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Home , Landfall, Tornado

SYNOPTIC-SCALE ANALYSIS OF -PRODUCING TROPICAL ALONG THE GULF COAST

Ariel E. Cohen

NOAA/National Service Weather Forecast Office Jackson,

Abstract

Composite mean synoptic analyses are presented for seven different meteorological variables to distinguish between landfalling tropical cyclones along the coast of the 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 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 ; 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] Prediction Center, Norman, OK Cohen

1. Introduction

with each Gulf of Mexico TC that makes 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 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 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 States prolific 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 for study 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 forecaster Centers forwith Atmospheric useful hints weak instability and strong low-level vertical 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 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 of the TC, where toassociated produce with tornadoes. TCs and to Numerous distinguish synoptic those that charts are more are limited diurnal heating between 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 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 fieldsoccurring in the in reanalysis.association Thiswith datethe 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 and the for substantial tornadic concern with a TC. The current from both digitized versions of the National Climatic searching through 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 threshold and after 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 is possible that discriminate between landfalling TC cases where there is a more substantial tornado risk from those which do reported. The date of analysis listed in Table 2 for Name Intensity at Landfall Date of Region Degree of (including hurricane category Analysis Tornado on the Saffir-Simpson Production Hurricane Wind Scale) Jerry Hurricane (1) 10/15/1989 WEST A Dean T.S. 7/30/1995 WEST A Frances T.S. 9/10/1998 WEST B Fay T.S. 9/6/2002 WEST B Emily Hurricane (3) 7/20/2005 WEST B

Danny Hurricane (1) 8/16/1985 CENTRAL D Juan Hurricane (1) 10/28/1985 CENTRAL B Andrew Hurricane (3) 8/25/1992 CENTRAL D Bill T.S. 6/30/2003 CENTRAL B Rita Hurricane (3) 9/24/2005 CENTRAL D

Agnes Hurricane (1) 6/18/1972 EAST C Florence T.S. 9/10/1988 EAST A Isidore T.S. 9/25/2002 EAST A Ivan (1st Hurricane (3) 9/15/2004 EAST D U.S. landfall) Alberto T.S. 6/12/2006 EAST C

Table 1.

ST-TC list providing the name, intensity at landfall, date of analysis, and region of landfall, leastalong 31 with tornado degree reports). of tornado production (“A” represents approximately 4-7 tornado reports, “B” represents 8 to 15 tornado reports, “C” represents 16 to 30 tornado reports, “D” represents at Volume 34 Number 2 ~ December 2010 101 Cohen

Fig. 1. .

Tornado reports (in red) associated with each of the TCs listed in Table 1

102 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

Name Intensity at Landfall Date of Region (including hurricane category Analysis NST-TCs corresponds to the period within on the Saffir-Simpson .24 hours of the TC landfall and generally Hurricane Wind Scale) when the TC was over land for the longest Chantal* Hurricane (1) 8/1/1989 WEST Arlene* T.S. 6/21/1993 WEST are Thestudied tornado separately reports for arethree evaluated regions regionally, such that the mean fields Beryl* T.S. 8/15/2000 WEST cases into regions was chosen to reduce Bertha (TX) T.D. 8/9/2002 WEST along the Gulf Coast. The separation of Grace T.S. 8/31/2003 WEST across the entire Gulf region when averagingthe amount variables of smoothing amongst of meanthe various fields Beryl T.S. 8/9/1988 CENTRAL cases. Figure 2 provides an approximate depiction of the spatial considerations for Danny Hurricane (1) 7/18/1997 CENTRAL Ivan as being a member of a particular region, (3rd U.S. T.S. 9/24/2004 CENTRAL this study. For an ST-TC to be considered landfall) (i.e., tornadoes occurring within 360 the majority of its associated tornadoes Matthew* T.S. 10/9/2004 CENTRAL of landfall) must have occurred in that Arlene T.S. 6/11/2005 CENTRAL km of the TC center and within one day to most consistently capture the aerial Kate Hurricane (2) 11/22/1985 EAST region. The distance of 360 km seemed Barry* T.S. 8/6/2001 EAST extent of the synoptic-scale influence Henri* T.D. 9/6/2003 EAST associatedof the TCs tornadoconsidered occurred in this (i.e., study. tornado The Tammy* T.D. 10/5/2005 EAST region for each NST-TC was where the one Claudette* T.S. 8/17/2009 EAST didoccurring not produce within any360 reportedkm of the tornadoes, TC center Table 2. theand reported within one region day correspondsof landfall). Ifto thewhere TC NST-TC list providing the name, intensity at landfall, date of analysis, and region of landfall (an asterisk indicates one tornado reported in association with 1the and TC 2.made Each landfall. of the Theregions regions in Fig. are 2also is representedthe TC). by ellipses,shown for and each the of regional the TCs centroid listed in will Tables often be referenced and assimilates numerous observational datasets (e.g., upperdimensional air sounding variation data (3DVAR) including data assimilation and scheme wind scale features. when describing distances from fundamental synoptic- predictors for tornadoes associated with landfalling velocity, vertical temperature soundings from NOAA To develop the spatial analysis of potential waspolar performed orbiters, satellite-derived every six hours at synoptic from geostationary (0000 satellites, and surface observational data). The reanalysis Gulf of Mexico TCs, the present study uses the NCAR/ grid, and the average of the four hours of data from each inNCEP a triangular reanalysis truncation, data. This as reanalysiswell as 28 projectvertical useslevels the in UTC, 0600 UTC, 1200 UTC, and 1800 UTC) on a 2.5° x 2.5° NCEP global spectral model, which includes 62 waves day were used to create daily mean plots. These were then sigma coordinates. The analysis scheme uses a three- further averaged to create mean composite synoptic fields

