U.S. Hurricane Landfalls: Climatology and Impacts



Abstract

Hurricanes are one of the larger and more devastating natural disasters on the planet. Before the aircraft and satellite era began, the only ways people knew about hurricanes were if an unlucky ship was in the path of the storm or if the storm made l andfall. Since 1944, aircraft observations have greatly improved records of the storms’ intensities, and satellites have been incessantly imaging the planet from space since 1960. Today, although every storm can be seen and forecasted before it makes la ndfall, forecasts are not always very accurate. Even if the forecast is accurate, coastal populations are so high that effective evacuations become increasingly difficult. There is a need to better understand the history of landfalls on the United State s in a hope that something can be learned about the future. Applying this knowledge can help coastal communities prepare for landfalls and in the end, save lives.

Multi-decadal patterns of tropical cyclone activity indicate some predictable, oscillatory behavior, but inter-annual variations are too great to be able to make generalizations on smaller time scales. Atlantic basin hurricane activity will be invest igated on annual and decadal time scales.

Coastal population is increasing exponentially, while forecasts and road networks are only slowly improving. The problem is simple: more lives and property are at risk during a landfall. This situation will be analyzed and potential improvements wil l be investigated in conjunction with the climatology of landfalls on the southern and eastern United States.

Finally, the socio-economic impacts of a hurricane landfall will be discussed. A major landfall affects livelihoods and economies on many time scales in many communities. Trends of these impacts will be shown as well as a general, subjective outlook for the coming years.


1. Introduction

Hurricane landfalls on the United States are inevitable; however, they are also sporadic and difficult to forecast. Some years there are no landfalls, other years there are several, ranging in intensity from minimal to catastrophic. A lack of landfa lls in a given region of the coast for several consecutive years can lead to a false sense of security, even if the region is one that is climatologically at high risk. Most high-risk areas get hit every 2-4 years on average, but can sometimes go ten yea rs or more without a landfall. Out of all of the Atlantic hurricanes in the past fifty years, 42% became intense hurricanes, 29% made U.S. landfall as hurricanes, and only 11% made U.S. landfall as intense hurricanes.

The dangers of a landfall are numerous, and increasing with time. The threat of storm surge flooding, rainfall flooding, high winds, and tornadoes have always existed, but the increasing population of coastal counties adds a growing risk, both in ter ms of lives and property. A larger coastal population places more lives in danger and makes evacuations more complex. With more people and more cars, inland-bound roads become gridlocked very quickly during evacuations. The key to safe and timely evacu ations is a combination of better forecasts, efficient warning, and public response. There are 168 counties in 18 states that border the Atlantic Ocean or the Gulf of Mexico; each one requires a unique disaster preparedness and evacuation plan.

This paper will begin by giving an overview of tropical cyclone activity in the Atlantic Basin, followed by a climatology of landfalling hurricanes. Finally, the impacts of landfall will be discussed in depth, including storm surge and the affects of landfall on lives and property.




2. Atlantic Basin Activity

The Atlantic Basin will be defined as the area of the Atlantic Ocean where tropical cyclones can exist. In general, it is the entire Northern Atlantic, from about 5°N to 50°N and from 20°W to 100°W. Using the past fifty years as a basis, there are t ypically 9.9 named storms (winds of 35 knots or higher), 5.9 hurricanes (winds of 65 knots or higher), and 2.5 intense hurricanes (winds of 95 knots or higher) each year. Therefore, of the 9.9 named storms, about 25% become intense hurricanes. An intens e hurricane is defined to be any hurricane that is a Category 3, 4, or 5 on the Saffir-Simpson Intensity Scale (Simpson, 1974). The scale is outlined in Appendix A. Figure 2.1 shows the numbers of named storms, hurricanes, and intense hurricanes in the past 56 years. Based on interpolation, a simple linear trend reveals that while the number of named storms has risen by about 1 every 83 years, the number of hurricanes has remained constant, and the number of intense hurricanes has actually been decreas ing at a rate of 1 every 75 years.



