The Atlantic’s Great Tropical Cyclones – Part 3

The Atlantic’s Great Tropical Cyclones – cont’d

by: Eric Brown

21st Century

“The storm is master. Man, as a ball, is tossed twixt winds and billows.”
Friedrich von Schiller – William Tell

Tropical Storm Allison, June 2001 – One of the worst misconceptions about tropical cyclones is that only the strong ones are deadly and destructive. Wind, however, isn’t the hurricane’s greatest killer…it’s water. And as the previous two entries demonstrated, it often isn’t the water from the sea that you should worry about, it’s the water that falls from the sky. In November, 1991, Tropical Storm Thelma drowned the Philippines in an astonishing 20 inches of rain in just three hours. This was one of the highest rainfall rates ever recorded and it unleashed devastating flash floods that wiped out entire villages. It’s believed that at least 7,000 people died with possibly over 1,000 more unaccounted for. Nearly 5,000 of those were in the city of Ormoc alone. Of all the incredible, devastating typhoons that have ravaged the Philippines over the years, this puny little tropical storm that never had winds stronger than 50 mph was the deadliest (at least until epic Typhoon Haiyan killed 7,400 in 2013). Ten years later, this painful lesson would be brought a little closer to home.

Houston, Texas is the fourth largest city in the United States, home to 1.9 million people at the turn of the millennium. It sits in a flat, marshy coastal plain, laced by rivers and bayous much like New Orleans. The city’s average elevation is only about 50 feet above sea level, with many areas lower than that and a very high water table. However, before Allison, few tropical systems had exploited this geography. In 1837, a hurricane known as Racer’s Storm reportedly caused up to four feet of flooding in downtown Houston. The historic Galveston hurricane in 1900 caused minimal damage in Houston, as did the 1915 hurricane. In 1983, Hurricane Alicia blasted downtown with wind gusts approaching 100mph, blowing out numerous skyscraper windows. However flooding was minimal and mainly confined to eastern suburbs. No one realized just how vulnerable the city was.

After World War II, the city’s population exploded. Home to less than 400,000 before the war, by 1970 Houston was home to 1.2 million. As the city grew, so did its need for land and resources. Marshy, wetland areas were reclaimed to build new subdivisions and dredging of the city’s rivers and bayous lowered the land’s elevation. It was a recipe for disaster.

Allison’s origins can be traced back to a tropical wave that left the coast of Africa on May 21, very early for an African wave. It traveled all the way across the Atlantic nearly devoid of thunderstorms, moving inland over northern Venezuela before crossing into the Pacific. Threat, gone…or so it was thought. On June 2, it stalled off the south coast of Mexico and developed a strong circulation, however strong wind shear prevented any development and the wave moved north, inland over Mexico. The wave disappeared, however the strong mid-level circulation remained and emerged over the Gulf of Mexico on June 4. Ripe conditions triggered strong thunderstorms and a disturbance developed beneath the strong mid-level circulation. It wasn’t long before a low-level center redeveloped and Allison was born.

This is when things got complicated. A low pressure system high in the atmosphere over south Texas southwest of Allison interacted with the storm’s circulation and helped fuel the explosion of thunderstorms around the center. It also helped Allison greatly broaden its wind field. Within hours of its official formation, tropical storm force winds (>40 mph) extended out up to 230 miles from its center and the storm itself, despite tepid water temperatures, had intensified with 60 mph sustained winds. Now the real question remained…where was it going?

Trapped beneath a large high pressure system over the southeast, Allison initially wasn’t going anywhere. Model forecasts were all over the place. Some took the storm west into southern Texas and northern Mexico. Others took the storm east into Louisiana. Instead, helped in part by the push from the upper low to the south, Allison drifted north toward east Texas. The storm actually weakened before making landfall, due in large part to cooler water near the coastline. However, it’s what happened after landfall that changed everything.

Once inland, Allison quickly weakened to a tropical depression. Now it had moved away from the influence of the upper low that had been helping it drift north and ran into a wall of high pressure. The storm came to a dead stop right over the Houston metro and all their bayous. Making matters worse, the circulation was so large that it was able to tap into the warm waters of the Gulf of Mexico, funneling a terrific deluge of rain into the Texas coast.

