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Tornado Outbreaks Associated with Landfalling Hurricanes in the North

Atlantic Basin: 1954–2004

STEPHANIE M. VERBOUT

, DAVID M. SCHULTZ

2,3

, LANCE M. LESLIE

HAROLD E. BROOKS

, DAVID KAROLY

AND

KIMBERLY L. ELMORE

2,3

School of Meteorology, University of Oklahoma, Norman, Oklahoma

NOAA/National Severe Storms Laboratory, Norman, Oklahoma

Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman,

Oklahoma

Submitted as an article for the Meteorology and Atmospheric Physics Special Issue on Tropical

Cyclones

Submitted 10 June 2006, Revised 14 August 2006

Corresponding author address

: Stephanie M. Verbout, University of Oklahoma, School of

Meteorology/National Weather Center, 120 David L. Boren Blvd, Norman, OK 73072

E-mail: stephanieverbout@hotmail.com

ABSTRACT

Tornadoes are a notable potential hazard associated with landfalling hurricanes. The

purpose of this paper is to discriminate hurricanes that produce numerous tornadoes (tornado

outbreaks) from those that do not (nonoutbreaks). The data consists of all hurricane landfalls

that affected the United States from the North Atlantic basin from 1954 to 2004 and the United

States tornado record over the same period. Because of the more than twofold increase in the

number of reported tornadoes over these 51 years, a simple least-squares linear regression (“the

expected number of tornadoes”) was fit to the annual number of tornado reports to represent a

baseline for comparison.

The hurricanes were sorted into three categories. The first category, outbreak hurricanes,

was determined by hurricanes associated with the number of tornado reports exceeding a

threshold of 1.5% of the annual expected number of tornadoes and at least 8 F1 and greater

tornadoes during the time of landfall (from outer rainbands reaching shore to dissipation of the

system). Eighteen hurricane landfalls were classified as outbreak hurricanes. Second, 37

hurricanes having less than 0.5% of the annual expected number of tornadoes were classified as

nonoutbreak landfalls. Finally, 28 hurricanes that were neither outbreak nor nonoutbreak

hurricanes were classified as midclass hurricane landfalls.

Stronger hurricanes are more likely to produce tornado outbreaks than weaker hurricanes.

While 78% of outbreak hurricanes were category 2 or greater at landfall, only 32% of

nonoutbreak hurricanes were category 2 or greater at landfall. Hurricanes that made landfall

along the southern coast of the United States and recurved northeastward were more likely to

produce tornadoes than those that made landfall along the east coast or those that made landfall

along the southern coast but did not recurve. Recurvature was associated with a 500-hPa trough

1. Introduction

Wind damage, storm surges, and inland flooding are hazards faced by coastal communities

in the southern and eastern United States due to landfalling hurricanes. Neumann et al. (1999)

found that the United States Atlantic and Gulf coasts experience an average of 1.7 landfalling, or

near-landfalling, hurricanes per year. Hurricane-related property losses accounted for nearly

40% of all insured losses in the United States from 1984 to 1993, exceeding earthquake losses by

a factor of four (Pielke and Pielke 1997).

A hurricane with watches or warnings in effect is considered a land-threatening hurricane

(Franklin et al. 2003). Tornadoes are a notable potential hazard associated with land-threatening

hurricanes. Ten percent of hurricane-related fatalities from 1948 to 1972 were attributed to

tornadoes near or during the time of landfall (Novlan and Gray 1974). Hurricane Allen’s (1980)

landfall in Texas produced well over $70 million in damage associated with hurricane-spawned

tornadoes alone (Gentry 1983). Other examples of tornadoes associated with tropical systems are

described in Gray (1919), Hills (1929), Tannehill (1938, 24–25), Malkin and Galway (1953),

Sadowski (1962), Rudd (1964), Pearson and Sadowski (1965), Orton (1970), Hoadley (1981),

Stiegler and Fujita (1982), McCaul (1987), Grazulis (1993, 124–127), McCaul et al. (2004), and

Watson et al. (2005).

Emergency managers typically treat landfalling hurricanes differently than tornadoes.

When a hurricane threatens landfall, coastal communities are evacuated inland to large buildings

with free-spanning roofs that can house many people, such as school gymnasiums and armories.

On the other hand, these types of buildings are more susceptible to collapse in a tornado. This

quandary proves problematic for organizations such as the Red Cross and the Federal Emergency

Management Agency, who work to shelter those evacuated from coastal regions.

Most tornadoes generated by hurricanes occur within 24 h of landfall (e.g., Hill et al. 1966;

Novlan and Gray 1974; Gentry 1983; McCaul 1991). Although some tornadoes can occur within

the inner rainbands or in the eyewall (“core” tornadoes; Gentry 1983; Weiss 1987; McCaul

1991), most tornadoes form in the outer rainbands where convection can be strong (e.g., Hill et

al. 1966; Novlan and Gray 1974). For example, Weiss (1987) found that 74% of postlandfall

tornadoes from 1964 to 1983 were generated by outer-rainband convection.

Tornadoes spawned by hurricanes can have different characteristics than tornadoes

spawned by midlatitude supercells or squall lines. For instance, the parent mesocyclones in

hurricanes tend to have shallower circulations than those associated with midlatitude supercells

in the central United States (average depth of 3–3.5 km vs 6 km, respectively) (McCaul 1987,

1991; McCaul and Weisman 1996; Spratt et al. 1997; McCaul et al. 2004). McCaul and

Weisman (1996) argued that the lack of strong surface cold pools (a source of horizontal

vorticity) associated with supercells in the moist tropical environment may limit the production

of strong tornadoes by tilting. Traditional tornadic radar signatures (e.g., bounded weak echo

regions, hook echoes, and appendages) may be subtle, or even nonexistent for hurricane-

spawned tornadoes (Spratt et al. 1997). In addition, some of the tornado-producing supercells

during Tropical Storm Beryl (1994) did not contain any cloud-to-ground lightning (McCaul et al.

2004). Finally, hurricane-related tornadoes tend to be weaker, and have half the path width and

path length as tornadoes formed by other systems (Smith 1965).

