Hurricanes are extreme storms. And when they rain, they can pour — hour after hour after hour. Just ask anyone in the vicinity of Harris County, Texas. It includes Houston, the fourth largest U.S. city. And since a storm named Harvey made landfall as a category 4 hurricane late August 25, torrents of rain have deluged the east-central Gulf coast of Texas.
The slow moving tropical cyclone stalled over the state — for days. So the water kept coming. And coming. And coming.
More than 1.2 meters (4 feet) of water fell in some parts of East Texas, according to the National Weather Service (NWS) office in Houston. Flooding prompted widespread water rescues across the greater Houston area (some 80 kilometers, or 50 miles, from the coast). This region is home to nearly 6 million people. And by the fourth day of Harvey’s rains, flood waters already covered an estimated one-third of Houston’s more than 1,550 square kilometer (600 square mile) area.
A rain gauge in Houston’s Cedar Bayou logged a staggering 131.78 centimeters (51.88 inches) of rain, the NWS tweeted out of its Houston office. That was as of 3:40 p.m. August 29. This surpassed the old record for a tropical storm in the continental United States. That record is 122 centimeters (48 inches). It was set in 1978, also in Texas, when tropical cyclone Amelia blew through.
The only U.S. storm to drop more water in one place was Hurricane Hiki. This 1950 cyclone unleashed 132 centimeters (52 inches) of water on the Hawaiian island of Kauai. (Those rains triggered dozens of mudslides and more.) When measurements of Harvey’s totals are complete, scientists expect it will have toppled Hiki’s record.
Meanwhile, even six days into the disaster, conditions in Texas had not abated. Since Harvey hit, at least 6 million Texans have seen more than 76 centimeters (30 inches) of rain, the NWS reports. It has summed up this storm as simply an “unprecedented and historic” rainfall “catastrophe.”
Flooding can be disastrous — and deadly
The rain didn’t just fall and then run back into the ocean. Much of it has been collecting throughout East Texas cities. First it filled low-lying streets and ditches. Then it swelled the bayous that wind through some of these towns. More water collected behind big lake-size reservoirs. (Those reservoirs began spilling over for the first time in history.) Eventually, there was nowhere else for the water to go. The result: Massive, catastrophic flooding.
Hurricanes are known for their extreme winds and devastating storm surges. But many people don’t appreciate the destructive force caused by the flooding that can occur inland — sometimes far from where a cyclone made landfall.
That is, however, something that a scientist with the National Hurricane Center in Miami, Fla., tried to drive home three years ago. Edward Rappaport is currently the acting director at the center. In a March 2014 report, he noted that when it comes to tropical cyclones in the United States, “Rainfall-induced freshwater floods and mudslides accounted for about one-quarter of the deaths.” (It’s actually a bit more than that: 27 percent.)
Storm surges can slam coasts with water that reaches 3.7 meters (12 feet) high or more. And, indeed, “Storm surge was responsible for about half of the fatalities,” he noted. What about wind-related impacts — such as roofing material and toys or porch chairs that can be lofted into the air and slung about like missiles? These accounted for a mere 8 in every 100 cyclone-linked deaths.
Rappaport’s analysis appeared in the Bulletin of the American Meteorological Society.
The science behind Harvey’s punch
A mix of factors made Harvey a recipe for disaster. For starters, Houston alone is home to 2.3 million people. It’s also America’s fastest growing urban area. Development there has been transforming thousands of hectares (acres) of exposed ground into concrete roads, buildings and parking lots. And these hard surfaces keep water from draining into the ground.
The water can quickly collect into raging floodwaters.
Because the ground is unusually flat in the greater Houston area, rainwater doesn’t rush to run off. This region also sits atop soil known to be relatively impermeable. The U.S. Geological Survey explains this as ground that has a hard time draining. Excess water tends to collect and stagnate atop the ground in Houston, rather than percolating deeply into it.
In addition, Harvey’s winds were accompanied by an exceptionally moist mass of air. One value meteorologists use to estimate the potential for every rainfall is called the PWAT. It stands for Precipitable WATer. It’s a gauge of how much water the clouds are holding.
A PWAT value of 8.3 centimeters (3.25 inches) means that the column of air holds that much water. And Harvey’s air had such a PWAT as it was sitting over Houston. That amount alone may not seem impressive. But consider a wet washcloth. If you wring it out, you won’t fill a bucket. But then you’re handed another washcloth to wring out. And then another. And another. And another. Soon enough, that bucket will be overflowing!