A useful index based on several parameters is the Net Tropical Cyclone Activity, or NTC. This index, developed by William Gray at Colorado State University makes use of not only the number of Named Storms (NS), Hurricanes (H), and Intense Hurricanes (IH) in a given season, but also the number of Named Storm Days (NSD), Hurricane Days (HD), and Intense Hurricane Days (IHD). A “day” is defined to be a 24-hour period during which the storm had at least a certain intensity. For example, a Category 4 (C AT4) storm will accrue 1 NSD, 1 HD, and 1 IHD if there are four 6-hourly advisories issued for it. Likewise, a Tropical Storm (TS) will only accrue 1 NSD in the same period. Also, the season’s NTC is based on a 50-year average in the following manner:

where the overbar denotes the long-term average of a quantity (Gray, 1998). Calculating the NTC for the past 56 years yields an interesting trend.Figure 2.2 shows the extreme interannual variations in NTC. Consecutive years can sometimes be a factor of six more active than one another. However, a best-fit 6th–order polynomial applied to the data brings out a fascinating multi-decadal trend. One can clearly see an NTC maximum in the 1940s and 1950s, an NTC minimum in the 1980s, and a recent increase in the late 1990s.

Landsea et al (1999) investigated an even longer record and noted that 1900 through the mid-1920s was relatively quiescent, the mid-1920s through 1960 was relatively active, and 1960 through the mid-1990s was once again relatively quiescent.



According to Landsea et al (1996), the 35-year lull in activity may be a result of the tropical tropospheric circulation changing.

Several factors could work in unison to deteriorate the necessary conditions for tropical cyclone formation. Three primary factors for development are low vertical wind shear, low environmental sea level pressures, and high sea surface temperatures. Both the ENSO (El Niño Southern Oscillation) cycle and African West Sahel rainfall seem to affect vertical shear over the basin; the strength and location of the ITCZ (Inter-Tropical Convergence Zone) affects the environmental sea level pressure; and the thermohaline circulation affects the basin’s sea surface temperatures (Landsea et al, 1996). If hurricane frequency is going to repeat the activity witnessed fifty years ago, coastal areas need to be prepared for more frequent landfalls. A key concern is that coastal populations in some areas have increased by several hund red percent since then.


3. Landfall Frequency

As one might expect, the frequency of landfalling hurricanes is directly related to the Net Tropical Cyclone Activity. In other words, the more hurricanes there are in a given season, the more likely it is that some of them will make U.S. landfall. Empirical relationships for landfalls have been identified by Gray (1998): 5.3 x 10-3 (NTC) for CAT 1,2 landfalls and 2.0 x 10-4 (NTC)1.6 for CAT 3,4,5 landfalls.

Figure 3.1 demonstrates a very important point. When the NTC is higher than 236%, the expected number of intense hurricane landfalls surpasses the number of expected non-intense hurricane landfalls. Factors that would contribute to an active season are low vertical wind shear, warm sea surface temperatures, and low environmental sea level pressure across the basin. If these conditions are met, then not only will tropical cyclone formation be likely, tropical cyclones will be more intense. At appro ximately 236% NTC, conditions across the Atlantic basin are so favorable that any hurricanes that do form are more likely to intensify to at least CAT3 storms and reach the U.S. coasts as intense hurricanes.



In terms of favored dates for landfall, the Gulf Coast leads the East Coast. There is an 8-week difference for the first landfall of the season between the Gulf Coast and the East Coast (16Jun and 12Aug). There is a 3-week lead for the last landfall of the season, with the Gulf still earlier (4Oct and 25Oct). Likewise, the median date for landfalls in each region differs by three weeks (5Sep and 24Sep) (Landsea, 1993). The reason for these systematic biases is simple… climatology. Conditions in t he Gulf become more favorable earlier in the season than in the Atlantic. Likewise, conditions in the Atlantic are favorable later in the year. These are all based on water temperature, synoptic weather patterns that affect the vertical wind shear, and the location of the ITCZ.