McCaul (1991) examined the number of tornadoes and thermodynamic properties

associated with all tropical cyclones (defined as all hurricanes, tropical storms, tropical

depressions, and subtropical storms) from 1948 to 1986, grouping tornado outbreaks into three

classifications:

minor

(not more than 8 tornadoes),

major

(more than 8 tornadoes), and

severe

(more than 24 tornadoes). He found that 413 of the 626 total tornadoes (66%) were produced by

18 major tornado outbreaks, of which four were severe.

One of the most prominent tornadic hurricanes was Hurricane Beulah (1967) (Orton 1970).

Beulah made landfall near Brownsville, Texas, and produced 117 tornadoes in the southern

portion of the state (Fig. 1). The majority of tornadoes occurred on 20 September when 67

tornadoes were reported with Beulah’s landfall (Orton 1970). Of those 67 tornadoes, 21 were

rated F1 and greater on the Fujita scale. Almost 40 years passed since any hurricane produced

nearly 100 tornadoes at landfall. Data from the Storm Prediction Center (SPC) showed that

Hurricane Frances (2004) produced 99 tornadoes and Hurricane Ivan (2004) produced 117

tornadoes (e.g., Watson et al. 2005). The large number of tornadoes associated with Beulah

(1967), Frances (2004), and Ivan (2004) clearly demonstrates the need to examine and

characterize these types of events for future hurricane preparedness.

The purpose of this paper is to examine the differences between outbreak and nonoutbreak

hurricane landfalls. Section 2 describes the dataset used in this study. Section 3 discusses our

criteria for identifying days with many tornadoes, as determined by the method in Verbout et al.

(2005). Section 4 outlines the three types of hurricanes identified by this study. Characteristics

such as intensity of the hurricane, landfall location, synoptic pattern, hurricane origin, date of

landfall, and El Niño–Southern Oscillation (ENSO) phase are analyzed in section 5.

Lastly,

section 6 summarizes the conditions favoring tornado outbreaks associated with landfalling

hurricanes.

2. Data

A main concern surrounding any tornadic event is the accuracy and availability of reports

(e.g., Pearson 1975; Doswell and Burgess 1988; Grazulis 1993, 187–195; Hagemeyer 1998;

Hagemeyer and Spratt 2002). This problem is amplified because hurricane-force winds, heavy

rains, and storm surge can obscure tornado verification. The present study inspected all tornado

reports from the United States tornado database (McCarthy 2003) from 1954 to 2004 two days

prior and four days following the landfall of a hurricane, similar to the criteria chosen in Weiss

(1987) and Fig. 15 in McCaul (1991). Because Weiss (1985) found 85% of tornadoes associated

with tropical systems were attributed to hurricane-strength storms, we chose to focus strictly on

hurricanes (category 1 or greater on the Saffir–Simpson scale). All landfalling hurricanes that

affected the United States originating from the North Atlantic basin from 1954 to 2004 were

identified from the National Hurricane Center/Tropical Prediction Center (NHC/TPC) archives.

These hurricanes included not only those that made landfall in the United States, but also those

that made landfall in northeastern Mexico and produced tornadoes in the United States.

Tornadoes were considered related to the hurricane if the tornado report occurred within 400 km

of the cyclone center. This distance is consistent with Spratt et al.’s (1997) definition of

maximum range. This definition may need to be expanded for future investigations to account

for Hurricane Katrina (2005) because of the extremely large, unparalleled size of Katrina’s outer

rainbands. At the time of this study, the 2005 hurricane season was ongoing and official

hurricane and tornado data had not been verified.

3. Defining a big tornado day

As stated earlier, the large number of tornadoes associated with Hurricanes Beulah (1967),

Frances (2004), and Ivan (2004) clearly demonstrates the need to examine these types of events

for future hurricane preparedness. With the aim of studying tornado outbreaks associated with

landfalling hurricanes, we first examined the daily United States tornado record from 1954 to

2004. Verbout et al. (2005) defined a “big tornado day” as a single day where numerous

tornadoes and/or many tornadoes exceeding a specified intensity threshold were reported. This

term was chosen with the intention of distinguishing a big tornado day from previous

applications of the word outbreak. Overall, the total number of tornadoes in the United States

tornado database (McCarthy 2003) had doubled from roughly 600 per year in the 1950s to

around 1200 per year in the 2000s (Fig. 2). A least-squares linear regression was fit to the

annual number of reported tornadoes over the period 1954–2004 in order to adjust for the general

increase in reports (Verbout et al. 2005). Such a procedure allows comparison of the tornado

record through the decades. Traditionally, the number of F2 and greater tornadoes has been used

to determine the significance of a tornado event. However, Verbout et al. (2005) demonstrated

that over this time period, the F2 and greater series had some overrating problems early in the

dataset. Therefore,

the F1 and greater time series is more stationary over this period in

comparison with the F2 and greater series.

Thus, a big tornado day is determined by a fraction

of the annual expected number of tornadoes associated with the linear regression and/or a

minimum number of F1 and greater tornado reports (Verbout et al. 2005).

In order to assess the daily probability of tornado occurrence in the United States, Verbout

et al. (2005) constructed a statistical model of the daily mean number of tornadoes reported from

1954 to 2004 using a kernel density estimation technique with a Gaussian smoother (

= 15

days) (Brooks et al. 2003). This technique was used to smooth the data in time and space and

estimate the probability of any tornado occurring anywhere in the United States on any given day

(Fig. 3; dashed line). The peak probability of 90% chance of any tornado occurring in the United

States is near June 12 (Brooks et al. 2003). However, once the data is constrained by thresholds

with the purpose of determining a big tornado day (for example, 1.5% of the linear regression

value and at least 8 F1 and greater tornadoes), the probability distribution changes. The black

curve in Fig. 3 illustrates the probability of these days from 1954–2004 and shows that the

largest peak occurs in mid-May, nearly three weeks earlier in the year than the any-tornado

curve. Hence, big tornado days are more likely to occur slightly earlier in the year than just any

day with a tornado. [See section 3e in Verbout et al. (2005) for further discussion of this point.]

