CLIMATE &
HURRICANE
REPORT
BROWNSVILLE, TEXAS
Comparative Assessment of Hurricane Threat,
Historical Patterns, and Established Flood
Protection Mechanisms for the Brownsville, Texas Area
By: Jude A. Benavides
TABLE OF CONTENTS
Introduction.....................................................................................................................................4
Section I: Comparison of Hurricane Strike Probabilities and Return Periods
along the U.S. Atlantic and Gulf of Mexico Coastlines......................................................................6
Section II: Important Factors Explaining the Reduced Threat of Impact
from Tropical Cyclones for the South Texas Area.............................................................................12
Section III – Brief Discussion of Coastal versus Inland Risk
for Historical Storms Impacting the Area...........................................................................................18
Section IV - The Rio Grande Valley Federal Flood Control Project
and its Role in Providing Flood Protection for the LRGV...................................................................21
References.......................................................................................................................................23
CLIMATE AND HURRICANE REPORT
3
Introduction
This report provides an independent review of existing studies, data, and information that assess the relative threat of a hurricane
or tropical storm impact on Brownsville, Texas, as compared to other locations along the U.S. Atlantic and Gulf of Mexico coastlines.
The report additionally provides information on factors influencing this relative threat through a review of peer-reviewed literature and
government agency reports (National Hurricane Center, National Climatic Data Center, etc.). A general review of inland hurricane
patterns, such as rates of inland decay for historical storms, is presented for comparison of typical storm impacts for the two Metropolitan Statistical Areas (MSA’s) in the Lower Rio Grande Valley (LRGV), namely the McAllen – Mission – Edinburg MSA and the
Brownsville – Harlingen MSA. Finally, due to its critical role in flood protection for both MSA’s, a brief discussion of the Lower Rio
Grande Flood Control Project (LRGFCP) is included.
The report was prepared in response to a request by the Brownsville Economic Development Council (BEDC) to investigate the
threat of hurricane impact in Brownsville relative to other coastal areas. Additional requested information included a discussion of
how storm threat varies with inland distance – particularly relative to the observed and expected impacts between the two MSA’s in
the LRGV. The perceived threat of damaging hurricane impacts often either underestimates or overestimates the true threat in nearly
every geographic area. The perception often is as cyclical as the pattern of hurricanes themselves, with overestimates of threats over
higher frequency periods and underestimates during hurricane “drought” periods for a region. This report attempts to present an unbiased review of established literature and technical findings to provide a statistically valid “best perception” for the threat of an impact
relative to other areas mostly in terms of return periods (period of time between events).
Report Structure (Table of Contents):
u
Section I – Comparison of Hurricane Strike Probabilities and Return Periods along the U.S. Atlantic and
Gulf of Mexico Coastlines
u
Section II – Important Factors Explaining the Reduced Threat of Impact from Tropical Cyclones for the South Texas Area
u
Section III – Brief Discussion of Coastal versus Inland Risk for Historical Storms Impacting the Area
u
Section IV – The Rio Grande Valley Federal Flood Control Project and its Role in Providing Flood Protection for the LRGV
Summary of Findings:
Section I – Comparison of Hurricane Strike Probabilities and Return Periods along the U.S. Atlantic
and Gulf of Mexico Coastlines
Findings show that the Brownsville, Texas area has a lower risk of being impacted by a hurricane than the majority of the U.S.
Gulf of Mexico coastline and much of the Eastern seaboard up to Chesapeake Bay. Brownsville’s average return period for tropical
storms and hurricanes of 5 years was longer than over 95% of the GOM coastline and 91 % of the coastline from Brownsville to Virginia Beach, VA. In fact, in both cases, Brownsville’s return period was tied for largest along the entire coastline up to Virginia Beach,
VA. Keim et al. (2007) found that the south Texas area had a return period of 5, 12, and 52 years for tropical storms, all hurricanes,
and major hurricanes respectively. The same source also determined a THI (based on likelihood of impact and damage susceptibility
of each area) of 67 for Brownsville. This THI was lower than the average of 104 for the Gulf Coast and 98 for the coastline from
Brownsville to Virginia Beach, VA. The THI was lower than 82 % of the combined coastlines.
Section II – Important Factors Explaining the Reduced Threat of Impact from Tropical Cyclones for the South Texas Area
Several reasons for this reduced threat of impact to the South Texas area were investigated and are briefly explained. Among the
most important is the advantageous geographic location and orientation of the South Texas coastline relative to the common track and
approach direction of tropical activity. Brownsville’s location appears to be a naturally reduced threat area from hurricanes based on
typical track patterns for tropical storms and hurricanes. Additionally, the east-facing South Texas coastline results in a narrow or
shallow approach angle for storms approaching from the southeast, the most common approach direction of tropical cyclones for this
area. This limits exposure to the more dangerous right front quadrant (RFQ) of storms that have not yet traversed at least some distance over land. Lastly, Mexico’s mountainous Yucatan Peninsula lies directly in the path of a common “track alley” for storms moving from the Caribbean to the Gulf of Mexico. The Yucatan serves as a critical buffer to many of the would be stronger storms
affecting the western Gulf – limiting the track alley to the Yucatan Strait, a narrow and fairly shallow channel separating Mexico and
Cuba.
