selected Kessler's words regarding the move to Norman, the community that evolved there, and how NSSL thrived there because of—and despite—the bureaucratic changes that federal agencies underwent at that time.—The Editors

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Stephen F. Corfidi, Michael C. Coniglio, Ariel E. Cohen, and Corey M. Mead

Abstract

The word “derecho” was first used by Gustavus Hinrichs in 1888 to distinguish the widespread damaging windstorms that occurred on occasion over the mid–Mississippi Valley region of the United States from damaging winds associated with tornadoes. The term soon fell into disuse, however, and did not appear in the literature until Robert Johns and William Hirt resurrected it in the mid-1980s.

While the present definition of derecho served well during the early years of the term’s reintroduction to the meteorological community, it has several shortcomings. These have become more apparent in recent years as various studies shed light on the physical processes responsible for the production of widespread damaging winds. In particular, the current definition’s emphasis on the coverage of storm reports at the expense of identifying the convective structures and physical processes deemed responsible for the reports has led to the term being applied to wind events beyond those for which it originally was intended.

The revised definition of a derecho proposed herein is intended to focus more specifically on those types of windstorms that are the most damaging and potentially life threatening because of their intensity, sustenance, and degree of organization. The proposal is not intended to be final or all encompassing, but rather an initial step toward ultimately realizing a more complete physically based taxonomy that also addresses other forms of damaging-wind-producing convective systems.

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Ariel E. Cohen, Michael C. Coniglio, Stephen F. Corfidi, and Sarah J. Corfidi

Abstract

The prediction of the strength of mesoscale convective systems (MCSs) is a major concern to operational meteorologists and the public. To address this forecast problem, this study examines meteorological variables derived from sounding observations taken in the environment of quasi-linear MCSs. A set of 186 soundings that sampled the beginning and mature stages of the MCSs are categorized by their production of severe surface winds into weak, severe, and derecho-producing MCSs. Differences in the variables among these three MCS categories are identified and discussed. Mean low- to upper-level wind speeds and deep-layer vertical wind shear, especially the component perpendicular to the convective line, are excellent discriminators among all three categories. Low-level inflow relative to the system is found to be an excellent discriminator, largely because of the strong relationship of system severity to system speed. Examination of the mean wind and shear vectors relative to MCS motion suggests that cell propagation along the direction of cell advection is a trait that separates severe, long-lived MCSs from the slower-moving, nonsevere variety and that this is favored when both the deep-layer shear vector and the mean deep-layer wind are large and nearly parallel. Midlevel environmental lapse rates are found to be very good discriminators among all three MCS categories, while vertical differences in equivalent potential temperature and CAPE only discriminate well between weak and severe/derecho MCS environments. Knowledge of these variables and their distribution among the different categories of MCS intensity can be used to improve forecasts and convective watches for organized convective wind events.

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Ryan C. Bunker, Ariel E. Cohen, John A. Hart, Alan E. Gerard, Kim E. Klockow-McClain, and David P. Nowicki

Abstract

Tornadoes that occur at night pose particularly dangerous societal risks, and these risks are amplified across the southeastern United States. The purpose of this study is to highlight some of the characteristics distinguishing the convective environment accompanying these events. This is accomplished by building upon previous research that assesses the predictive power of meteorological parameters. In particular, this study uses the Statistical Severe Convective Risk Assessment Model (SSCRAM) to determine how well convective parameters explain tornado potential across the Southeast during the months of November–May and during the 0300–1200 UTC (nocturnal) time frame. This study compares conditional tornado probabilities across the Southeast during November–May nocturnal hours to those probabilities for all other November–May environments across the contiguous United States. This study shows that effective bulk shear, effective storm-relative helicity, and effective-layer significant tornado parameter yield the strongest predictability for the November–May nocturnal Southeast regime among investigated parameters. This study demonstrates that November–May southeastern U.S. nocturnal predictability is generally similar to that within other regimes across the contiguous United States. However, selected ranges of multiple parameters are associated with slightly better predictability for the nocturnal Southeast regime. Additionally, this study assesses conditional November–May nocturnal tornado probabilities across a coastal domain embedded within the Southeast. Nocturnal coastal tornado predictability is shown to generally be lower than the other regimes. All of the differences highlight several forecast challenges, which this study analyzes in detail.