Tornadoes embedded within hurricanes can occur within a one-day period (e.g. Hurricane

Edith 1971) to over a three-day period (e.g. Hurricane Frances 2004). Because a landfalling

hurricane can spawn tornadoes over several days, this paper considers the number of tornadoes

over the duration of landfall (from outer rainbands reaching shore to dissipation of the system) as

a natural temporal limit.

4. Hurricane classifications

In order to determine if an outbreak hurricane occurred, thresholds must again applied to

the tornado data. As again discussed in Verbout et al. (2005), thresholds must be chosen

arbitrarily depending on the number of events one wishes to analyze.

For this study, a threshold

of 1.5% of the linear regression value and at least 8 F1 and greater tornadoes

was chosen to

distinguish between outbreak and nonoutbreak hurricanes in order to create a sizeable outbreak

dataset. As a result, a total of 83 hurricane landfalls were classified as

outbreak

(18 cases),

nonoutbreak

(37 cases), or

midclass

(28 cases) hurricanes.

4.1 Outbreak hurricanes

Hurricanes with tornado reports exceeding a threshold of 1.5% of the annual expected

number and at least 8 F1 and greater tornadoes were classified as

outbreak

hurricanes. For

instance, the 1.5% annual expected value for 1954 was 8; therefore, at least 8 tornadoes were

needed for an outbreak (Fig. 4). For 2003, however, the 1.5% annual expected value was 18

(Fig. 4). These thresholds generated 18 outbreak hurricane landfalls (Table 1). When the 18

outbreak hurricane cases were plotted in Fig. 3 (again using the kernel density estimation

technique; gray curve), the peak occurs in early to mid-September (most active hurricane time in

the North Atlantic basin). These results show that the outbreak hurricane tornadoes peak

probability (gray curve; ~0.005) comprises nearly all the daily big tornado day signal (1.5% and

8 F1+ black curve; ~0.006) for early to mid-September. Thus, if a big tornado day occurred in

early to mid September, chances are it occurred within an outbreak hurricane.

Eleven of the 18 cases defined by McCaul (1991) are included in this study, while seven

additional storms since 1986 have been added to this dataset (Gilbert 1988, Andrew 1992, Opal

1995, Georges 1998, Frances 2004, Ivan 2004, and Jeanne 2004). Hurricane Alicia (1983) was

not considered an outbreak hurricane in our study because only two of the total 22 reported

tornadoes were rated F1 or greater. Similarly, Curtis (2004) created a dataset of 13 outbreak

hurricanes from 1960 to 1999 based on 20 or more reported tornadoes (Galway 1975, 1977).

Ten of the 13 cases in Curtis’s (2004) dataset appeared in our dataset— the excluded three were

Hurricane Alicia (1983), described above, and Tropical Storms Beryl (1994) and Josephine

(1996). Beryl (1994) and Josephine (1996) were omitted from this dataset because of the strict

requirement of the storm needing to be of hurricane strength (according to Saffir–Simpson scale)

at some point in its lifetime. Additionally, Curtis’s (2004) dataset did not include Hurricanes

Audrey (1957), Cleo (1964), Hilda (1964), and Babe (1977) because of his higher threshold that

required at least 20 tornadoes. Finally, 10 of the 13 (77%) outbreaks identified by Curtis (2004)

occurred in the last 25 years (1979 to 2004), indicating that the temporal increase in reports (Fig.

2) was perhaps reflected in his dataset.

4.2 Nonoutbreak hurricanes

Hurricanes with less than 0.5% of the annual expected number of tornado reports were

classified as

nonoutbreak

hurricane landfalls (Table 2). For example, no more than three

tornadoes could have occurred in 1954, and no more than six in 2004 (Fig. 4). From 1954 to

2004, there were 37 cases of nonoutbreak hurricane landfalls. Several of these cases include

hurricanes that made multiple landfalls and either had no reported tornadoes or very few (Table

2). Note that five nonoutbreak cases (Diane 1955, Ione 1955, Cindy 1959, Gloria 1985, Charley

1986) narrowly made landfall and quickly turned back out to sea, allowing little time for the

hurricane to produce tornadoes over land (Table 2). Most recently, 1999 had three nonoutbreak

hurricane landfalls (Bret, Dennis, and Irene). Bret (1999) was the first hurricane to make landfall

on the Texas coastline since Hurricanes Chantal and Jerry (1989) and the first intense hurricane

(category 3 or greater on Saffir–Simpson scale) to hit Texas since Hurricane Alicia (1983).

4.3

Midclass hurricanes

Many landfalling hurricanes produced several tornadoes but could not be labeled strictly

outbreak or nonoutbreak. Those cases that fell between the two thresholds were classified as

midclass

hurricanes. There were 28 midclass hurricanes identified from 1954 to 2004 (Table 3).

A few of these midclass hurricanes failed to make outbreak classification because of too few F1

and greater tornadoes reported. For example, Hurricanes Alicia (1983) and Lili (2002) both had

the minimum number of tornadoes needed (22 and 26, respectively), yet not enough F1 and

greater tornadoes were reported (2 and 5, respectively). On the other hand, Hurricanes Isbell

(1964) and Juan (1985) did have 8 F1 and greater reported tornadoes, but the 9 and 12 total

tornadoes (respectively), however, fell short of the 1.5% criterion (Fig. 4). Another midclass

case is Hurricane Gracie (1959). Gracie only had six reported tornadoes associated with landfall,

but three of the six tornadoes were rated F3.

5. Discriminating between outbreak and nonoutbreak hurricanes

Of the 83 hurricane landfalls in this study (Tables 1–3), only 14 (17%) had no reported

tornadoes. Of those 14, only four have occurred since 1973, suggesting that early tornado

records, especially for weaker tornadoes, may be incomplete (e.g., Smith 1965; Hill et al. 1966;

Gentry 1983; Hagemeyer 1998; Verbout et al. 2005). Thus, the threat of tornadoes faces

forecasters with nearly every hurricane. What determines the conditions for especially prolific

tornado-producing hurricanes when tornado watches may be required? In this section, we

examine various characteristics that may prove useful in discriminating outbreak cases from

nonoutbreak cases. These include the intensity of the hurricane, landfall location, synoptic

pattern, hurricane origin, date of landfall, and ENSO phase.