Fig. 2.

Graphical view of regional sectorization for (the regions from left to right are west (a), central (b), and east (c)) (map backgrounds from Storm PredictionVolume Center 34 2010). Number 2 ~ December 2010 103 Cohen

sea level pressure, etc.) are certainly directly related to for Gulf Coast landfalling TCs and to make comparisons between substantially tornadic TCs (ST-TCs) and non- itsthe spatial TC itself, scale the atinfluence least approaches of the TC onthe many synoptic mass scale and analysis.substantially tornadic TCs (NST-TCs). Two individual ST- kinematic fields is typically expansive enough such that TC cases are also presented for additional illustration and 3. Discussion (i.e., several hundred to several thousand kilometers). Thus, a major influence governing the synoptic-scale a. Mean synoptic fields factors.environment More in importantly, the vicinity a of distinction the TC tornadoes is being ismade the hereTC itself, between and it isthe prudent tornado, to considerfor which these the larger-scale associated

1) 200-hPa flow toflow be fields of a substantially are considered larger to be scale. of “storm Arguably, scale,” in and some the TC, for which the associated flow fields are considered Figure 3 provides the mean 200-hPa wind vector circulation and lower pressures and heights around the fields across the United States with the left column of the smaller TCs, although the size of the cyclonic middleindicating row ST-TCs, (b) central and the region right columncases, andcontaining the bottom NST- TCs. The top row (a) includes western region cases, the TC are small enough to be characterized as “mesoscale,” supportivethese cases of are tornadoes. being considered as “synoptic-scale” row (c) eastern region cases. This general format is to highlight the larger-scale kinematic environment followed for all remaining figures through Fig. 8. The Coast,most remarkable which is particularly feature in evident Fig. 3 is amongst the placement the central and The first of the variables which shows a definitive andstrength eastern of the region 200-hPa cases jet (b streak and c). relative For tothese the Gulftwo response to the presence of the TC is the 850-hPa flow , as seen in Fig. 4. The most noticeable difference between the ST-TC and NST-TC cases is the considerably placesregions, the the centerrespective of the inland jet streak locations is approximately over the larger size and greater organization and directional southeast1100 km United northeast States of in the the regional favorable centroids, right entrance which symmetry of the 850-hPa storm-induced cyclonic wind field in the tornado events, in addition to the stronger ascending branch of the ageostrophic circulation in 850-hPa flow in the northeast semicircle of the cyclonic region of the jet streak. In this entrance region, the envelope. Neither the strength nor organization of the positive advection and upper level divergence. 850-hPa flow is necessarily proportional to the size of the jet entrance region is driven by strong differential the storm. Thus, the importance of cyclone size and especiallyorganization as more associated historical with studies ST-TCs (e.g., are Novlan important and Note the southward placement of this jet streak in the findings, consistent with those in Belanger et al. (2009), tornado events as opposed to the non-tornado events. strength of the storm as being a critical determining This pattern is not as apparent in the western region Gray 1974) have placed more of an emphasis on the events (Fig. 3a). However, for the western region ST-TCs southernthe weak Louisianajet maximum in the over right , entrance centered region around of this factor for tornadogenesis. These mean fields also show 750 km east of the regional centroid, places portions of the strongest 850-hPa flow is found in the northeastern semicircle of the cyclone in a fixed reference frame. south,jet streak. and In is contrast,displaced the considerably corresponding to the jet east maximum of the Since none of these calculations take into account the in the non-tornado events is oriented more north-to- motion of any storm, this finding suggests that tornado risk identification on a storm-sector basis could be more western region. The former jet pattern is somewhat Hillefficiently et al. (1966), done in and a Cartesian-basedboth can provide reference similar results. frame reminiscent of an enhanced subtropical . Additionally,than a storm-relative comparison reference between frame the tornadoas suggested reports in