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CLIMATE AND HURRICANE REPORT
While storms do form in the Gulf of Mexico, a majority of these storms move in a northward direction and impact the east coast of
Texas or Louisiana. Storms that form in the Bay of Campeche are more likely to move in an easterly or south-southeasterly direction
and often impact Mexico well south of Brownsville (Islam et al., 2009). Lastly, but just as importantly, the City of Brownsville is located 25 miles inland from the coast, providing a small, but seemingly important buffer from storm surge effects and an approaching
storm’s most intense winds. This buffer increases to 30-35 miles in the SSE to SE direction, the most common direction of approaching major storms.
Section III – Discussion of Coastal versus Inland Risk for Historical Storms Impacting the Area
The current literature supports the claim that predicting inland decay rates and inland storm activity (direction, speed of advance,
wind speed decay, rainfall intensity and accumulation, tornado formation, etc.) are still not possible with any high degree of certainty.
Blanket forecasts for wind and wind gusts, precipitation amounts, etc. often offer a broad range of predicted values, reflecting this uncertainty. As such, any comparison of the relative inland threat of storms between the two MSA’s in the LRGV should remain general
in nature.
This section presents findings of published literature on the topic of inland decay and specifically reviews the decay rate of 21
storms that made landfall within 50 nautical miles of Brownsville from 1850-2010. Findings show that despite the relatively fast
decay rates of tropical cyclones once inland (particularly for slower moving, higher intensity storms), the distance of 35 linear miles
separating the centers of the two MSA’s results in only minor differences in storm intensity for storms affecting both MSA’s. In the
case of fast moving minor hurricanes (Category 1 and 2) and tropical storms, both MSA’s historically experience very similar wind
speeds, rainfall intensities and accumulations. The only substantive threat difference between the two is that stemming from storm
surge; however, only the far eastern portions of the Brownsville area are within storm surge risk areas and only in the case of powerful
Category 4 and 5 storms. Again, the low number of events over the period of record, the wide variability in tropical cyclone behavior
(particularly inland), and the wide variability in storm size and speed of advance should limit the application of these findings to a
general discussion and comparison only.
Section IV – The Rio Grande Valley Federal Flood Control Project and its Role in Providing Flood Protection for the LRGV
The historical threat of flooding from the Rio Grande river was substantial enough for the creation of a system of levees, diversion
dams, and diversion floodways specifically designed to prevent such an occurrence. A review of the system shows that substantial
flood protection as been provided from river flooding and high flows in the Rio Grande resulting from tropically-induced heavy rainfalls in upstream portions of the watershed. In particular, the two diversion dams and floodways provide the ability to drastically reduce and carefully control flood flows through the Rio Grande past the cities of Brownsville and Matamoros.
A review of the operation of the system through one of the wettest years on record (2010) revealed minor concerns with floodway
conveyance capacity and levee integrity along the floodways. The diversion dams and levees along the Lower Rio Grande channel
functioned as designed, resulting in flood flows being effectively managed. However, the normal use of the floodways for drainage
needs stemming from local rainfall (as opposed to upstream rainfall) was restricted during the several weeks of floodway operation for
some areas. This presented some challenges for the McAllen MSA and upper valley, particularly during TS Hermine’s passage at the
end of a very wet year. This resulted in long-term ponding, some property flooding, and storage issues in the upper Valley until water
levels in the floodways were lowered by the International Boundary and Water Commission (IBWC). The Brownsville area does not
rely on these floodways at all for local drainage, and only relies in a limited fashion on the Rio Grande. As a result, local drainage issues were addressed quickly after the wet period ended.
CLIMATE AND HURRICANE REPORT
5
Section 1 - Comparison of Hurricane Strike Probabilities along
the U.S. Atlantic and Gulf of Mexico Coastline
From a review of various sources containing strike probability data, hurricane hazard indices, and return period analyses, it is evident that the Brownsville, Texas area has a lower threat of hurricane impact compared to most other portions of the U.S. Gulf of Mexico and Southeast Atlantic coast. Data presented in this section were collected from National Hurricane Center technical memos,
peer-reviewed literature, and coastal hazard studies. Additional analysis was conducted in a geographic information system (GIS)
framework by analyzing the tracks of historical hurricanes impacting the South Texas coast.
The National Weather Service and National Hurricane Center’s Technical Memo No. 46 (NWS NHC 46, 1992, updated 2010) (Jarrell et al., ) provides a review of direct and indirect strikes by county along the U.S. Gulf of Mexico and Atlantic coastline. While the
original version was written in 1984, the study has been updated repeatedly since that time, with storm strike counts last updated in
2010. Using GIS, Jarrell et al. (2010) compiled the text and tabular data of this original report into a much more user-friendly set of
maps (Figures 1-4).
Figures 1-4 illustrate the total number of hurricane strikes per county for the period of record from 1900-2009 as per data collected
for NWS NHC 46. Figure 1 provides a color-coded comparison of strike counts for the entire coast. The cumulative strike counts are
shown from low (blue) to high (red). The highest strike count counties are limited to four main areas: the Houston / Galveston Texas
area; a stretch of coastline from eastern Louisiana (New Orleans) to western Alabama; the southern Florida coastline; and the barrier
islands of the North Carolina coast. The South Texas coastline is not part of these higher strike count regions.
Figures 2-4 provide closer views of the same data presented in Figure 1 with the notable exception that exact counts for direct and
indirect strikes for each county are provided via a county label. Nine storms are shown to have struck Cameron County from 19002009. (An indirect strike is counted when hurricane force winds are experienced in any part of the county as a result of a nearby
storm.)