5.1 Hurricane intensity

Previous research has indicated that stronger hurricanes are more likely to produce

tornadoes than weaker ones (Hill et al. 1966; Novlan and Gray 1974; Gentry 1983; Weiss 1985;

McCaul 1991). In this study, the Saffir–Simpson scale, a measure of the maximum near-surface

wind speed, was used as a measure of hurricane intensity. The Saffir–Simpson scale ranges from

1 to 5, with 5 having the strongest winds. A category 0 was included in this study to account for

sub hurricane-force winds. Most hurricanes are on the low end of the Saffir–Simpson scale. For

example, whereas 34 of 83 hurricane landfalls were category 1 strength at landfall, only two

hurricanes were category 5 at landfall: Hurricane Camille (1969; nonoutbreak case) and

Hurricane Andrew (Florida 1992 landfall; nonoutbreak case) recently upgraded to a category 5

(Landsea et al. 2004).

Figure 5 shows the distribution of Saffir–Simpson category of each classification of

hurricane (outbreak, nonoutbreak, or midclass) at landfall. Forty hurricanes were of at least

category 2 at landfall and 43 were rated categories 1 or 0. Thirty-five percent (14 of 40) of all

category 2 or greater hurricanes at landfall were associated with outbreaks, whereas 30% (12 of

40) were associated with nonoutbreaks (Fig. 5). In addition, 78% (14 of 18) of outbreak

hurricanes were category 2 or greater at landfall, and only 32% (12 of 37) of nonoutbreak

hurricanes were category 2 or greater at landfall (Tables 1 and 2). In order to assess the

statistical significance of hurricane intensity as a discriminator between outbreak and

nonoutbreak cases, a chi-square test was used. In essence, the chi-square test tests associations

between two variables and can determine if the association is stronger than random chance alone

(i.e., statistically significant; Wilks 1995, 133–134). The chi-square test statistic depends on the

degrees of freedom. At one degree of freedom, a statistically significant relationship exists

between outbreak and nonoutbreak hurricanes and Saffir–Simpson categories 1 and 0 and

categories 2 and higher at landfall. A

value equal to 0.002 was found; thus indicating that

intensity is a significant parameter at

=0.01 level. Clearly, these results show that outbreak

hurricanes are more likely to occur if the hurricane is a category 2 or higher at landfall. On the

other hand, hurricane intensity at time of landfall is not the only variable that should be

considered when assessing the risk of tornado outbreaks embedded within the system.

5.2. Landfall location

Previous authors have suggested that hurricane landfalls over the southern United States

are more likely to produce tornadoes than those making landfall over the eastern United States

(e.g., Hill et al. 1966; Novlan and Gray 1974; Gentry 1983; Weiss 1985; McCaul 1991). Such a

relationship, however, was not apparent from the figures comparing tracks for tornado-producing

hurricanes with tracks of non-tornado-producing hurricanes presented by Novlan and Gray

(1975). We wished to examine our dataset for such a purported relationship.

Landfall locations for the 83 hurricanes in our dataset were separated into two categories:

southern-coast landfalls (Texas, Louisiana, Mississippi, Alabama, and west coast of Florida) and

eastern-coast landfalls (east coast of Florida, Georgia, South and North Carolina, Virginia, and

New England states). Overall, 14 of 51 (27%) landfalls on the southern coast were outbreaks,

whereas 4 of 32 (13%) landfalls on the eastern coast were outbreaks. Again, using a chi-square

test with one degree of freedom, a

statistically significant relationship exists between outbreak

and nonoutbreak hurricanes and landfall location at

=0.10 level (

value=0.06).

Tracks of the various hurricanes for outbreak, nonoutbreak, and midclass categories were

plotted in Fig. 6. When presented in this manner, it is clear that outbreak hurricanes tended to

make landfall on the southern coast compared to the eastern coast (Fig. 6a). In addition, nearly

all landfalling outbreak hurricanes recurved northeastward after landfall, supporting the results of

Smith (1965) and Hill et al. (1966). In contrast, nonoutbreak hurricanes tended to fall into one of

two categories (Fig. 6b). Although the majority of nonoutbreak hurricanes tended to be those

that made landfall on the eastern coasts and recurved northward, there were a lesser group of

hurricanes that made landfall over the southern coast, but did not recurve (Fig. 6b). Midclass

hurricanes were more evenly divided between southern and east coast landfalls (Fig. 6c). When

the distinction is drawn between hurricanes that made landfall on the southern coast that

recurved versus hurricanes that made landfall on the southern coast that did not recurve, the

statistics produce a different result. In this case, 10 of 21 (48%) hurricanes that made landfall on

southern coasts and recurved were outbreaks, whereas 4 of 21 (19%) hurricanes that made

landfall on southern coasts and did not recurve were outbreaks.

This presentation suggests that

simple rules of thumb such as “landfalling hurricanes on the east coast do not produce

tornadoes” can be a serious oversimplification of the actual results.

To understand why recurving hurricanes that made landfall on the southern coast are more

likely to produce tornado outbreaks, we need to understand why hurricanes spawn tornadoes.

Most tornadoes in hurricanes are spawned in the right-front quadrant of the hurricane relative to

the direction of motion of the hurricane (e.g., Malkin and Galway 1953; Pearson and Sadowski

1965; Smith 1965; Orton 1970; Novlan and Gray 1974; Gentry 1983; McCaul 1991; Hagemeyer

and Hodandish 1995), the quadrant where the vertical wind shear and helicity tend to be most

favorable for tornado production (McCaul 1991; Bogner et al. 2000). Thus, hurricanes that place

their right-front quadrant over land for a longer period of time are more likely to produce greater

numbers of tornadoes. Hurricanes approaching southern United States coastlines have more time

to interact with land surfaces and have more exposure for the preferred right half of the hurricane

to generate tornadoes (Sadowski 1962; Smith 1965). Most east coast landfalls, on the other

hand, do not penetrate far inland, recurve back over the ocean more quickly, and are less likely to

produce a large number of tornadoes (Hill et al. 1966; Gentry 1983; McCaul 1991). One notable

exception is Hurricane Beulah (1967), which produced 117 tornadoes while tracking cyclonically

(Fig. 1). Hagemeyer and Hodanish (1995) found that 87% of tornado-producing hurricanes that

made landfall in Florida approached from the west coast of Florida, which placed the right-front

quadrant over the state and thus provided more opportunity for the hurricane to produce

tornadoes over land (e.g., Hill et al. 1966). As a result, whether storms recurve over land appears

to be an important distinction in order to determine whether or not an outbreak may occur. To

address the nature of recurving, we look to the next section.