Parts of the Houston metro received over two feet of rain in 12 hours. Rainfall rates as high as six inches per hour were recorded as the storm parked over the city. The effect was catastrophic. Five of the six bayous in the city burst their banks. Four of them reached the highest water levels in over a century of record keeping. By the morning of June 9th, downtown and sprawling neighborhoods north of Interstate 610 were inundated. Despite the worst of the flooding miraculously missing Friday rush hour the previous afternoon, thousands of vehicles were stranded on the city’s freeways, which were under several feet of water in places. Hundreds of drivers were rescued by helicopter or were forced to seek higher ground on foot. Thousands more had to be airlifted off of rooftops as homes flooded into the upper floors.

The sprawling complex of hospitals that make up the Texas Medical Center, the largest complex of its kind in the world, was also inundated. Multiple hospitals lost all power, including backup generators, and were forced to evacuate thousands of patients, many of them in critical condition, down dark stairwells in sweltering 100+ degree heat. Patients who could not breathe on their own had to be manually ventilated for hours before they could be airlifted to other hospitals by Coast Guard and National Guard helicopters. Miraculously, no patients died during the evacuation. Multiple medical science centers lost decades of priceless research, including the life’s work of many scientists. All three major universities in the city also suffered significant flooding, none more so than the University of Houston, whose law library lost 174,000 books.

The flooding was arguably exacerbated by Houston’s system of underground tunnels that connect many of the city’s buildings downtown. Floodwaters spilled into the tunnels and inundated the basements of numerous buildings. This included parts of the upscale theater district, shopping centers, government buildings, underground parking garages, and several high rise office towers.

After spending about two days parked over Houston, Allison began a slow drift back to the south as two competing high pressure systems to the northwest and northeast battled for dominance. Finally, Allison emerged back over the Gulf of Mexico on June 10. The high pressures had begun to weaken and an upper level trough to the south began to move the storm east-northeast toward Louisiana. As it interacted with the trough, its circulation expanded even more and began to take on the characteristics of a non-tropical low pressure system, becoming something of a hybrid.

Allison made landfall over the Louisiana bayous, unleashing another deluge of rain. Parts of Louisiana got nearly 30 inches of rain. Luckily this was over much more sparsely populated areas. As it was, over 1,000 homes were flooded in St. Tammany Parish alone. And so Allison began its weeklong march across the southeast that everyone pretty much forgot about. The storm dropped over ten inches of rain on nine states, causing widespread flooding in all of them. Extensive river flooding struck Pennsylvania, where over 200 homes were destroyed and numerous roads and bridges were washed out. Flooding occurred as far away as New England. Finally, on June 17, Allison moved off the New England coast and became extratropical.

The devastation in Houston was incredible. Over 73,000 homes were flooded, nearly 3,000 of which were total losses, and 300,000 people were left homeless. Also, roughly 95,000 vehicles were flooded. Hospitals at the Texas Medical Center were shut down for over a month. A temporary hospital with 88 doctors was ultimately set up to compensate for the loss of medical service for the city. 22 people died in the Houston area, most of them in cars that became trapped in floodwaters. Miraculously, none of the deaths occurred in homes. This is likely due to the worst of the flooding happening in daylight. Damage in Houston alone came to $5.2 billion. Overall, Allison killed 41 people, making it the deadliest storm in US history that never became a hurricane. Eight people died in Florida and seven died in Pennsylvania, evidence of the far reaching effects of the storm. Damage came to $8.5 billion.

In the wake of Allison, however, little was done to beef up the city’s flood control systems. The Harris County Flood Control district actually saw funding cuts that caused some improvement projects to be shelved entirely. Allison was seen as Houston’s “perfect storm.” Few with actual authority to make decisions seemed to think it could ever happen again. Sadly, 16 years later, an even greater catastrophe struck the city of Houston.

Allison should remind us that any storm can be devastating, no matter how strong it is, and it’s up to us to understand the vulnerabilities of our communities and prepare accordingly. Unfortunately, it usually takes a tragedy to bring about meaningful change. Mother Nature teaches us hard lessons. It’s up to us to listen.

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