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Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

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Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast SYNOPTIC-SCALE ANALYSIS OF TORNADO-PRODUCING TROPICAL CYCLONES ALONG THE GULF COAST Ariel E. Cohen NOAA/National Weather Service Weather Forecast Office Jackson, Mississippi Abstract Composite mean synoptic analyses are presented for seven different meteorological variables to distinguish between landfalling tropical cyclones along the coast of the Gulf of Mexico that are substantially tornadic (tropical cyclones that produce at least four tornadoes) and those that are not substantially tornadic (tropical cyclones that produce no more than one tornado). 30From tropical 1985 cyclonesto 2006, 15are landfalling considered tropical separately cyclones amongst were three identified sectors as along being the substantially Gulf Coast in tornadic, while an additional 15 were identified as being non-substantially tornadic. These were: (1) the southeastern United States was broadly located within the right entrance region computing mean synoptic variables. Some findings for substantially tornadic tropical cyclones of an upper-level jet streak centered over the northeastern United States; (2) organized, large, and directionally-symmetric 850-hPa flow was maximized in the northeastern semicircle of the cyclonic envelope; (3) a distinct, organized, and symmetric mean sea level pressure pattern over the Gulf was associated with the tropical cyclones; (4) a strong 600-hPa relative humidity commonalitiesgradient was present associated in the with northeastern the substantially semicircle tornadic of the tropical cyclone; cyclones and (5) werea dry notintrusion found wasto be found in the eastern semicircle in some cases of substantially tornadic tropical cyclones. These associated with the non-substantially tornadic tropical cyclones. Corresponding Author: Ariel Cohen National Weather Service,Flowood, Jackson MS 39232MS Weather Forecast Office 234 Weather Service Dr. Currently with the NOAA/NWSe-mail: [email protected] Storm Prediction Center, Norman, OK Cohen 1. Introduction with each Gulf of Mexico TC that makes landfall on the operational forecasters with a challenge to assess the United States coastline. The inputs for these models Numerous tropical cyclones (TCs) have tasked include TC intensity indicated by the 10-m wind speed, horizontal size of the cyclone measured by the radius of tornado risk as they make landfall. Tornadoes associated the outermost closed isobar, recurvature of the TC, and with TCs are known to result in a major loss of life formulationmiddle-tropospheric of their model specific that humidity incorporates measured all four by of the and property and account for 4% of the total deaths aforementioned500-hPa specific variableshumidity (i.e.,gradient a regression upon TC model landfall. named The associated with TCs that make landfall in the United Recon States (Rappaport 2000). However, some TCs have landfallbeen responsible along the for United more Statesprolific Gulf tornado Coast production and was ) was found to explain 70% of the actual tornado associatedthan others. with For anexample, extraordinary in 2004, 117 Hurricane tornadoes Ivan across made frequency associated with each landfall of a Gulf of Mexico TC during the period from 1998 to 2008. While Belanger et al. (2009) specify meteorological the United States, which were responsible for killing eight parameters to forecast TC tornado frequency with an withinindividuals the severe (Stewart convective 2005). stormsThese tornadoes research community associated emphasis on synoptic-scale variables, and other studies with landfalling TCs are receiving increasing attention document the mesoscale environment supporting TC environmentstornadoes; the favorable current forstudy producing is intended tornadoes to expand with andfor those ongoing reasons research (e.g., McCaul has focused 1991; Molinari on the andmesoscale Vollaro our qualitative understanding of the synoptic-scale environments2008; and Rasmussen associated and with Blanchard these tornadoes, 1998). Previous with a particular focus on the distributions of instability and welllandfalling as several TCs. other This studyones. exploresMore importantly, a handful ofthe variables present related to those investigated by Belanger et al. (2009), as is critical in pattern recognition, using reanalysis data shear influencing these environments. providedwork provides by the spatial National patterns Centers of these for variables, Environmental which The majority of tornadoes associated with TCs have been found to occur in the right-front quadrant of the cyclones in an environment characterized by relatively SuchPrediction a process (NCEP)/National provides the forecasterCenters forwith Atmospheric