5.3 Synoptic patterns for landfalling hurricanes that affected Texas

As mentioned previously, the ability of hurricanes making landfall over the southern

United States to produce a tornado outbreak apparently is tied to its ability to recurve

northeastward. To explore the reason for this recurvature, we investigate the differences in

composite synoptic patterns between outbreaks and nonoutbreaks associated with landfalling

hurricanes that affected Texas. We chose to perform this composite on Texas hurricanes for two

reasons. First, such hurricanes are relatively abundant in the outbreak and nonoutbreak

categories (5 and 8, respectively). Second, these hurricanes reach landfall in a relatively focused

geographic region, arriving in a nearly consistent storm track from the southeast (Figs. 6a, b).

(Developing such a composite analysis for Florida hurricanes was not robust for this reason.)

These two criteria allow for the construction of meaningful composite patterns. Composite

analyses on the day of landfall were produced using the NOAA–CIRES Climate Diagnostics

Center daily-averaged NCEP/NCAR Reanalysis (Kalnay et al. 1996) web page

(http://www.cdc.noaa.gov/Composites/Day). Composite analysis using the 6-h data did not

produce significantly different results and are not considered here.

Of the five outbreaks that affected Texas, there was one category 3 and four category 4.

In contrast, of the eight nonoutbreaks, all were category 1 hurricanes, except one category 4 and

one category 0. Thus, the synoptic composite might also provide some insight into why the

outbreak storms were more intense than the nonoutbreaks.

Although several different composite parameters show differences between outbreaks

and nonoutbreaks, we illustrate these differences with 500-hPa geopotential height and surface–

850-hPa wind shear parameters (Fig. 7). Composite 500-hPa geopotential height indicates a

substantial difference between the Texas outbreaks and nonoutbreaks at landfall (Figs. 7a, b).

Whereas the Texas outbreak composite has a 500-hPa trough in the north-central United States,

the nonoutbreak composite has a ridge where the jet stream is far poleward of the landfalling

hurricanes (Figs. 7a, b). The anomalies of these composite means (based on 1968–1996

climatology) extend over a large area in the northern United States and southern Canada (Figs.

7c, d). Furthermore, the 500-hPa height anomaly over the landfalling hurricane is over twice as

deep, with a gradient of geopotential height (geostrophic wind) twice as great, for the outbreak

events (Figs. 7c, d).

Each of the five outbreak events had a trough in the northern United States and each of

the eight nonoutbreak events had a ridge in the northern United States (not shown), supporting

the composites. However, there remain two questions: 1) is the difference between the means at

each grid point significant (local significance), and 2) does the collection of grid points with

mean differences have field significance? Elmore et al. (2005) presented a nonparametric

approach to this problem. Unlike the data considered in Elmore et al. (2005), however, serial

correlation does not need to be considered in our data. Thus, a permutation test (Efron and

Tibshirani 1995; Elmore et al. 2002) is more accurate than a bootstrap test of the difference

between the means. The permutation test is performed at each grid point using 10 000 replicates.

All grid points for which the difference between the means is significant at the

= 0.05 level

are counted and divided by the total number of grid points, which yields the proportion of the

grid containing significant differences. Following Elmore et al. (2005), spatial correlation is

accommodated using a Monte Carlo process first described in Livezey and Chen (1989), and

dubbed the B method by Wang and Shen (1999). The B method for this work uses 10 000 trials

and is performed by combining the two fields (5 outbreaks and 8 nonoutbreaks) into a single data

matrix with 925 rows and 13 columns. Hence, 13 values are associated with each grid point. The

correlation between a uniform random variate at each grid point, as described in Elmore et al.

(2005), is tested for significance at

= 0.05. The proportion of grid points for which a

significant correlation occurs purely by chance yields the minimum proportion of grid points that

must possess significant difference between the means for field significance at

= 0.05. For

the height data, 14.38% of the gridpoints display statistically significant mean differences, and

12.97% are needed for field significance. Thus, the difference between these two fields is

significant at the

= 0.05 level.

These results suggest that landfalling hurricanes that affected Texas require greater deep-

layer (surface–500-hPa) shear over the right-front quadrant in order to spawn a large number of

tornadoes, as noted by Wills (1969). This greater deep-layer shear is due to several factors. First,

the greater geostrophic wind anomaly at 500-hPa for the outbreak composite (Fig. 7c) suggests

that greater deep-layer shear is present compared with the nonoutbreak events (Fig. 7d). Second,

the jet stream dipping equatorward over the central United States toward Texas (Fig. 7a)

suggests that some interaction between the trough and the hurricane may be helping to produce

more intense hurricanes at landfall. As reviewed in Jones et al. (2003, their section 3d), a

moderate amount of shear associated with midlatitude troughs in the westerlies is sometimes

believed to be responsible for hurricane intensification. For nonoutbreak hurricanes, the jet is

much farther poleward, effectively isolating the landfalling storm underneath a large-scale 500-

hPa ridge (Fig. 7b) in much weaker deep-layer shear. Finally, the differences in the 500-hPa

flow over the southeastern United States result in greater deep-layer shear in the outbreak cases

than the nonoutbreak cases. The results from these composites support previous research

showing that the presence of large deep-layer shear is important for the generation of midlevel

mesocyclones in supercell thunderstorms (e.g., McCaul and Weisman 1996; McCaul et al. 2004),

which can spawn tornadoes in the hurricane environment.

