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Roger Edwards
and
Stephen J. Hodanish

Abstract

Anticyclonic left-moving supercells are observed each year in the United States, emanating both discretely and from storm splitting processes. Such thunderstorms often produce severe hail and wind gusts and, on rare occasion, tornadoes. The body of documentary literature on this subset of supercells is relatively scant compared with right-moving storms, and this is especially true regarding visual characteristics and conceptual models. Here a characteristic example of the anticyclonic supercell is presented using an intense and well-defined specimen that passed over Aroya, Colorado, on 15 June 2002. Photographic and radar documentation is provided in original and mirrored forms, for aid in conceptualizing the left-moving supercell and associated structures and processes. A summary overview is presented of the environment, development, evolution, and effects of this remotely located but noteworthy event.

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Roger Edwards
and
Richard L. Thompson

Abstract

This study tests hypothetical correspondences between size of severe hail, WSR-88D derived vertically integrated liquid water (VIL), and an array of thermodynamic variables derived from computationally modified sounding analyses. In addition, these associations are documented for normalized VIL using various sounding parameters, and statistical predictive value is assigned to the various VIL-based and sounding variables. The database was gathered from Weather Service Radar-1988 Doppler (WSR-88D) units nationwide from cases identified during real-time operations and consists of over 400 hail events, each associated with a radar-observed VIL value and a modified observational sounding.

Some parameters are found to increase in the mean with larger hail-size categories. Specific hail size, however, varies widely across the spectra of VIL, thermodynamic sounding variables, and combinations thereof, with only a few exceptions. No operationally useful parameters of value in hail-size prediction were discovered in the database of VIL and thermodynamic sounding data. These largely antihypothetical findings are compared with hail forecasting and warning techniques developed in the WSR-88D era—few in number and mostly regionalized and informal in nature—and with more widespread and empirical forecasting assumptions involving many of the same variables.

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Richard L. Thompson
and
Roger Edwards

Abstract

An overview of conditions associated with the Oklahoma–Kansas tornado outbreak of 3 May 1999 is presented, with emphasis on the evolution of environmental and supercellular characteristics most relevant to the prediction of violent tornado episodes. This examination provides a unique perspective of the event by combining analyses of remote observational data and numerical guidance with direct observations of the event in the field by forecasters and other observers. The 3 May 1999 outbreak included two prolific supercells that produced several violent tornadoes, with ambient parameters comparable to those of past tornado outbreaks in the southern and central Great Plains. However, not all aspects leading to the evening of 3 May unambiguously favored a major tornado outbreak. The problems that faced operational forecasters at the Storm Prediction Center are discussed in the context of this outbreak, including environmental shear and instability, subtle processes contributing to convective initiation, the roles of preexisting boundaries, and storm-relative flow. This examination reveals several specific aspects where conceptual models are deficient and/or additional research is warranted.

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Roger Edwards
and
Richard L. Thompson

Abstract

On the local afternoon of 29 May 2012, a long-lived, right-moving (RM) supercell formed over northwestern Oklahoma and turned roughly southeastward. For >3 h, as it moved toward the Oklahoma City, Oklahoma, metro area, this supercell remained nontornadic and visually high-based, producing a nearly tornadic gustnado and a swath of significantly severe, sometimes giant hail up to 5 in. (12.7 cm) in diameter. Meanwhile, a left-moving (LM) supercell formed over southwestern Oklahoma about 100 mi (161 km) south-southwest of the RM storm, and moved northeastward, with a rear-flank gust front that became well defined on radar imagery as the LM storm approached southern and central parts of the metro. The authors, who had been observing the RM supercell in the field since genesis, surmised its potential future interaction with the LM storm’s trailing gust front about 1 h beforehand. We repositioned to near the gust front’s extrapolated collision point with the RM mesocyclone, in anticipation of maximized tornado potential, then witnessed a small tornado from the RM mesocyclone immediately following its interception of the boundary. Synchronized radar and photographic images of this remarkable sequence are presented and discussed in context of more recent findings on tornadic supercell–boundary interactions, with implications for operational utility.

Significance Statement

Supercells—well-organized, rotating thunderstorms mainly found in midlatitudes—commonly produce the largest hail, along with damaging gusts and most tornadoes. In radar imagery and photographs, we show the characteristics and merger of two supercell types: left-moving and right-moving, with respect to winds aloft. As the left-moving storm’s trailing gust front interacted with the right-mover’s mesocyclone, the latter strengthened quickly, soon producing a tornado. Observed evolution of these storms supports idealized numerical and conceptual models for supercell behavior and interactions with storm-scale boundaries, and may be useful in short-fused tornado forecast and warning operations.