2) 850-hPa flow tornadoes occurred within the northeastern semicircle in Fig. 1 and ST-TC tracks (not shown) suggests that most In addition to considering the synoptic-scale environment unaffected by the TC, there are critical sof the ST-TC tracks. The strength of the 850-hPa wind synoptic-scale mass and kinematic field responses to the maximum-1 is also important, with speeds of-1 10 to 14 m TC, which, in turn, influence the propensity for a TC to be for the ST-TCs and speeds of 6 to 10 m s for the NST- a substantial tornado-producer. While the variables that TCs. Continued page 107 describe this response (e.g., 850-hPa flow field, mean 104 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

Fig. 3(a).

Fig. 3(b).

Fig. 3(c).

Fig. 3.

Mean 200-hPa flow for ST-TCs (left column) and NST-TCs (right column) for west (a), central (b), and east (c) regions. Shading depicts in m s-1. Arrows indicate .

Volume 34 Number 2 ~ December 2010 105 Cohen

Fig. 4(a).

Fig. 4(b).

Fig. 4(c).

Fig. 4. regions. Shading depicts wind speed in m s . Arrows indicate wind direction. Mean 850-hPa flow for ST-TCs (left -1column) and NST-TCs (right column) for west (a), central (b), and east (c)

106 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

3) 850-200-hPa bulk shear In particular, the specific reanalysis hour (i.e., 00Z, were also computed (not shown), and no substantial respectively,06Z, 12Z, or was 18Z) used closest to compute to the tornado the radii. reports For cases and The mean 850-200-hPa mean bulk shear vectors wherelandfall the on outermostthe date of closed analysis isobar listed was in Tablesmore elliptical1 and 2, as opposed to circular, half the lengths of the minor axes differences were found in this variable between the ST- were chosen to represent the radii. For these cases, the TC and NST-TC environments. This suggests that large- scale, deep-layer shear plays an inconsequential role in enhancing the propensity of a TC to produce tornadoes. minor axes appeared to more consistently vary from TC foundThis isto consistentbe relatively with shallow the (e.g., finding Spratt that et al. tornado- 1997) to TC than the major axes, which is why the minor axes andproducing thus unable convection to strengthen in TC environments due to the presence has been of were chosen for the analysis of the ROCI. Amongst the ST-TCs, the average ROCI was around 390 km, whereas the average ROCI amongst the NST-TCs was around deep-layer shear. 230 km; and these differences highlight the spatial differences between the ST-TCs and NST-TCs. 4) Mean sea level pressure In addition to investigating TC size, we can also further gauge the average strength amongst the ST- The importance of system size and symmetry are investigatedTCs and the as NST-TCs. they made To do landfall this, numerical (i.e., tropical ranks also apparent in the mean sea level pressure (MSLP) were assigned to the intensities for each of the TCs fields, which are plotted in Fig. 5. Of particular note, the area of lower pressure associated with the ST-TCs depression assigned a rank of 1, tropical storm a rank of appears to be well-defined and symmetric, as opposed 2, category 1 hurricane a rank of 3, category 2 hurricane to the broader trough in the NST-TCs. Also of note, a rank of 4, and category 3 hurricane a rank of 4). Using mean minimum pressures below 1006 hPa are noted the TC intensities listed in Tables 1 and 2, the mean in each of the tornado events with the MSLP minima rank amongst the ST-TCs is 3.1 and is 2.1 amongst the within 500 km of the regional centroids. The pressure NST-TCs, thus highlighting the ST-TCs as being stronger gradient appears to be maximized over the northeast than the NST-TCs upon landfall. Both the larger size semicircle of the cyclone in the ST-TCs, approximately and greater strength of the ST-TCs explain the ability most500 km of thefrom tornado the mean reports center were of the found TCs, to which occur isin also the for the NCAR/NCEP reanalysis fields to better resolve where the 850-hPa flow is also strongest. Once again, their circulations as seen in some of the figures. This, in itself, is an important finding, in that the large-scale northeastern semicircle of the ST-TCs. influence of the NST-TCs is less pronounced than for ST- As with the 850-hPa flow pattern, the larger-scale TCs. MSLP distribution is driven by the presence of the aTC, simple which gradient forces wind the kinematic balance would field suggest that can that locally the 5) 600-hPa relative humidity enhance the tornado potential with TCs. In particular, with a more pronounced easterly component, where Figure 6 displays the mean 600-hPa relative aforementioned gradient would drive a low-level flow humidity fields, which reveal a spatially-broad, yet strong, horizontal gradient in mid-level moisture storm-relative helicity would be maximized due, in part, over the northwestern semicircle of the cyclone. The scaleto low-level circulation veering plays of a flow.crucial While role inhelicity locally typically enhancing is strongest gradient is found around 300 km northwest considered to be a mesoscale variable, the synoptic- envelopof the centerthe storm of theintruding 600-hPa into TC-inducedits eastern semicircle moisture inmaximum. the central In and,fact, tothis a drysmaller air is degree, found toin cyclonicallythe eastern these helicity values around the tornado. This maximization in storm-relative helicity is consistent with results discussed in Molinari and Vollaro (2008). region cases (Fig. 6b-c). This intrusion extends within This pressure gradient is substantially weaker in the the500 numerouskm of the tornadoescenter of thereported circulation. in association This process with NST-TCs, particularly in the west and central regions occurred with and likely contributed to (Fig. 5a-b). To further investigate the similar occurrences of Ivan (Baker et al. 2009). larger TC size being associated with ST-TCs, as opposed This gradient was also found by Curtis (2004), and to NST-TCs, the radius of the outermost closed isobar supports the theory that the presenceContinued of a mid-levelpage 110 (ROCI) for each TC was computed from the reanalyses. Volume 34 Number 2 ~ December 2010 107 Cohen