Wills (1969), Novlan and Gray (1974), and McCaul (1991) also found that high low-level

wind shear was present in tornado-producing hurricanes. The surface–850-hPa (hereafter, low-

level) wind shear also shows considerable differences between the outbreak and nonoutbreak

composites (Figs. 7e, f). The mean low-level wind shear is 4–8 m s

-1

in an onshore direction

northeast of the landfall location for the outbreak events (Fig. 7e), but is about half as strong (2–

4 m s

-1

) for the nonoutbreak events (Fig. 7f). To test the statistical significance of this data, only

4.0% of the data display significantly different shear values, yet as many as 7.4% could display a

significant difference by chance alone. Hence, the difference between the surface–850-hPa shear

for the outbreak and nonoutbreak cases does not possess field significance.

If we increase

0.10 from 0.05, then field significance is obtained with

= 0.05.

Thus, the synoptic composites bring some insight into the potential causes for the

outbreaks: greater deep-layer shear and greater low-level shear, both associated with more

intense hurricanes, favors mesocyclogenesis and tornadogenesis, respectively. What causes the

recurvature? The composite of outbreak hurricanes that affected Texas suggests that a trough in

the jet stream (Fig. 7a) may transport them northeastward, whereas the nonoutbreaks remain

isolated from the jet (Fig. 7b). To test this hypothesis on a different set of cases, we examine the

strongly recurving nonoutbreak hurricanes over the eastern United States. Seven cases were

chosen from Table 2 and their 500-hPa geopotential height fields were composited (Fig. 8).

These results show that a 500-hPa trough lies to the northwest of the hurricane at landfall (Figs.

8a, c), which, by a day after landfall, has moved eastward (Figs. 8b, d). These results are

reminiscent of the northwest composite of extratropical transition over the western North Pacific

noted by Harr et al. (2000). Furthermore, the results of Fig. 8 support those of the synoptic

composites from the Texas storms in Fig. 7 that hurricane interaction with the jet stream results

in recurvature. Whereas this interaction and recurvature may lead to tornado outbreaks in the

Texas cases, the right-front quadrant tends to be offshore in the east-coast recurvature cases, so

tornadoes are not likely to be observed over land, if they are even produced at all.

To summarize, simple statistical tests on the location of landfall only provide part of the

explanation for whether an outbreak occurs. When the statistical methodology is combined with

synoptic composites, physical insight is obtained. Despite the relative success of these

composites on the synoptic scale, mesoscale and storm-scale effects may produce an

environment favorable for tornadoes or a tornado outbreak in an atypical synoptic-scale

environment, as might have been the case with Beulah’s unusual track.

Before leaving this discussion, this paper is not the first attempt to relate the track of

landfalling hurricanes to tornadogenesis. Novlan and Gray (1975) plotted landfall location and

tracks for hurricanes that produced tornadoes versus those that did not. They did not see as clean

a distinction as we did. This apparent discrepancy suggests that our approach of distinguishing

outbreak from nonoutbreak is more successful than attempting to distinguish tornado from no-

tornado.

5.4 Hurricane origin and date of landfall

Three areas of the North Atlantic Basin were identified as hurricane origin sites. First, the

Atlantic Ocean was defined as the area north and east a diagonal line from Key West, FL,

southeastward towards Cuba, Haiti, and the Dominican Republic ending near Trinidad and

Tobago. Next, the Caribbean Sea was defined as the area south and west of the same diagonal

line and south of ~23°N latitude near the Yucatan Peninsula. Last, the Gulf of Mexico was

identified as the area north and west of the Yucatan Peninsula and Cuba and waters west of Key

West, FL. The Atlantic Ocean is the largest and most active body of water and produced 38

United States hurricane landfalls from 1954–2004. Furthermore, the Caribbean Sea and Gulf of

Mexico formed 34 hurricane landfalls that affected the United States. Hurricane origin was

investigated as a possible parameter that may distinguish between outbreak and nonoutbreak

hurricanes, yet no significant relationship was found.

Additionally, date of landfall was inspected in order to determine if outbreak and

nonoutbreak hurricanes favor a certain time of year. To distinguish between outbreak hurricanes

and nonoutbreak hurricanes, Fig. 9 shows the ranked distribution of landfall date for each

classification. There appears to be a small gap in outbreak hurricanes between June and August

(Fig. 9), consistent with Hagemeyer (1997) who showed that the months of July and August had

a relative deficiency in tornado reports for Florida peninsular outbreaks. One possible

explanation for this shortage was noted by Hagemeyer and Schmocker (1991). They found that,

in the middle of the wet season (July and August in Florida), tornado environments were

characterized by weak lower-tropospheric winds and very low shear; consequently contributing

to the lack of tornadogenesis in the region. Even though date of landfall was not found to be a

statistically significant parameter in distinguishing between outbreak and nonoutbreak

hurricanes, it is interesting to note the general lack of outbreak cases in July.

5.5 El Niño-Southern Oscillation (ENSO) phase

There have been many studies on the effect of ENSO on hurricanes in the North Atlantic

basin (e.g., Gray 1984; Bove et al. 1998; Pielke and Landsea 1999; Larson et al. 2005). These

studies show that El Niño events suppress hurricane and tropical storm activity because of the

abnormally strong upper-tropospheric westerlies in the western Atlantic and Caribbean. La Niña

events, on the other hand, increase the number of hurricanes and tropical storms in the Atlantic

basin, increase the frequency of damaging landfalling storms, and increase the amount of

damage per storm. Hagemeyer (1999) found the relationship between ENSO and hurricane-

spawned tornadoes in Florida to be unclear. To attempt to resolve some of these discrepancies,

we examined the effect of ENSO on tornado outbreaks associated with hurricanes with our

dataset.

Rasmusson and Carpenter (1982) demonstrated that sea-surface temperature (SST)

anomalies can identify the onset of an El Niño event in late spring to early summer, and the peak

in SST anomalies occur in December, January, and February (i.e., Northern Hemisphere winter).