Cameron County has a lower strike count total than 70 % of the coastal counties from the Rio Grande River to the Chesapeake
Bay. Cameron County has a lower strike count total than 80% of all counties among coastal counties in the Gulf of Mexico (Cameron
to Miami-Dade). Among the 17 Texas coastal or near coastal counties shown in Figure 2, Cameron County is tied for the lowest
strikes with Jackson County. While this is strong support for the claim that Brownsville has a lower strike probability compared to the
majority of other counties along the GOM and Atlantic coastline, the approach used for this study did have some limitations.
One of the shortcomings of NWS NHC 46 is that the size of the coastal county affects the strike count tally shown in Figures 1-4.
In other words, a larger county will present a larger target for both direct and indirect strikes by approaching storms. This study also
only figured wind speed in its evaluation of a storm strike. As a result, additional sources of data and studies were considered necessary.
Commencing in 1978, the National Climatic Data Center (NCDC), in conjunction with the NHC, commenced publishing a series
entitled, “Tropical Cyclones of the North Atlantic Ocean.” This series has been updated every few years since then with a most recent
update of July 2009. The July 2009 version is hereafter referred to as McAdie et al. (2009). The report is arguably one of the most
comprehensive and routinely updated series about Atlantic and GOM hurricanes, providing climatological, meteorological, and general data about storms in addition to a storm-season specific accounting of tracks, intensities, and landfalls. In contrast to the direct
and indirect counts used in NWS NHC 46, McAdie et al. 2009 studied both the variation of tropical cyclone frequency and intensity
along the same extent of coastline using a different counting procedure. Data were collected at fifty-seven, equidistant coastal locations from Port Isabel, TX to Eastport, ME (approximately 50 nautical miles apart from each other). Counts were made of the number of cyclones whose centers passed within 75 nautical miles of each sampling point from 1886-2008. The count was additionally
divided into standard tropical cyclone intensity thresholds: 34 knots (all tropical storms and hurricanes), 64 knots (hurricanes) and 96
knots (major hurricanes). Figure 5 shows the results of their tally, with the vertical axis showing the number of storms counted by this
process normalized to the number of storms over a 100 year period. The horizontal axis shows well-known geographic locations for
convenient determination of the location of the sampling point.
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CLIMATE AND HURRICANE REPORT
Figure 1: Total number of hurricane strikes by counties/parishes/boroughs, 1900-2009 Source: NWS NHC 46
Figure 2: Total number of hurricane strikes by county/parish/boroughs, 1900-2009. Texas and Louisiana.
Source: NWS NHC 46
CLIMATE AND HURRICANE REPORT
7
Figure 3: Total number of hurricane strikes by counties/parishes/boroughs, 1900-2009, for Florida.
Source: NWS NHC 46
Figure 4: Total number of hurricane strikes by counties/parishes/boroughs, 1900-2009, for South Carolina,
North Carolina, and Virginia. Source: NWS NHC 46
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CLIMATE AND HURRICANE REPORT
Analysis of the figure reveals the low incidence count for Port Isabel (Cameron County area) in comparison to the vast majority of
the coastline from Texas to Maine. In fact, the graph clearly illustrates, when considering all tropical cyclone activity (tropical storms
and hurricanes of any category), only the extreme portions of the Northeast United States, specifically the coast of Maine, had fewer
storms pass within the given radius.
Owing to the infrequent return periods for tropical cyclones in the northeastern United States, it is helpful and appropriate to reduce the sample size to only those points between South Texas and the Chesapeake Bay before further analyzing the data shown in
Figure 5. The exclusion of these 13 stations along the New England and Northeast coastline results in a total set of 45 observation
points. Ranking these 45 points from highest incidence to lowest incidence, Port Isabel ranks as follows for the three different intensity categories analyzed: (Note: a higher ranking indicates a lower incidence rate for the corresponding intensity categories)
u
u
u
Ranks 44th of 45 locations with respect to all tropical cyclones (tropical storms and all category hurricanes.)
Ranks 34th of 45 locations with respect to all category hurricanes
Ranks 27th of 45 locations with respect to all major hurricanes
Presenting this in terms of the percentage of coastline with an equal or higher chance for storms to pass within 75 nautical miles of
the respective location results in:
u
u
u
When considering a tropical cyclone event of any intensity (tropical storm, minor hurricane, or major hurricane) 98% of
locations along the coast between South Texas and the Chesapeake Bay had a higher number of storms passing within a 75
nautical miles of the respective location.
For the same area, but considering only hurricanes of any category (minor and major hurricanes), 76% of locations had a
higher incidence of strikes.
And finally, considering only major hurricanes, 60% of locations had a higher incidence of strikes.
A study by Keim et al. (2007) entitled,
“Spatiotemporal patterns and return periods of tropical storm and hurricane
strikes from Texas to Maine” presented a
return period based analysis of 105 years
of tropical cyclone strikes at 45 locations
from Brownsville, Texas to Eastport. ME.
A return period is the inverse of frequency or strike counts over a given period – in other words, a return period is
the average time it takes for a storm of a
specific intensity to return or be experienced again at any location of interest.
For example, if an area experienced four
major hurricanes over the period of 100
years, the return period for major hurricanes in that location would be 25-years
(or 100/4). The report also summarizes
the literature and research completed to
date on return period analyses of hurricane strikes in the United States and presents a detailed discussion of their
improvements on these studies. Namely,
the Keim study avoids the target size bias
that plagues the county-based analysis of
NWS NHC 46 by adopting a methodology that focuses on specific coastal
places (towns or cities) rather than sectors of coasts.