In terms of favored locations for landfall, both the Gulf Coast and the East Coast make substantial contributions. The Gulf Coast has high-risk areas near Houston, TX and New Orleans, LA. The East Coast has its high-risk areas near Miami, FL and Wil mington, NC. However, there is virtually nowhere along the entire U.S. coastline that is “safe” from a hurricane landfall. As mentioned earlier, there are 168 counties in 18 states that lie along either the Gulf Coast or the East Coast. Nearly every co unty has witnessed a hurricane landfall in the past 50 years.




4. Landfall Impacts

A hurricane landfall affects an area on many scales. A single hotel could be washed away or an entire city could be leveled. There are the immediate effects of winds and flooding, then there are the subsequent effects of clean-up and rebuilding. Pielke et al (1998) describes three tiers of impacts: direct, secondary, and tertiary. Primary impacts would be those related to wind and water damage; these are realized immediately. Secondary impacts are those such as disease and other medical problems and are realized days to weeks after the landfall. Finally, tertiary impacts are those that take months to years to be realized, such as insurance rates and property taxes. An impact that cannot be quantified is the tremendous life-long psychological effect that such a disaster has on people. For example, the following is an excerpt of a newspaper’s description of a landfall on Last Island, LA (Anon., 1856):

Men, women, and children were seen running in every direction, in search of some means of salvation. The violence of the wind, together with the rain, which fell like hail, and the sand blinded their eyes, prevented many from reaching the objects they had aimed at.
Many were drowned from being stunned by scattered fragments of the buildings, which had been blown asunder by the storm; many others were crushed by floating timbers and logs, which were removed from the beach, and met them on their journey. To attempt a description of this sad event would be useless.

Another graphic example comes from the infamous Labor Day Hurricane of 1935 that made landfall near Miami, FL and killed at least 400 people (TWC, 2000):

On the afternoon of September 2, 1935, hundreds of people stood on the train platform in Matecumbe Key, Florida, anxiously awaiting the arrival of an evacuation train. The U.S. Weather Bureau had sent warning that a powerful hurricane was moving in from the Caribbean, and no one along the low-lying beaches of the Keys wanted to be in its path.
For hours, they waited at Islamorada Station as the train was held up in Homestead, delayed by red tape. By the time it finally arrived, so too had the most intense hurricane to strike the U.S. coastline in recorded history.
As the Labor Day Hurricane blew in, its winds roared with unimaginable force - more than 200 mph. Those waiting on the train platform were literally sandblasted beyond recognition, their clothes and skin sheared away. Even the smallest objects became deadly projectiles. Sheet metal and lumber were hurled through the air, impaling some victims against trees, decapitating others.
It was a gruesome scene, and one, thankfully, which has not been replayed to that extent since.

There have been several landfalls that wake the nation to the reality of a catastrophic disaster. Besides the two referenced previously, there was the 1900 Galveston Hurricane, the 1926 Miami Hurricane, and the 1938 Long Island Hurricane. However, it was Hurricane Hazel in 1954 that awakened people to the need for a forecasting and warning system. Since then, major landfalls included Camille in 1969 and Andrew in 1992, just to name a couple. The important point is that since the 1950s, very few lives have been lost in the U.S. due to hurricanes. Instead of death tolls in the 1,000s or 10,000s, the numbers have fallen to the 10s or 100s.

It is easy to show how significant intense hurricanes are to landfall statistics. Fifty percent of all landfalling storms are Tropical Storms that are responsible for 3% of the damage. Thirty percent of landfalling storms are non-intense hurricanes that cause 10% of the damage. That leaves intense hurricanes: they make up only 20% of all landfalling storms yet cause 87% of the damage (Gray, 1998).