To determine the phase of ENSO during the months of peak hurricane activity (August,

September, and October), we used the Climate Prediction Center’s Oceanic Niño Index (ONI)

for

Niño

3.4

SST

anomalies

based

the

1971–2000

period

(http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml). The

ONI indicates an El Niño event when the Niño 3.4 SST anomaly is greater than or equal to

0.5

C, and a La Niña event when the anomaly is less than or equal to –0.5

C. Anomalies

between –0.5

C and 0.5

C are considered neutral years. Figure 10 illustrates each hurricane

classification and the respective El Niño, neutral, or La Niña phase during August, September,

and October of the hurricane year. More hurricanes occurred during non El Niño years (26 in

neutral phases; 28 in La Niña phases) than in El Niño years (18), in agreement with previous

studies (e.g., Gray 1984; Bove et al. 1998; Pielke and Landsea 1999; Larson et al. 2005). Six

outbreak hurricanes occurred during El Niño events and seven occurred during La Niña events.

Therefore, if a landfalling hurricane occurred during an El Niño year, the event had a higher

chance of being associated with a tornado outbreak (6 outbreaks of 18, or 33%), as opposed to if

the hurricane occurred during non El Niño (neutral or La Niña) years (12 outbreaks of 54, or

22%). However, a statistically significant relationship was not found between outbreak and

nonoutbreak hurricanes and El Niño versus non-El Niño years using a chi-square test and one

degree of freedom (

value=0.26). A much longer period of observations will be required to

assess whether this relationship is significant or just due to random variations and the small

sample size.

6. Summary and Conclusions

This study examined all landfalling hurricanes that affected the United States from 1954 to

2004. Criteria developed by Verbout et al. (2005) was used to examine days with a large number

of tornadoes across the United States (big tornado days) and later to develop a dataset of tornado

outbreaks associated with hurricanes and nonoutbreak hurricanes. The results of the present

study, as well as a review of the previous literature, indicates the following characteristics are

capable of distinguishing landfalling hurricanes that produce tornado outbreaks from those that

do not.

More intense hurricanes are more likely to produce tornadoes (Novlan and Gray 1974;

Gentry 1983; McCaul 1991; this study). Hurricanes that are weakening are more likely to

produce tornadoes (Hill et al. 1966; Novlan and Gray 1974), although this effect may be more

likely due to the fact that most hurricanes are weakening as they approach land.

Previous studies have argued that hurricanes recurving to the northeast were more likely to

produce tornadoes than those moving westward (Smith 1965; Hill et al. 1966; Novlan and Gray

1974). By exception, one of the most prolific tornado-producing storms, Hurricane Beulah

(1967), did not behave in this manner (Fig. 1). Although there is some skepticism about the

validity of this rule (Novlan and Gray 1974), nearly all our outbreak cases (Fig. 6a) exhibited

recurvature over the southern or eastern United States. By contrast, nearly all our nonoutbreak

cases (Fig. 6b) did not recurve or recurved along or off the eastern United States. Composite

synoptic analyses for landfalling hurricanes that affected Texas show that distinct synoptic

patterns distinguish outbreaks from nonoutbreaks due to the location of the jet stream.

Outbreaks were associated with a 500-hPa trough over the north-central United States, allowing

the jet stream to dip equatorward over the central United States (Fig. 7a). With such a synoptic

pattern, landfalling hurricanes that affected Texas experienced recurvature quickly (Fig. 6a).

Nonoutbreaks, on the other hand, were associated with a 500-hPa ridge over the north-central

United States and a jet stream in southern Canada (Fig. 7b). As such, recurvature was less likely,

and the tracks of Texas nonoutbreak hurricanes showed a westward or northwestward track once

onshore (Fig. 6b).

Hurricanes that recurve along or off the eastern United States do not produce tornado

outbreaks because their right-front quadrant is not over the land for an extended period of time

(Hill et al. 1966; Novlan and Gray 1974; McCaul 1991; this study). Most tornadoes are found in

the right-front quadrant relative to the motion of the hurricane (Malkin and Galway 1953;

Pearson and Sadowski 1965; Smith 1965; Orton 1970; Novlan and Gray 1974; Gentry 1983;

McCaul 1991). This quadrant is where the greatest deep-layer shear and helicity are found

(McCaul 1991; Bogner et al. 2000), typical ingredients for mesocyclogenesis and

tornadogenesis, respectively. Air parcels in the right-front quadrant also have a recent history of

being over water, and the right-front quadrant is where convection tends to be strongest (e.g.,

Gentry 1983). Outbreak hurricanes have stronger low-level wind shear than nonoutbreak

hurricanes (Wills 1969; Novlan and Gray 1974; McCaul 1991; this study). This result is

consistent with the requirement for strong low-level wind shear for tornadogenesis.

Gray (1984), Bove et al. (1998), Pielke and Landsea (1999), Larson et al. (2005) and this

study have shown that Atlantic basin hurricanes tend to occur during non El Niño years. The

number of outbreak hurricanes, however, was nearly evenly split between El Niño and La Niña

years. This study also found the following characteristics were not useful for distinguishing

outbreaks from nonoutbreaks: storm origin and date of landfall.

The results of this study allow forecasters to identify the factors that affect tornado

outbreaks with landfalling hurricanes, with the goal of being better able to anticipate these

events. As our results show, the statistics suggest better discrimination between outbreak and

nonoutbreak hurricanes rather than the occurrence of tornadoes or no tornadoes. This effect may

explain some contradictory results in the previous literature.

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TABLE CAPTIONS

Table 1. Outbreak cases listed by landfall date. Hurricane name, landfall date, Saffir–Simpson

category at time of landfall, total number of reported tornadoes, and number of F1 and greater

tornadoes are given.

Indicates landfall that affected Texas (section 5c); * indicates multiple

landfalls and location.

Table 2. Same as Table 1 except for nonoutbreak cases. ! Indicates hurricanes that narrowly

made landfall;

indicates landfall that affected Texas;

indicates east coast recurver (section

5c).

Table 3. Same as Table 1 except for midclass cases.

FIGURE CAPTIONS

Fig. 1. Track of Hurricane Beulah (1967) from 20–22 Sept. Hollow circles indicate center of

circulation at 0000 UTC on each day. Thin line denotes a distance of 100 nm from shore.

Individual tornado reports are marked with small plus signs.

Fig. 2. Annual number of reported tornadoes (triangles) and annual number of F1 and greater

reported tornadoes (circles) from 1954 to 2004. Black line and dashed line indicate linear

regression fit to each series.