CLIMATE AND HURRICANE REPORT
9
Keim et al. selected 45 coastal cities along the Gulf and East coasts of the U.S. from South Padre Island, TX to Eastport, ME, and
produced return-periods for various intensity level storms. Strike intensities at these locations were estimated from reported maximum
sustained winds when approaching centers were within 80 km of the specific location – regardless of whether or not the storm actually
made landfall in that location.
Figure 6 shows the average return periods for tropical storms, hurricanes, and major hurricanes according to Keim et al. The figure shows that South Padre Island, TX, has return periods of 5, 12 and 52 years respectively for tropical storms and hurricanes combined, all hurricanes, and major hurricanes. Converting these numbers to frequency of events over 100 years results in incident rates
lower than those found by McAdie et al. Keim’s study shows that South Padre should expect approximately 20 tropical cyclone
events, 8.5 hurricanes of any magnitude, and just under 2 major hurricanes to pass within 80 nautical miles over a period of 100 years.
Section 2 of this report shows the results of my independent review of storms passing within 50 nautical miles of Brownsville, Texas.
My results are more closely aligned with the return period and incidence rates of those found by Keim et al. Additionally, other studies analyzing the frequency and distribution of hurricane impacts on these portions of the U.S. coastline are more in-line with Keim’s
results. These studies include Elsner and Kara (1999) and Simpson and Lawrence (1971).
Figure 6: Average return periods for tropical storms, hurricanes, and major hurricanes.
Source: Keim, Barry D. and Robert A. Muller. Spatiotemporal Patterns and Return Periods of Tropical Storm and Hurricane Strikes from
Texas to Maine.
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CLIMATE AND HURRICANE REPORT
Regardless of the differences between the Keim and McAdie studies with respect to incident rates (return periods), both studies
show the same comparatively lower risk for impact by a tropical cyclone for the South Texas area when compared to many other portions of the GOM and Atlantic U.S. coastlines.
A Tropical Hazard Index (THI) was also created by Keim et al. in order to provide “additional geographic perspective on the vulnerability of the Atlantic and Gulf Coast regions.” Strikes of tropical storm intensity were assigned two points, minor hurricanes given
four points, and major hurricanes 8 points. The points were tallied over the 105-year period of data analysis – the same as their return
period analysis. Figure 7 shows Keim’s THI graphed for the coastal locations between Brownsville, TX and Nags Head, NC. The
Brownsville THI is the 7th lowest index along this portion of the coast. This additional, third manner of illustrating relative hurricane
threat along the U.S. Gulf and Atlantic coastlines confirms the lower, comparative threat of a storm strike to the Brownsville, Texas region. The following section of this report investigates some of the reasons behind this reduced threat.
Figure 7: Tropical Hazard Index by location – Brownsville, TX to Nags Head, NC Source: Keim et al.(2007)
CLIMATE AND HURRICANE REPORT
11
Section II - Important Factors Explaining the Reduced Threat of Impact
from Tropical Cyclones for the South Texas Area
This section presents a brief discussion of some of the reasons for the reduced threat of impact from tropical storms to the South
Texas area as illustrated and supported in Section I. While several reasons for this reduced threat were discovered through research,
only those well-supported in the literature and government studies are discussed in any detail.
Some of the well-documented and supported reasons for the diminished threat are listed below:
a)
b)
c)
Advantageous geographic location relative to common historical track patterns for tropical cyclones resulting in a lower than
average strike probability relative to coasts both north and south of the Brownsville, Texas area as well as other areas as
discussed in Section 1;
Orientation of the South Texas coastline relative to the most common southeasterly approach direction of storms resulting in
less frequent exposure to the more dangerous right front quadrant (RFQ) of storms that have not yet traversed at least some
overland distance;
The critical buffering effect of Mexico’s mountainous Yucatan Peninsula for storms approaching the Gulf of Mexico from the
Caribbean.
Other reasons that could possibly explain the reduced threat to South Texas were discovered through discussions with expert personnel, experienced weather watchers, and anecdotal evidence; however, little to no substantive, peer-reviewed research was found for
these additional explanations and as such, will not be detailed here. Examples of these as of yet unsupported theories include: the role
of the Rio Grande river deltaic protrusion in diverting storms around Brownsville (Hurricane Dolly of 2008); the shallower waters of
the western Gulf of Mexico causing decay of approaching storms (data both supporting and refuting this were found); and the role of
more persistent, larger scale weather patterns (steering currents) on the mesoscale and synoptic scales diverting storms either northward or southward depending on the storm’s area of genesis.
It is important to note that this section does not, nor does any part of this report, review the well-established literature relating the
geographic cycles of hurricanes in the GOM and Atlantic to much larger weather patterns and events such as El Nino or La Niña in the
Pacific. Relationships between hurricane heavy (poor) seasons and La Niña (El Nino) years have been observed and are becoming
better understood; however, these multi-year oscillations affect much larger geographic regions and would not explain the longer return periods seen at Brownsville over the period of 140 years of data. This being said, the cyclical nature of hurricane strikes from one
portion of the coastline to another plays an important role in the groups of years during which Brownsville experiences below or
above average hurricane activity. Readers wishing to look more carefully at this topic are referred to Elsner et al. (2000) and Kushnir
(1994).
a)
Advantageous geographic location relative to common historical track patterns for tropical cyclones resulting in a lower than
average strike probability relative to coasts both north and south of the Brownsville, Texas area as well as other areas as
discussed in Section 1;
There are a wide-array of on-line databases and tools available for efficient perusal of historical storm tracks. These databases can
be queried so that analysis can be performed and storms grouped by basin, area of impact, intensity, year, etc. An analysis of all
storms passing in close proximity (50 miles) to Brownsville, Texas, between the years of 1860-2010 was completed using
http://www.csc.noaa.gov/hurricanes/ and will be discussed and summarized later in this section.