Fig. 5(a).

Fig. 5(b).

Fig. 5(c).

Fig. 5.

Mean MSLP for ST-TCs (left column) and NST-TCs (right column) for west (a), central (b), and east (c) regions. Shading depicts MSLP in hPa.

108 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

Fig. 6(a).

Fig. 6(b).

Fig. 6(c).

Fig. 6. and east (c) regions. Shading depicts relative humidity in percent. Mean 600-hPa relative humidity for ST-TCs (left column) and NST-TCs (right column) for west (a), central (b),

Volume 34 Number 2 ~ December 2010 109 Cohen

dry intrusion, evident in thermodynamic soundings,

the non-tornado events. This feature is consistent with likely contributes to the development of cold pools in finding ST-TCs to be larger and stronger than NST-TCs. whichstorms canlocated further in the deepen outer rainbands.the lift in Thesethe presence cold pools of The positive upper-level temperature perturbation thecan unstableenhance theand horizontalparticularly vorticity moist boundary in the low layer levels, of associated with the warm core of the ST-TCs is also more pronounced at 200 hPa than for NST’s, again reflecting the stronger and larger ST-TCs as seen in Fig. coveragethe outer of rainbands. convection Also,therefore the intrusion increasing of insolation. mid-level 7. Both 500-hPa height minima and 200-hPa warm core dry air can increase low-level buoyancy by limiting the centers are generally associated with the MSLP minima, environment can also be downscaled to smaller scales reflecting the vertically-stacked nature of the landfalling The presence of this dry air in the synoptic-scale b. TCs.Individual cases it is speculated that the dry air could enhance the to explain local convective enhancements. In particular, further investigate the synoptic environments associated aidingdegree in of the potential tornadogenesis instability process. and also While enhance the relative rear- with The the NCEP/NCAR approach of Reanalysis Hurricane Project Rita towarddata were southeast used to effectsflank downdrafts of evaporational (RFDs) cooling associated due to withthe presence of