useful hints weak instability and strong low-level vertical wind shear asResearch to what (NCAR) spatial distributionsReanalysis Project of variables (Kalnay are et conducive al. 1996). mesocycloneswith the latter within providing the severehigh values local stormsof low-level are typically storm- for observed phenomena, including tornadoes associated relative helicity (Molinari and Vollaro 2008). As a result, shallow, with depths of around 3.5 km (Spratt et al. 1997). forecasterswith landfalling with TCs.tools to better anticipate tornadic events These mesocyclones are also transient and associated The overall goal of this work is to provide operational with weaker tornadoes. Most of the associated tornadoes are found within the outer rainbands of the TC, where toassociated produce with tornadoes. TCs and to Numerous distinguish synoptic those that charts are more are limited diurnal heating between rain bands increases low- likely to produce tornadoes from those that are less likely level instability. Previous work (i.e., Curtis 2004) has also these tools, and forecasters are encouraged to employ identified that mid-level dry air entrainment within the synopticprovided pattern in this work recognition from the in reanalysisthe forecast data process to develop when 700-500-hPa layer supports tornado outbreaks (defined as producing at least 20 tornadoes) for landfalling TCs substantial number of tornadoes. relativealong the humidity Atlantic andgradients Gulf of Mexicoand dry Coasts. intrusions, The present which identifying the likelihood for a landfalling TC to produce a study focuses on the 600-hPa level for the analysis of 2. Data and Methodology potential.is representative of the 700-500-hPa layer identified in CurtisWhile (2004) our as being understanding critical for analyzing of the tornadogenesis mesoscale investigated Table 1 providesin this study,a list ofincluding all substantially the name tornadic of the TCs (ST-TCs, those producing at least four tornadoes) areenvironment the characteristics supporting of tornadic the synoptic-scale TCs continues environment to grow, a major question remains somewhat unaddressed: what TC, its intensity at landfall, and the date that was used numberfor computing of tornadoes mean fields occurring in the in reanalysis. association Thiswith date the associated with tornadoes produced by landfalling TCs? corresponds to the period within 24 hours of the greatest Belanger et al. (2009) took some of the first steps at selected date generally encompassed the initial reports investigating this problem. They created two regression TC. For tornadoes occurring over longer durations, the models100 National to explain Weather the frequencyDigest of tornadoes associated Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast to represent preconditions supporting the tornado event. SeverePlot program (Hart and Janish 2006), which includes reports not produce such a risk. While entirely arbitrary, the Tornado reports were queried using the studythreshold is intended of four tornadoesto focus on is a smallerused in thissubset work of landfalling as a proxy Storm Data and the for substantial tornadic concern with a TC. The current from both digitized versions of the National Climatic searching through severe weather databases, the number anyData report Center database (NCDC) contains publication uncertainty, it is assumed ofTCs four that seemed are more to be prolific an appropriate tornado-producers, minimum thresholdand after Storm Prediction Center online database (2010). While of reported tornadoes from those associated with a lower that they can reasonably determine whether a TC is a to separate the TCs associated with a greater frequency substantial tornado-producer or not. Table 1 also lists the degree of tornado production from each of the ST- frequency. The strength of tornado played no role in inTCs the based tornado on the reporting aforementioned database reports. and potential These errordegrees in the definition of a tornadic TC, as most of the tornadoes are generalized into categories due to the uncertainties intensityspawned reports.by TCs are weak (F1 intensity or less) lending little opportunity for robust categorization of higher- associating reports with a particular TC. Plan view maps SeverePlotof tornado program.reports associated with each of the TCs listed For a TC to be considered non-substantially tornadic in Table 1 are provided in Fig. 1 based on output from the (NST-TC), the TC had to have produced no more than one tornado. These cases are listed in Table 2. However, A minimum of four tornadoes was required for a TC tornadoesgiven the substantially-reducedwere produced in these visibilities cases but with were greater not to be considered a ST-TC. This threshold is intended to low-level moisture in TC environments, it
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