The National Hurricane Center has analyzed the prevailing tracks and common areas of occurrence for tropical cyclones for each
of the six months in the Atlantic hurricane season. The results of this analysis have been summarized in a series of six images available on their home website: www.nhc.noaa.gov. Figures 8 and 9 show the six images in succession, illustrating the areas within the
North Atlantic, Caribbean, and Gulf of Mexico that are likely, more likely, and most likely to experience tropical cyclone activity in
each of the respective months. While these images were not created for explicit interpretation of the threat of a storm impact between
any two specific point locations, the figures do show the prevalent tracks observed over a long period of record.
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CLIMATE AND HURRICANE REPORT
Figure 8: Typical areas of origin and tracks for tropical cyclones
over the first half of the hurricane season.
Figure 9: Typical areas of origin and tracks for tropical cyclones
over the latter half of the hurricane season.
Source: National Oceanic and Atmospheric Administration
http://hurricanes.noaa.gov/prepare/season_zones.htm.
Source: National Oceanic and Atmospheric Administration
http://hurricanes.noaa.gov/prepare/season_zones.htm.
Analysis of Figures 8 and 9 shows that Brownsville is never shown in the “most likely” zone / category during any month in the
hurricane season. Brownsville is just on the border between the “more likely” and “likely” zone once, during the traditionally active
month of September. Brownsville is shown in the “likely” zone for the first month of hurricane season, June, and again in August.
Brownsville is not in any of the likely occurrence categories during the months of July, October, and November.
Analysis of the prevalent track patterns shows that the more common directions of approach for storms affecting the South Texas
area move from the South-Southeast early in the hurricane season, to the Southeast in August, and finally the Southeast and possibly
more easterly direction in September. By October, the influence of larger scale weather patterns, namely frontal systems from the
northwest and a weakening of the Bermuda high that peaks in the summer, permits the prevalent track pattern to shift more to the eastern half of the Gulf of Mexico. A comparison of these generalized track patterns with the actual approach directions of storms impacting the Brownsville, Texas area follows later in this report.
b)
Orientation of the South Texas coastline relative to the most common southeasterly approach direction of storms resulting in
less frequent exposure to the more dangerous right front quadrant (RFQ) of storms that have not yet traversed at least some
overland distance.
CLIMATE AND HURRICANE REPORT
13
A hurricane’s angle of approach to a coastline greatly affects the intensity of winds, storm surge, rainfall, and associated damages
that is experienced by coastal and inland areas. The right side of a tropical storm relative to its direction is considered the “more dangerous” semi-circle. In particular, the right-front quadrant (RFQ) is typically the strongest portion of a tropical cyclone. This right
side typically has stronger winds, more intense rainfall both near the eye and in rain bands extending miles from the center, and the
strongest storm surge. A tropical cyclone striking a coast “head on”, or perpendicular, exposes the coastline to the direct force of the
right front quadrant and the more dangerous semi-circle. Storms with a shallow angle of approach to the coastline that permits the
weaker left side of the storm to impact the coastline first usually results in reduced storm surge and lower inland damages from high
winds. This is due to the fact that some decay occurs prior to the RFQ making landfall in these situations.
A review of the direction of approach for all tropical cyclones passing within 50 miles of Brownsville, TX was completed for the
years 1850-2010. Figure 10 illustrates that of the 25 storms that passed within that radius over the 160 year period, the majority approached from the southeasterly direction – with the strongest storms approaching solely from that direction. The south-southeasterly
direction was the next most common approach direction. This means that approach angles of 45 degrees or less are more common for
South Texas. The east facing coastline of South Texas, combined with the common approach direction from the Southeast and southsoutheast results in less frequent exposure to the RFQ for storms that have not yet traversed at least some overland distance.
Figure 10 also shows that there may be a relationship between approach direction and the intensity of approaching storms. The
two most recent major hurricanes to impact Brownsville (Beulah-1967 and Allen-1980) approached from the SSE and SE respectively
after having passed through or near the Yucatan Strait. In fact, Brownsville only experienced the left-side of hurricane Allen and in
conjunction with its rapid decay from a Category 5 to a Category 3 right before landfall, possibly explains the relatively lower winds,
rain, and damages experienced in the region during that event.
Figure 11 shows the distribution of the 25 storms by category and month of landfall. As is well understood by people living in
South Texas, September and August are the high frequency months for hurricanes. Additionally, it can be seen that the three major
hurricanes to strike the area occurred in those two months as well. An early season peak in June can be seen with the month of June
containing 2 tropical storms, 3 Category One storms, and 1 Category Two storm. While the hurricane season officially extends
through November, only one storm after September 30th has passed within 50 miles of Brownsville, an unusual late season Category 2
that approached Brownsville from the northeast.
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CLIMATE AND HURRICANE REPORT
WIND SPEED
(Knots)
>= 135
114 - 135
96 - 114
84 - 96
65 - 84
0 - -64
Calms: 0.00%
Figure 10: Radical schematic showing number of storms by approach angle
and intensity passing within 50 nautical miles of Brownsville between the
years 1850 and 2010. (Note: TS Hermine, which approached from the south
southeast is not shown)
Source: Graphic designed by Alejandro Lara, Jr. under supervision of author. data obtained from HURDAT and NOAA Coastal Services Center online historical hurricane
data tracker (www.csc.noaa.gov/hurricanes)
10
9
8
Category 5
7
Category 4
6
Category 3
5
Category 2
4
3
Category 1
2
Tropical Storm
1
0
June
July
August
September
October
November
Figure 11: Distribution of the 25 storms by category and month of landfall near Brownsville.