are not investigated in the present study, it is speculated (analyses at 0000 UTC on 24 September 2005) dry air and adiabatic warming of RFDs as they descend theand aforementioned the approach ofcriteria, Tropical both Storm of these Alberto systems toward are Florida (analyses at 1800 UTC on 12 June 2006). Using that the dry, relatively denser air associated with ST-TCs enhance RFDs and tornadogenesis through associatedconsidered upper ST-TCs. level Figure divergence 9 highlights in its right the presenceentrance low-levelMinimum vorticity relative stretching humidities and transport,west of the as wellstrong as of the northeastern United States upper level jet, with low-level convergence. theregion strongest supporting core synoptic-scalewinds are found ascent to andbe relatively potential gradient fall into the 25 to 35% range. Interestingly, dislocatedlarge-scale fromdestabilization the initial forlandfalling the tornado area, events. these windsWhile this strong gradient is present in both the ST-TCs and are found to gradually trail off toward the south, which NST-TCs for the eastern region (Fig. 6a). However, as in all tornado events, the size of the relative humidity enhance convective activity. enhancement associated with the TC appears to span could still support sufficient upper level divergence to a much larger region in as compared to the NST- (Fig.TCs. 6b).The relative humidity gradient is less defined in semicircle Figure of10 Hurricane illustrates Ritathe well-defined,(Fig. 10a), along broad with relative a dry the NST-TCs, particularly for the central region events humidity gradient at 600-hPa in the northwestern dry intrusion is less pronounced, but still present for intrusion into its eastern semicircle from the south. The 6) 500-hPa height field and 200-hPa temperature field stillTropical present Storm in the Alberto northern (Fig. semicircle. 10b); and the orientation of Mid- to upper-level mass and thermodynamic fields the relative humidity gradient is tilted quasi-zonally, but respectively.were investigated, Figure and7 reveals the 500-hPa a typical heightsynoptic field pattern and 200-hPa temperature fields are plotted in Figs. 7 and 8, The 850-hPa pattern (Fig. 11) for both the , with an upper ridge over the and Tropical Storm Alberto is fairly similar to that seen during hurricane , with a mid-level trough over in the mean fields for ST-TCs with a relatively symmetric Stormand well-organized Alberto, though height both and cases wind feature field. However, the strongest some Plains. However, the depression in the 500-hPa heights eccentricity is noted in the 850-hPa pattern for Tropical in the vicinity of the TC is more pronounced in the ST- envelope. Continued page 112 TCs with a closed 5825 geopotential meter contour in flow in the northeastern semicircle of the cyclonic mean fields for the ST-TCs, and no closed contours for

110 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

Fig. 7(a).

Fig. 7(b).

Fig. 7(c).

Fig. 7. east (c) regions. Shading depicts height in geopotential meters. Mean 500-hPa heights for ST-TCs (left column) and NST-TCs (right column) for west (a), central (b), and

Volume 34 Number 2 ~ December 2010 111 Cohen

Fig. 8(a).

Fig. 8(b).

Fig. 8(c).

Fig. 8. for west (a), central (b), and east (c) regions. Shading depicts temperature in degrees Celsius. Mean 200-hPa for ST-TCs (left column) and NST-TCs (right column)

Conclusions

Much previous research has been performed to detail of the Gulf of Mexico from 1985 to 2006 were identified the buoyancy and shear distributions, as well as the as substantially tornadic TCs (ST-TCs), producing at least four tornadoes, while an additional fifteen were identified as being non-substantially tornadic TCs (NST-TCs), mesoscale environments, supporting tornado outbreaks producing no more than one tornado. Each of the fifteen associated with landfalling tropical cyclones (TCs). The cases was classified as occurring in one of three sectors purpose of this study is to fit these mesoscale environments twoadjacent event to types the Gulffor each coast, sector. and several mean composite in the context of the synoptic-scale environment. synoptic fields were computed to discriminate among the 112 FifteenNational landfalling Weather DigestTCs along the United States coast Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

Fig. 9(a). Fig. 9(b).

Fig. 9(c). Fig. 9(d).

Fig. 9.

Flow at 200 hPa (a and b) and MSLP (c and d) in the bottom row during. Arrows the indicate approach wind of Hurricane direction. Rita (left column) and the approach of Tropical Storm Alberto (right column). The black-1 dot depicts the approximate center of the surface circulation associated with the TC. Shading depicts wind speed in m s

Fig. 10(a). Fig. 10(b).