Fig. 3. Dashed line (scale on left side of graph) is the probability of having any tornado reported

somewhere in the United States by day of year, based on the 1980–1999 record (Brooks et al.

2003). Solid black line (black scale on right side of graph) displays the probability of a single

day having at least 1.5% of the expected annual value and at least 8 F1 and greater tornadoes

reported, based on the 1954–2004 record. Solid gray line (gray scale on right side of graph)

displays the probability distribution of all outbreak hurricane cases from 1954–2004. Solid black

curve is scaled by a factor of 9 and solid gray curve is scaled by a factor of 30. Curves were

generated using kernel density estimation with a Gaussian smoother with

= 15 days (Brooks et

al. 2003).

Fig. 4. Graph of the number of tornadoes associated with 1.5% of the linear regression value

(triangles) and 0.5% of the linear regression value (circles) for each year (1954–2004).

Fig. 5. Number of outbreak (black), midclass (gray hatched), and nonoutbreak (gray) hurricanes

listed by Saffir–Simpson category (1-5) at time of landfall. Category 0 represents storms that

were not hurricane strength at time of landfall.

Fig. 6. Tracks of hurricanes that affected the United States (1954–2004). (a) outbreak hurricane

tracks; (b) nonoutbreak hurricane tracks; and (c) midclass hurricane tracks.

Fig. 7. Composite fields for landfalling hurricanes that affected Texas with tornado outbreaks

(left) and nonoutbreaks (right) (Tables 1 and 2). (a) and (b) mean 500-hPa geopotential height

(every 25 m); (c) and (d) anomaly 500-hPa geopotential height from 1968–1996 climatology

(every 10 m); and (e) and (f) mean surface–850-hPa wind-shear magnitude (shaded every 2 m s

) and direction (vectors). Composite maps were provided by the NOAA–CIRES Climate

Diagnostics Center, Boulder, Colorado (http://www.cdc.noaa.gov).

Fig. 8. Composite fields for east coast landfalling hurricanes associated with nonoutbreaks for

(a) and (c) day of landfall and (b) and (d) day after landfall (Table 2). (a) and (b) mean 500-hPa

geopotential height (every 25 m); (c) and (d) anomaly 500-hPa geopotential height from 1968–

1996 climatology (every 10 m). Composite maps were provided by the NOAA–CIRES Climate

Diagnostics Center, Boulder, Colorado (http://www.cdc.noaa.gov).

Fig. 9. Ranked distribution of outbreak (triangles) and nonoutbreak (circles) hurricanes by date

from 1954 to 2004.

Table 1. Outbreak cases listed by landfall date. Hurricane name, landfall date, Saffir–Simpson

category at time of landfall, total number of reported tornadoes, and number of F1 and greater

tornadoes are given.

Indicates landfall that affected Texas (section 5c); * indicates multiple

landfalls and location.

Name

Date of Landfall Cat. @ Landfall

Total Tors

F1+

Audrey

6/27/1957

Carla

9/11/1961

Cleo

8/27/1964

Hilda

10/4/1964

Beulah

9/20/1967

117

Edith (LA) *

9/16/1971

Agnes (FL) *

6/19/1972

Babe

9/5/1977

David

9/3/1979

Allen

8/10/1980

Danny

8/15/1985

Gilbert

9/16/1988

Andrew (LA) *

8/26/1992

Opal

10/4/1995

Georges

9/28/1998

Frances

9/5/2004

Ivan

9/16/2004

117

Jeanne

9/26/2004

Table 2. Same as Table 1 except for nonoutbreak cases. ! Indicates hurricanes that narrowly

made landfall;

indicates landfall that affected Texas;

indicates east coast recurver (section

5c).

Name

Date of Landfall Cat. @ Landfall

Total Tors

F1+

Alice

6/25/1954

Hazel

10/15/1954

Diane

8/17/1955

Ione

9/19/1955

Cindy

7/9/1959

Debra

7/25/1959

Donna (S. FL)*

9/10/1960

Donna (CT)*

9/12/1960

Cindy

9/17/1963

Dora

9/10/1964

Isbell (NC)*

10/16/1964

Betsy (S. FL)*

9/8/1965

Alma

6/9/1966

Gladys

10/19/1968

Camille

8/18/1969

Fern (LA)*

9/4/1971

Fern (TX)*

9/10/1971

Edith (S. TX) *

9/14/1971

Agnes (NY) *

6/22/1972

Diana

9/13/1984

Bob (S. FL)*

7/23/1985

Bob (SC)*

7/24/1985

Gloria

9/27/1985

Kate

11/21/1985

Charley

8/17/1986

Florence

9/10/1988

Chantal

8/1/1989

Hugo

9/22/1989

Bob (MA)*

8/19/1991

Andrew (FL) *

8/24/1992

Fran

9/6/1996

Bret

8/23/1999

Dennis

9/5/1999

Irene

10/15/1999

Gordon

9/18/2000

Claudette

7/15/2003

Isabel

9/18/2003

narrowly made

landfall

Texas landfall

east coast recurver

* multiple landfall

Table 3. Same as Table 1 except for midclass cases.

Name

Date of Landfall Cat. @ Landfall

Total Tors

F1+

Connie

8/12/1955

Flossy

9/24/1956

Gracie

9/29/1959

Donna (NC)*

9/11/1960

Ethel

9/15/1960

Isbell (S. FL)*

10/14/1964

Betsy (LA)*

9/10/1965

Celia

8/3/1970

Carmen

9/8/1974

Eloise

9/23/1975

Bob

7/11/1979

Frederic

9/13/1979

Alicia

9/18/1983

Elena

9/2/1985

Juan

10/29/1985

Bonnie

6/26/1986

Jerry

10/16/1989

Bob (NC)*

8/18/1991

Gordon

11/16/1994

Allison

6/5/1995

Erin

8/2/1995

Bertha

7/13/1996

Danny

7/18/1997

Bonnie

8/27/1998

Floyd

9/16/1999

Lili

10/3/2002

Charley (S. FL)*

8/13/2004

Charley (SC)*

8/14/2004

* multiple landfall