CLIMATE AND HURRICANE REPORT
15
Another mechanism that provides some “protection” to the South Texas coastline from more frequent tropical cyclone events (particularly larger storms) is the buffering effect of the Yucatan Peninsula of Mexico. While there is ample evidence in the hurricane
track record of the mountainous portions of the Yucatan disrupting and weakening stronger storms, it is not impossible for storms to
survive this disruption and continue their path into the GOM, and possibly experiencing re-intensification while in the Gulf.
An analysis of the 25 storms influencing the Brownsville region between 1850-2010 with respect to their intensity level at impact
and their area of initial formation yields a strong pattern. Figures 12 and 13 show that storms originating in the GOM and impacting
Brownsville tend to be weaker compared to storms originating in the Caribbean or the Atlantic.
Of the 25 storms impacting the Brownsville area, 14 formed in the Gulf of Mexico. A simple intensity scoring system was established based off of standard hurricane categories. A Category One hurricane was given an intensity score of 1, a Category Two an intensity score of 2, etc. Tropical storms were given an intensity score of zero. After assigning these intensity scores to the 14 storms
that formed in the GOM, their average intensity score resulted in 0.54. The 14 storms were comprised of 2 Category Two storms, 3
Category One storms, and 9 Tropical Storms.
In contrast, the 11 storms that formed outside of the GOM and impacted the Brownsville area between 1850-2010 had an average
intensity score of 2.00, consisting of 2 Category Fours, 1 Category Three, 5 Category Two’s, two Category One’s and two Tropical
Storms.
Figure 12 also shows that storms originating in the GOM tend to approach the Brownsville area from a more southeast to southsoutheasterly direction. Islam et al. (2009) conducted focused research on tropical cyclone strikes for the coast of Texas and confirmed that a majority of storms that form in the GOM move in a more northerly direction, more likely impacting the east coast of
Texas or Louisiana. Storms forming in the extreme southern portion of the GOM, the Bay of Campeche, often move more westerly
and frequently impact the Mexican coastline well south of Brownsville.
Figure 13 shows that the 11 storms that formed in the Atlantic and influenced the Brownsville area were in general much stronger
and had a direction of approach slightly more toward the southeast and east-southeast as compared to storms that formed in the GOM.
(Note: As of the time of this report, Hurricane Beulah’s status at landfall remains under review. Several literature sources list it as
a high Category 3 at landfall; however, post-event analysis is trending toward a higher category rating – possibly even a Category 5.
The author realizes that this is in contradiction to many older data reports as well as commonly agreed upon convention. However,
HURDAT, the ‘best track’ dataset for hurricanes in the North Atlantic maintained by the Tropical Prediction Center, currently lists
Beulah as a low range Category 5 just prior to landfall. According to this dataset, post-analysis review of the storm at landfall held it
to have sustained 1-minute wind speeds of approximately 160 mph, 5 mph over the 155 mph threshold for a Category 5. The maximum wind speeds recorded at South Padre of 138 mph do not directly support this; however, this could have been a result of sudden
and rapid weakening just prior to landfall. For the purpose of this report, Beulah (1967) is listed as a Category 4 – a reasonable compromise between the considered range from a high Category 3 to a low Category 5.)
Lastly, but just as importantly, the geometric center of the City of Brownsville is located 25 miles inland from the coast in a
straight east – west direction , providing a small, but seemingly important buffer from storm surge effects and an approaching storm’s
most intense winds. This inland buffer increases to 30-35 miles in the SSE to SE direction, the most common direction of approaching major storms. The next section of the report will discuss the decay rates of both historical storms that have struck in the vicinity of
the Brownsville area and decay rates of storms in general.
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CLIMATE AND HURRICANE REPORT
Tropical Storm (<73mph)
Category 3 (111
130 mph)
Category 1 (74
95 mph)
Category 4 (131
Category 2 (96
155 mph)
110 mph)
Category 5 ( >155 mph)
Figure 12 (above): Angle of approach and intensity for storms passing within 50 nautical miles of Brownsville that originated in the Gulf of Mexico. Figure 13 (below) shows the same but for storms originating in the Gulf of Mexico.
CLIMATE AND HURRICANE REPORT
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Section III – Discussion of Coastal versus Inland Risk for Historical Storms Impacting the Area
Tropical cyclones not only pose a threat to life and property along coastal counties, but also to inland counties and regions as they
move inland and dissipate. In many cases, aside from storm surge and wind damages resulting directly from the hurricane, damages
resulting from high winds due to tornados, heavy rains, and flooding for inland areas can equal or even exceed those experienced by
the coastal region where the hurricane made landfall (Kruk et al. (2010) and Rappaport (2000). In fact, Rappaport discovered that a
large portion of storm-related fatalities occurred inland, associated with decaying storm activity such as high winds, heavy rains, etc.
Additionally, Rappaport found that in the 1970s, 1980s, and 1990s, freshwater floods accounted for 59% of the recorded deaths from
tropical cyclones.
This section presents a summary of previous work as well as independent analysis of post-landfall historical tracks for storms impacting the LRGV. The section commences with a brief summary of the state of the science of inland intensity decay and in particular,
mentions one document that provides an excellent estimation of inland winds and associated damages from a comprehensive, GISbased analysis of historical tracks and inland intensity data.