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Roger Edwards
,
Harold E. Brooks
, and
Hannah Cohn

Abstract

U.S. tornado records form the basis for a variety of meteorological, climatological, and disaster-risk analyses, but how reliable are they in light of changing standards for rating, as with the 2007 transition of Fujita (F) to enhanced Fujita (EF) damage scales? To what extent are recorded tornado metrics subject to such influences that may be nonmeteorological in nature? While addressing these questions with utmost thoroughness is too large of a task for any one study, and may not be possible given the many variables and uncertainties involved, some variables that are recorded in large samples are ripe for new examination. We assess basic tornado-path characteristics—damage rating, length, width, and occurrence time, as well as some combined and derived measures—for a 24-yr period of constant path-width recording standard that also coincides with National Weather Service modernization and the WSR-88D deployment era. The middle of that period (in both time and approximate tornado counts) crosses the official switch from F to EF. At least minor shifts in all assessed path variables are associated directly with that change, contrary to the intent of EF implementation. Major and essentially stepwise expansion of tornadic path widths occurred immediately upon EF usage, and widths have expanded still farther within the EF era. We also document lesser increases in pathlengths and in tornadoes rated at least EF1 in comparison with EF0. These apparently secular changes in the tornado data can impact research dependent on bulk tornado-path characteristics and damage-assessment results.

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Benjamin A. Schenkel
,
Roger Edwards
, and
Michael Coniglio

Abstract

The cyclone-relative location and variability in the number of tornadoes among tropical cyclones (TCs) are not completely understood. A key understudied factor that may improve our understanding is ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS), which impacts both the symmetry and strength of deep convection in TCs. This study conducts a climatological analysis of VWS impacts upon tornadoes in TCs from 1995 to 2018, using observed TC and tornado data together with radiosondes. TC tornadoes were classified by objectively defined VWS categories, derived from reanalyses, to quantify the sensitivity of tornado frequency, location, and their environments to VWS. The analysis shows that stronger VWS is associated with enhanced rates of tornado production—especially more damaging ones. Tornadoes also become localized to the downshear half of the TC as VWS strengthens, with tornado location in strongly sheared TCs transitioning from the downshear-left quadrant in the TC inner core to the downshear-right quadrant in the TC outer region. Analysis of radiosondes shows that the downshear-right quadrant in strongly sheared TCs is most frequently associated with sufficiently strong near-surface speed shear and veering aloft, and lower-tropospheric thermodynamic instability for tornadoes. These supportive kinematic environments may be due to the constructive superposition of the ambient and TC winds, and the VWS-induced downshear enhancement of the TC circulation among other factors. Together, this work provides a basis for improving forecasts of TC tornado frequency and location.

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Benjamin A. Schenkel
,
Michael Coniglio
, and
Roger Edwards

Abstract

This work investigates how the relationship between tropical cyclone (TC) tornadoes and ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS) varies between coastal and inland environments. Observed U.S. TC tornado track data are used to study tornado frequency and location, while dropsonde and radiosonde data are used to analyze convective-scale environments. To study the variability in the TC tornado–VWS relationship, these data are categorized by both 1) their distance from the coast and 2) reanalysis-derived VWS magnitude. The analysis shows that TCs produce coastal tornadoes regardless of VWS magnitude primarily in their downshear sector, with tornadoes most frequently occurring in strongly sheared cases. Inland tornadoes, including the most damaging cases, primarily occur in strongly sheared TCs within the outer radii of the downshear-right quadrant. Consistent with these patterns, dropsondes and coastal radiosondes show that the downshear-right quadrant of strongly sheared TCs has the most favorable combination of enhanced lower-tropospheric near-surface speed shear and veering, and reduced lower-tropospheric thermodynamic stability for tornadic supercells. Despite the weaker intensity farther inland, these kinematic conditions are even more favorable in inland environments within the downshear-right quadrant of strongly sheared TCs, due to the strengthened veering of the ambient winds and the lack of changes in the TC outer tangential wind field strength. The constructive superposition of the ambient and TC winds may be particularly important to inland tornado occurrence. Together, these results will allow forecasters to anticipate how the frequency and location of tornadoes and, more broadly, convection may change as TCs move inland.

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Benjamin A. Schenkel
,
Roger Edwards
, and
Michael Coniglio
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John P. Monteverdi
,
Roger Edwards
, and
Gregory J. Stumpf

Abstract

This manuscript documents the tornado in the Rockwell Pass area of Sequoia National Park, California, that occurred on 7 July 2004. Since the elevation of the tornado’s ground circulation was approximately 3705 m (~12 156 ft) MSL, this is the highest-elevation tornado documented in the United States. The investigation of the storm’s convective mode was performed mostly inferentially on the basis of an analysis of the radar imagery from Edwards Air Force Base (which was in clear-air mode on this day), objectively produced soundings and/or CAPE estimates from two mesoscale models, an objectively produced proximity sounding and hodograph, and analyses of satellite imagery. The nearest Weather Surveillance Radar-1988 Doppler (WSR-88D) in Hanford, California, could not be used to observe this storm because of terrain blockage by the Sierra Nevada, and the nearest sounding sites were too distant and in a different meteorological environment on this day. The near-storm environment may have been favorable briefly for a supercell in the upper portion of the Kern River Canyon. The limitations of the radar data precluded the authors from making a definitive conclusion on the convective mode of the storm but do not rule out the possibility that the storm briefly might have been a supercell. There was insufficient evidence, however, to support the notion that the tornado itself was mesocyclone induced. High LCL heights in the proximity sounding also suggest that the tornado was formed by processes not associated with a mesocyclone (popularly known as a “landspout”), but do not allow us to dismiss the possibility that the tornado was mesocyclone induced.

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