Based on the value of coastal property, the population, and the likelihood of a hurricane landfall, Gray (1998) locates the five highest-risk areas, or what he calls the greatest “damage potential”: the Miami area (including Key West, Miami, Fort Lauderdale, and West Palm Beach), the Houston area (including Brownsville, Corpus Christi, Galveston, and Houston), the New Orleans area (including New Orleans, Gulfport, Mobile, and Pensacola), the Lake Charles area, and the Wilmington area (including Savannah, Charleston, and Wilmington). Based on the extreme cases of “damage potential”, the author created storm surge maps, or maps showing the effect of raising sea level by some prescribed amount, mimicking the effect of a storm surge. Appendix B contains some of the maps made to analyze the immense flooding potential of these high-risk areas.

Despite the severe weather that occurs during landfall (storm surge, rain, high wind, tornadoes), hurricanes claim relatively few lives when compared to other natural disasters. Figure 4.1 shows the distribution of deaths attributed to natural disasters.



However, the low death toll is compensated for by the contribution to property damage. Figure 4.2 shows the significance of hurricane landfalls to the insurance business.



Figures 4.1 and 4.2 encompass a relatively small dataset, but looking at the past 100 years, an interesting trend can be found: in general, the amount of property damage is increasing exponentially with time while the number of lives lost is decreasing exponentially with time.

There are simple reasons for these trends. The property damage increase is due entirely to larger coastal populations and more expensive houses/hotels/resorts. The decrease in lives lost could be due to at least two factors: better forecasts and better evacuations. One could make an argument as well that there has not been a major landfall on a populated area in a long time; should that happen, the “lives lost” curve could spike upward.



A key word when addressing landfalls is exposure (Pielke et al, 1997). Exposure is a function of the three P’s: population, property, and preparedness. The population of an area at risk affects how many people will be directly impacted and how many people need to be evacuated. Property refers to how many buildings, airplanes, ships, and cars could potentially be destroyed when the hurricane makes landfall. Finally, preparedness is basically how well the at-risk communities are prepared to handle a landfall (adequate building codes, construction supplies, hygiene/emergency materials, and a robust evacuation plan).

Three links make up the Chain of Risk Reduction: more accurate forecasts, effective emergency management, and public education. Each of these depends on each other completely… if one fails, the other two fail. For example, if forecasts are 100% accurate and emergency management personnel are aware of the threat and issue timely warnings, but the public either never receives the warnings or ignores them, the first two steps were futile. Likewise with a weak link anywhere else in the chain. Scientists, emergency personnel, and the public must work together to make coastal communities a safer place to live. Landsea (1999) questions whether the nation’s risk is matched by its response; unfortunately, the only way to answer that is to experience an intense hurricane landfall on a populated area.


5. Conclusions

Long-term trends of hurricane activity may be changing, but the effects that a landfall has on the coast have always been devastating. If the 60-year NTC oscillation is real, then the Atlantic Basin is about to experience tropical cyclone activity like it has not seen since the 1940s. The decreased activity lately combined with exploding coastal populations is setting the stage for a catastrophic landfall. People have let their guard down, and most likely underestimate the power of a major landfall. The number of people who would remember the infamous 1900, 1926, or 1938 landfalls is decreasing, and therefore decreasing society’s appreciation for such a disaster.

If the NTC is on the rise again as it appears to be, then the U.S. must be better prepared for more frequent and more intense landfalls, possibly placing areas of the coast under higher risk than they normally would be. Using the past to predict the future is not a new concept, but combined with the prototype seasonal forecasts from Colorado State University’s Gray Research Team (Gray, 198), it is the only tool scientists have to peer into the coming years. Although the property damage caused by a landfall will inevitably continue to rise, the death toll can be diminished through a combination of better forecasts, effective emergency management, and an educated public.