It is important to note that of the various uncertainties and difficulties associated with tropical event prediction, the prediction of
inland damages is perhaps the most difficult. The current literature supports this claim repeatedly (Kruk et al., 2010), by acknowledging that hurricane intensity forecasting, rates of inland decay and geographically specific predictions of rainfall accumulations and resulting damages remain beyond the ability to predict with a high degree of certainty. This is particularly true as the geographic
specificity or precision of the forecast increases. Most of the work completed to date in this area has focused on numerical and computer simulations attempting to predict the inland decay of storms such as in Tuleya et al. (1984) and Kaplan and Demaria (1995,
2001). Zandbergen (2009) examined the inland exposure of U.S. counties to tropical storm and hurricane force winds, concluding that
despite relatively fast decay rates of hurricane winds, inland damages due to rainfall, flooding, tornados and general storm activity
were significant in many areas of the Deep South – sometimes extending as far inland as the Ohio Valley and New England.
Most of these studies, such as Emmanuel (2005), focus only on wind speed decay. For wind speed directly related to hurricanes
(excluding tornado damage) Emmanuel showed that the decay rate of a land falling hurricane is rapid, losing half its wind speed value
in roughly 7 hours of inland travel, 75% in 15 h, and nearly 90% after just 1 day inland; however, this and other studies like the wide
variability of inland decay rates for storms.
In general, the variety of studies on inland decay rates and associated damages can loosely be summarized as follows: decay rates
are a function of storm intensity and the rate of motion of the storm and damages are not limited to coastal communities or even nearcoastal communities. Slower moving, more intense storms tend to decay faster just after landfall, while faster moving, less intense
storms tend to decay more slowly. (Kruk et al., 2010)
Historical track records of the 21 storms passing and making landfall within 50 miles of the Brownsville area between 1850 and
2010 were analyzed for the purpose of obtaining a rough estimate of hurricane intensity decay rates after landfall. While a detailed inland-risk assessment for the Lower Rio Grande Valley was beyond the scope of this report, this cursory analysis yielded useful and interesting results; however, the low number of events over the period of record, combined with the wide variability in storm behavior
should limit the application of these findings to a general discussion and comparison only.
One of the primary reasons this analysis was conducted was to approximate the difference in hurricane intensity (category) for the
same storm affecting both the McAllen-Edinburg-Mission and Brownsville – Harlingen MSA’s. The Brownsville-Harlingen MSA is
approximately 25 miles inland, while the McAllen-Edinburg-Mission MSA is approximately 70 miles inland. Results of the three
major hurricanes making landfall in the above area (Allen, Beulah, and the Category 4 Unnamed storm of 1880) maintained their
major hurricane strength (Category 3 or greater) for 85, 65, and 65 miles respectively – an average of 72 miles. Comparing this to
other major storms along the Texas coastline with similar topographies and land roughness resulted in similar findings that large, fast
moving, higher intensity storms can maintain greater than hurricane force winds inland for over 125 miles.
Tropical cyclones making landfall as a hurricane of any category (n=12) maintained hurricane status for an average of 129 miles
after landfall. Tropical cyclones making landfall as any category hurricane or tropical storm maintained at least tropical storm status
for an average of 216 miles after landfall. Cyclones making landfall as a tropical storm (n=9) maintained that status for an average of
152 miles inland. Cyclones making landfall as a Category 1 or 2 (n=9) maintained hurricane strength for an average of 102 miles inland.
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CLIMATE AND HURRICANE REPORT
Kruk et al. (2010) conducted a very comprehensive analysis of the entire HURDAT track and intensity database with the primary
aim of producing a series of maps with tremendous value to forecasters, emergency managers, and the general public. Their maps
show a series of frequency distributions and estimations of return intervals for inland tropical storm and hurricane force winds.
Figure 13 shows the return period for tropical storm force winds for inland areas. Figure 14 shows the same for hurricane force
winds. Figure 14 shows that return periods for hurricane force winds do not drop off significantly with inland distances on the scale
of 40-50 miles from the coast. In other words, the two MSA’s in the LRGV do not differ substantially in return periods for hurricane
force winds – with a return period of about 6-8 years for the Brownsville – Harlingen MSA and a return period of about 8-9 years for
the McAllen MSA.
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19
There is sufficient evidence in the literature as well as our study of the decay rates of historical storms having impacted the South
Texas area to make a few general, but well-supported conclusions. First and most importantly, hurricane damages are not limited to
coastal areas. In particular, a fast moving storm (greater than 15 mph speed over land), would likely expose both MSA’s to hurricane
force winds. Secondly, aside from storm surge threat, both MSA’s experience about equal risk from heavy rains, localized flooding
from heavy rains, tornado formation during hurricane passage, and high winds.
From a review of the historical track data for storms making landfall in the South Texas area, it is apparent that storms, on average,
maintain their intensity for a long enough distance inland to affect the two MSA’s in the LRGV with only small changes in wind speed
intensity and practically no apparent difference in rainfall intensities and total rainfall accumulation. Wind speed and rainfall records
for historical storms for cities in both MSA’s support this claim as do a number of independent, peer reviewed publications regarding
the inland decay rates of hurricanes in general.
The threat of tornado formation during hurricane passage is actually somewhat higher for areas just inland of coastal areas and for
major storms, often at inland distances of 100-200 miles. Hurricane Beulah held the record for the number of tornadoes spawned during hurricane inland transit, with the vast majority of these tornados forming in areas as far inland as 100-150 miles.