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Richard L. Thompson, Bryan T. Smith, Jeremy S. Grams, Andrew R. Dean, Joseph C. Picca, Ariel E. Cohen, Elizabeth M. Leitman, Aaron M. Gleason, and Patrick T. Marsh

Abstract

Previous work with observations from the NEXRAD (WSR-88D) network in the United States has shown that the probability of damage from a tornado, as represented by EF-scale ratings, increases as low-level rotational velocity increases. This work expands on previous studies by including reported tornadoes from 2014 to 2015, as well as a robust sample of nontornadic severe thunderstorms [≥1-in.- (2.54 cm) diameter hail, thunderstorm wind gusts ≥ 50 kt (25 m s−1), or reported wind damage] with low-level cyclonic rotation. The addition of the nontornadic sample allows the computation of tornado damage rating probabilities across a spectrum of organized severe thunderstorms represented by right-moving supercells and quasi-linear convective systems. Dual-polarization variables are used to ensure proper use of velocity data in the identification of tornadic and nontornadic cases. Tornado damage rating probabilities increase as low-level rotational velocity V rot increases and circulation diameter decreases. The influence of height above radar level (or range from radar) is less obvious, with a muted tendency for tornado damage rating probabilities to increase as rotation (of the same V rot magnitude) is observed closer to the ground. Consistent with previous work on gate-to-gate shear signatures such as the tornadic vortex signature, easily identifiable rotation poses a greater tornado risk compared to more nebulous areas of cyclonic azimuthal shear. Additionally, tornado probability distributions vary substantially (for similar sample sizes) when comparing the southeast United States, which has a high density of damage indicators, to the Great Plains, where damage indicators are more sparse.

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Matthew A. Campbell, Ariel E. Cohen, Michael C. Coniglio, Andrew R. Dean, Stephen F. Corfidi, Sarah J. Corfidi, and Corey M. Mead

Abstract

The goal of this study is to document differences in the convective structure and motion of long-track, severe-wind-producing MCSs from short-track severe-wind-producing MCSs in relation to the mean wind. An ancillary goal is to determine if these differences are large enough that some criterion for MCS motion relative to the mean wind could be used in future definitions of “derechos.” Results confirm past investigations that well-organized MCSs, including those that produce derechos, tend to move faster than the mean wind, exhibiting a significantly larger degree of propagation (component of MCS motion in addition to the component contributed by the mean flow). Furthermore, well-organized systems that produce shorter-track swaths of damaging winds likewise tend to move faster than the mean wind with a significant propagation component along the mean wind. Therefore, propagation in the direction of the mean wind is not necessarily a characteristic that can be used to distinguish derechos from nonderechos. However, there is some indication that long-track damaging wind events that occur without large-scale or persistent bow echoes and mesoscale convective vortices (MCVs) require a strong propagation component along the mean wind direction to become long lived. Overall, however, there does not appear to be enough separation in the motion characteristics among the MCS types to warrant the inclusion of a mean-wind criterion into the definition of a derecho at this time.

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Ariel E. Cohen, Richard L. Thompson, Steven M. Cavallo, Roger Edwards, Steven J. Weiss, John A. Hart, Israel L. Jirak, William F. Bunting, Jaret W. Rogers, Steven F. Piltz, Alan E. Gerard, Andrew D. Moore, Daniel J. Cornish, Alexander C. Boothe, and Joel B. Cohen

Abstract

During the 2014–15 academic year, the National Oceanic and Atmospheric Administration (NOAA) National Weather Service Storm Prediction Center (SPC) and the University of Oklahoma (OU) School of Meteorology jointly created the first SPC-led course at OU focused on connecting traditional theory taught in the academic curriculum with operational meteorology. This class, “Applications of Meteorological Theory to Severe-Thunderstorm Forecasting,” began in 2015. From 2015 through 2017, this spring–semester course has engaged 56 students in theoretical skills and related hands-on weather analysis and forecasting applications, taught by over a dozen meteorologists from the SPC, the NOAA National Severe Storms Laboratory, and the NOAA National Weather Service Forecast Offices. Following introductory material, which addresses many theoretical principles relevant to operational meteorology, numerous presentations and hands-on activities focused on instructors’ areas of expertise are provided to students. Topics include the following: storm-induced perturbation pressure gradients and their enhancement to supercells, tornadogenesis, tropical cyclone tornadoes, severe wind forecasting, surface and upper-air analyses and their interpretation, and forecast decision-making. This collaborative approach has strengthened bonds between meteorologists in operations, research, and academia, while introducing OU meteorology students to the vast array of severe thunderstorm forecast challenges, state-of-the-art operational and research tools, communication of high-impact weather information, and teamwork skills. The methods of collaborative instruction and experiential education have been found to strengthen both operational–academic relationships and students’ appreciation of the intricacies of severe thunderstorm forecasting, as detailed in this article.

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