Appendix A : Saffir-Simpson Scale
CATEGORY MAX SUSTAINED SFC WINDS
[in kts (mph)]		 STORM SURGE
[in m (ft)]		 MSLP
[in mb ("Hg)]
Tropical Depression <35 (<40) - -
Tropical Storm 35-63 (40-73) - -
CAT 1 64-82 (74-95) 1.2-1.5 (4-5) >980 (>28.94)
Damage primarily to shrubbery, trees, foliage, and unanchored mobile homes. No real damage to other structures. Some damage to poorly constructed signs. Low-lying coastal roads inundated, minor pier damage, some small craft in exposed anchorages torn from moorings.
CAT 2 83-95 (96-110) 1.8-2.4 (6-8) 965-979 (28.50-28.91)
Considerable damage to shrubbery and tree foliage, some trees blown down. Major damage to exposed mobile homes. Extensive damage to poorly constructed signs. Some damage to roofing materials of buildings; some window and door damage. No major damage to buildings. Coastal roads and low-lying escape routes inland cut by rising water 2-4 hours before arrival of hurricane center. Considerable damage to piers. Marinas flooded. Small craft in unprotected anchorages torn from moorings. Evacuation of some shoreline residences and low-lying island areas required.
CAT 3 96-113 (111-130) 2.7-3.7 (9-12) 945-964 (27.91-28.47)
Foliage torn from trees, large trees blown down. Practically all poorly constructed signs blown down. Some damage to roofing materials of buildings; some window and door damage. Some structural damage to small buildings. Serious flooding at coast and many smaller structures near coast destroyed; larger structures near coast damaged by battering waves and floating debris. Low-lying escape routes inland cut by rising water 3-5 hours before hurricane center arrives. Flat terrain 5 feet or less above sea level flooded inland 8 miles or more. Evacuation of low-lying residences within several blocks of shoreline possibly required.
CAT 4 114-134 (131-154) 4.0-5.5 (13-18) 920-944 (27.17-27.88)
Shrubs and trees blown down, all signs down. Extensive damage to roofing materials, windows, and doors. Complete failure of roofs on many small residences. Complete destruction of mobile homes. Flat terrain 10 feet or less above sea level flooded inland as far as 6 miles. Major damage to lower floors of structures near shore due to flooding and battering by waves and floating debris. Low-lying escape routes inland cut by rising water 3-5 hours before hurricane center arrives. Major erosion of beaches. Massive evacuation of all residences within 500 yards of shore possibly required, and of single-story residences on low ground within 2 miles of shore.
CAT 5 >134 (>154) >5.5 (>18) <920 (<27.16)
Shrubs and trees blown down, considerable damage to roofs and buildings; all signs down. Very severe and extensive damage to windows and doors. Complete failure of roofs on many residences and industrial buildings. Extensive shattering of glass in windows and doors. Some complete building failures. Small buildings overturned or blown away. Complete destruction of mobile homes. Major damage to lower floors of all structures less than 15 feet above sea level within 500 yards of shore. Low-lying escape routes inland cut by rising water 3-5 hours before hurricane center arrives. Massive evacuation of residential areas on low ground within 5-10 miles of shore possibly required.



Appendix B : Storm Surge Maps

1m : TS - weak CAT1 (60kts)
2m : weak CAT2 (85kts)
3m : weak CAT3 (100kts)
4m : weak CAT4 (115kts)
5m : med CAT4 (125kts)
6m : weak CAT5 (140kts)
7m : strong CAT5 (150kts)

The winds, categories, and storm surge depth relationship is based on the Saffir-Simpson Scale of Hurricane Intensity.
The topographic data is courtesy of the USGS ETOPO30 dataset (30-arcsecond horiz res, 1-meter vert res) created in 1996.
The map and river data is courtesy of the CIA World Map database created in 1993.
Visualization was done using IDL v5.4 released in 2000.


References


Brian McNoldy for MESO, June 2001

http://www.mcwar.org


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