While the threat of storm surge is by its nature limited to coastal areas, the Brownsville – Harlingen MSA is largely outside the
threat zone for Category 1, 2, and 3 storms. For Category 4 and 5 storms, generalized maps show portions of the eastern half of
Brownsville within the storm surge area. However, the prediction skill in this area is admittedly low – particularly for higher intensity
storms. Additionally, the storm surge predictions often assume the worst-case strike angle of perpendicular to the coast. As discussed
earlier, perpendicular storm strikes in the South Texas area are not common.
Given the above, it is again important to recall that the low sample size (n=25), the dependence of inland decay rates on storm
speed of advance, and the wide variability of storm decay rates as a whole mean that the above conclusions should be taken only as a
general guide and should never be used for planning purposes, emergency management decisions, and/or by the general public.
One aspect not yet discussed, and an important difference between the two is the risk due to riverine flooding from a storm impacting the South Texas and Northeastern Mexico region – such as Hurricane Alex in 2010. In fact, the series of storms impacting the
broader region around this area in conjunction with the wet season of 2010 highlights the important role of flood protection in the
LRGV as well as its limitations. The next and final section of this report will discuss a few important specifics about the Lower Rio
Grande Flood Control Project.
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CLIMATE AND HURRICANE REPORT
Section IV - The Rio Grande Valley Federal Flood Control Project
and its Role in Providing Flood Protection for the LRGV
This section presents a brief summary of the Lower Rio Grande Flood Control Project (LRGFCP). The LRGVCP consists of a series of diversion dams, dikes, levees, and interior floodways that serve to protect the Rio Grande Valley from river-related flooding
stemming from tropically-induced rainfall upstream of the Valley. The project is detailed here to highlight the operations of the system during high-flow events and how this standard operation affects local drainage operations in the two MSA’s in the LRGV. Interestingly, while the project design is to protect both MSA’s from riverine flooding, the operation of the system during a high-flow event
restricts some local drainage options for the McAllen-Edinburg-Mission MSA, but does not restrict those operations for the city of
Brownsville. Additionally, the systems two diversion dams – Anzalduas and Retamal – function to divert sufficient flood flows so that
the design flow past Brownsville / Matamoros is 20,000 cfs, a mere 8% of the design flood flow into Anzalduas dam.
The LRGFCP covers 180 river miles from Penitas, TX to just downstream of Brownsville, TX. Approximately 270 miles of levees
are located along the Rio Grande, the Arroyo Colorado, and interior floodways. Major operational systems include the Banker Floodway, Main Floodway, North Floodway, the Arroyo Colorado, Anzalduas Dam, and Retamal Dam.
While levees are an integral part of the system, perhaps the most vital components are the two diversion dams and the systems of
interior floodways designed to handle diverted water – preventing this water from travelling downstream and impacting the
Brownsville – Matamoros area. Anzalduas dam is located just south of Mission, Texas and has the primary function of diverting the
U.S. share of floodwaters in the U.S. floodways. Retamal dam, located straight south of Donna, TX, diverts Mexico’s share of floodwaters into the Mexican floodways. Both diversion dams operate in concert to limit the flows in the Rio Grande downstream of the
dams so that flood flows in the Rio Grande remain at safe levels in the Brownsville – Matamoros area.
Figure 15 shows the location of the diversion dams, interior floodways on both the U.S. and Mexican sides, the component portions of the U.S. interior floodways (Main, Arroyo Colorado, and North floodways) and the location of major cities, notably
Brownsville / Matamoros, McAllen and Harlingen.
The system operates under specific design flood flows that were updated after Hurricane Beulah in 1967, owing to the devastating
floods experienced in the McAllen and Harlingen areas after that storm. The current design flows are as follows:
a)
b)
c)
d)
e)
f)
250,000 cubic feet per second (cfs) at Rio Grande City (inflow to Anzalduas Dam);
105,000 cfs for diversion by Anzalduas to the U.S. floodway (Main floodway)
105,000 cfs for diversion by Retamal to the Mexican floodway
84,000 cfs in the North Floodway (diverted from the Arroyo Colorado)
21,000 cfs in the Arroyo Colorado at Harlingen
20,000 cfs in the Rio Grande at Brownsville / Matamoros.
As can be seen from Figure 15 and from the list of design flood flows, the LRGFCP can control flows in the Rio Grande past
Brownsville and Matamoros at numerous upstream points. The system successfully maintained flood flows below design levels for
the Brownsville / Matamoros area during the 2010 hurricane season – one of the wettest water years on record.
Unfortunately, one of the significant drawbacks of this system is the that the interior floodways are contained by levees out of necessity – in order to not flood the areas the floodways pass through. The levee contained floodways, while preventing the Rio Grande
from flooding downstream, severely hinder the local drainage of areas near the floodways. The reason for this is that the normal
drainage conduit for the upper valley MSA is the same floodways used by the LRGFCP. As a result, if the LRGFCP is in operation
and diverting floodwaters, the upper valley MSA cannot utilize the floodways because they are already full with Rio Grande diversion
flood water. Continuing, and as occurred in 2010, several upper valley municipalities were forced to retain and/or store significant
floodwaters in streets, property, and under-designed detention ponds for several weeks. In contrast, the city of Brownsville utilizes a
series of drainage canals that drain to the Brownsville Ship Channel. As such, the flood condition of the Rio Grande River does not
affect the drainage operations of Brownsville as significantly.
CLIMATE AND HURRICANE REPORT
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Mexican Floodway
22
Matamoros
CLIMATE AND HURRICANE REPORT
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