Fig. 10.

relative humidity Relative humidityin percent. at 600 hPa during the approach of Hurricane Rita (a) and the approach of Tropical Storm Alberto (b). The black dot depicts the approximate center of the surface circulation associated with the TC. Shading depicts Firstly, the right entrance region of an enhanced

United States was found to enhance the tornado potential organized, larger, directionally-symmetric, with the and 200-hPa jet streak centered over the northeastern strongest flow found in the northeast semicircle of the associated cyclonic envelope. In this region, low-level with TCs across the southeastern United States. Any storm-relative helicity is maximized, enhancing the upper-level jet streak associated with the NST-TCs was symmetrictornadogenesis mean potential.sea level pressure Likewise, pattern composite over the mean Gulf substantially weaker. In the lower levels, the 850-hPa sea level pressure fields show a distinct, organized, and flow associated with the ST-TCs was found to be more Volume 34 Number 2 ~ December 2010 113 Cohen

Fig. 11(a). Fig. 11(b).

Fig. 11. . Arrows indicateFlow at 850wind hPa direction. during the approach of Hurricane Rita (a) and the approach of Tropical Storm Alberto (b). The black-1 dot depicts the approximate center of the surface circulation associated with the TC. Shading depicts wind speed in m s

Author in a tight pressure gradient supporting strong easterly associated with the ST-TCs. This leaves the Gulf States Ariel Cohen gradient winds further enhancing low-level storm relative recently became a Mesoscale Assistant/ helicities. A much weaker pressure gradient is identified Weather Forecaster at the Storm Prediction Center for the non-tornado events. Also, the ST-TCs were found after working as a General Forecaster at the National humidityto be larger gradient and stronger is foundthan the over NST-TCs. the northeastern Weather Service Weather Forecast Office (WFO) in Additionally, the presence of a 600-hPa relative theJackson, Graphical Miss., Forecast since August Editor 2009. for a At wide the WFOarray Jackson,of local he was involved with numerous projects to enhance semicircle of the cyclonic envelope. The dry air driving forecasts. He was also involved in a handful of local this gradient has been speculated to enhance low-level interests including convective, fire, , and lake weather buoyancy in the vicinity of rain bands through mid-level dry air entrainment. ST-TCs are associated with deeper projects and case studies with an emphasis on severe circulations extending above 500-hPa with evidence of convection. Prior to working in Jackson, Ariel was a stronger 200-hPa positive temperature perturbations. Surface Analyst and Marine Forecaster with the Tropical Individual case studies highlight these characteristics in thatAnalysis developed and Forecast the Graphical Branch Forecast of the National Editor for Hurricane national particularlythe mean fields. valuable to forecasters is that the features centerCenter marinein Miami, domains Florida. and There, led research he worked initiatives with ain team What makes the results in the present work to Miami, Ariel started his characterizing the synoptic-scale environments careerwind forecasting as a General in the Forecaster Gulf of California. at the Weather Prior to Forecast moving theseassociated environments, with ST-TCs the were results not foundcan provide to exist guidance in the NST- to TC environments. Because of the differences between StateOffice University in Great Falls, in 2006, Montana he received in 2007. his AfterM.S. degree receiving in scaleoperational patterns forecasters presented in inanticipating this study the with potential observations for ST- his B.S. degree in Atmospheric Sciences from The TCs. Forecasters are encouraged to compare the synoptic- where he performed research with the National Severe from the University of in 2008 and model forecasts of the patterns associated with land- falling TCs as a means for determining their potential to Laboratory in storm electrification modeling. be substantially tornadic. Future work could develop cluesquantitative for which guidance variables for evaluating could TCbe tornadoinvestigated potential in developingbased on large-scale such guidance. patterns; and this study provides

114 National Weather Digest Synoptic-Scale Analysis of Tornado-Producing Tropical Cyclones Along the Gulf Coast

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1954–1978. Acknowledgments

Hurricane Center in Miami, Florida, for numerous discussions and motivation to further I thank the staff of the National Weather Service in Jackson, Mississippi, and at the National

study synoptic environments associated with land-falling TCs along the Gulf of Mexico Coast, particularly Mr. Alan Gerard and Mr. Greg Garrett of the National Weather Service Jackson, Mississippi; Dr. Christopher Landsea of the National Hurricane Center in Miami, Florida; and Mr. Russell Pfost, formerly of the National Weather Service in Miami, Florida. I also thank Laura Kanofsky of the National Weather Service in St. Louis, , for her review of this work, along with all editors for their input in bettering this work. All of this feedback greatly improved the quality of this work.

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