With onshore winds continually replenishing Harvey with more moisture picked up over the warm Gulf of Mexico, this storm had no shortage of humidity to tap into — and release!
Where does Harvey rank against other storms?
Marc Chenard is with the National Oceanic and Atmospheric Administration. He’s at the agency’s Weather Prediction Center in College Park, Md. This meteorologist has been closely monitoring Harvey for more than a week. “While some [rain] totals soared to record levels approaching 50 inches [127 centimeters], what was really remarkable was just how large of a region in southeast Texas saw amounts climbing to above 20 inches [51 centimeters],” he said. “The flooding has been so much worse given the magnitude of the area impacted.”
His center was even forced to add two new colors of purple to the maps it uses when forecasting rainfall. It points to how this event has literally been “off the charts.”
Harvey has underscored the importance of taking all tropical cyclones seriously, even those that may not be classified as a tropical storm or full-blown hurricane. In Rappaport’s study, “six of the 10” most deadly tropical cyclones that he studied were not major hurricanes. Instead, they registered as “tropical storms or category 1 hurricanes at landfall.” (Major hurricanes are those rated category 3 or higher.)
One of the biggest factors that contributed to Harvey being a worst-case nightmare for the Gulf Coast has been its speed, or lack thereof. As of August 29, the storm’s center was still fewer than 322 kilometers (200 miles) from where its rampage began. Its average forward speed was under 3.2 kilometers (2 miles) per hour. This means broad areas found themselves trapped underneath the same enormous bands of rain for agonizingly long periods.
And here’s why storms with extreme winds or rains are likely to be increasingly common in years to come. As climate change takes hold, upper-atmosphere winds slacken. This ups the chance a storm will stall. Warming temperatures also increase the air’s ability to hold moisture. But what goes up must come down, eventually. And as Harvey has shown, that moisture can pour out by the bucket-full. When those storms move at a snail's pace, they can foster flooding.
Scientists worry that storms rivaling the intensity of Harvey are more likely in the future. It’s alarming to consider that Harvey may have given people a taste of what the future of weather might hold. They say that’s yet another reason why the changing climate should be on everybody’s radar. And as past storms have made clear, hurricanes are not a threat merely to coastal towns. Their danger zone can extend far, far inland.
atmosphere The envelope of gases surrounding Earth or another planet.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
bayou The French-Indian-derived name for smallish, natural rivers that meander through Louisiana.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
development (in economics and social sciences) The conversion of land from its natural state into another so that it can be used for housing, agriculture, or resource development.
factor Something that plays a role in a particular condition or event; a contributor.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
gauge A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.
geological Adjective to describe things related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.
humidity A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
impermeable An adjective for something that will not let a liquid flow through it.
literally A term that means precisely what it says. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.
meteorologist Someone who studies weather and climate events.
reservoir A large store of something. Lakes are reservoirs that hold water. People who study infections refer to the environment in which germs can survive safely (such as the bodies of birds or pigs) as living reservoirs.
society An integrated group of people or animals that generally cooperate and support one another for the greater good of them all.
storm surge A storm-generated rise in water above normal tidal level. In most cases, the largest cause of storm surge is strong onshore winds in a hurricane or tropical storm.
survey (v.) To ask questions that glean data on the opinions, practices (such as dining or sleeping habits), knowledge or skills of a broad range of people. Researchers select the number and types of people questioned in hopes that the answers these individuals give will be representative of others who are their age, belong to the same ethnic group or live in the same region. (n.) The list of questions that will be offered to glean those data.
Texas The second largest state in the United States, located along the southern border with Mexico. It is about 1,270 kilometers (790 miles) long and covers an area of 696,000 square kilometers (268,581 square miles).
tropical cyclone A strong, rotating storm. These usually form over tropical areas around the equator where the water is warm. Tropical cyclones have strong winds of more than 119 kilometers (74 miles) per hour and usually have heavy rain. Large ones in the Atlantic are known as hurricanes. Those in the Pacific are termed typhoons.
U.S. Geological Survey (or USGS) This is the largest nonmilitary U.S. agency charged with mapping water, Earth and biological resources. It collects information to help monitor the health of ecosystems, natural resources and natural hazards. It also studies the impacts of climate and land-use changes. A part of the U.S. Department of the Interior, USGS is headquartered in Reston, Va.
unique Something that is unlike anything else; the only one of its kind.
urban Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: E.N. Rappaport. Fatalities in the United States from Atlantic tropical cyclones: New data and interpretation. Bulletin of the American Meteorological Society, Vol. 95, May 1, 2014, p. 341. doi:10.1175/bams-d-12-00074.1.