Hurricane Harvey might have beget rain in biblical volume and billions in flood damage when it stalled over Houston, but it’s not the wettest storm delivered by the Atlantic.
Not by far.
Cuba got hammered by more than 100 inches of rain when Hurricane Flora sat over the island for four days in 1963. And even earlier, in 1909 before hurricanes were named, a storm dropped more than 96 inches of rain on Jamaica. In more recent history, Wilma dumped more than 62 inches of rain on Mexico in 2005 and Hurricane Mitch, blamed for killing more than 11,000 in Central America in 1998, soaked Nicaragua with more than 62 inches, according to records compiled by National Oceanic and Atmospheric Administration forecaster David Roth.
As Texas digs its way out of what’s likely to be the state’s worst natural disaster in history, the widespread flooding that submerged whole towns and sent 30,000 people to shelters serves as a sobering reminder that hurricanes may be defined by wind. But their most lethal weapon is usually water.
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In 1979, Claudette produced torrential rainfall in Texas and Louisiana, dropping 42 inches in a single day on Alvin, Texas, which got slammed again by Harvey. Alvin still holds the U.S. record for the most rain over 24 hours, although Harvey might surpass that. Texas also got slammed with 40 inches of rain from Tropical Storm Allison in 2001.
“Both were fairly weak tropical storms, but they stalled over the Texas area and dumped copious amounts of rain,” NOAA meteorologist Neal Dorst said in an email.
In the coming days, as flood waters subside and Texas wrestles with a clean-up expected to take years, meteorologists will begin confirming rain totals and rolling out new records. So far, Roth said at least five locations along Harvey’s path have recorded levels above 52 inches, breaking the 67-year-old U.S. record set by Hurricane Hiki in Hawaii.
“Houston has had really, really bad events, but not only was it more rain, it was over such a broad area that nobody could really prepare for this,” said Roth, who helped predict Harvey’s rain at NOAA’s Weather Prediction Center. “Beaumont never had more than 20 inches of rain in a seven-day period before. That shows you how small past storms were.”
What makes a storm generate so much rain often is determined by what’s going on around it. Harvey swept over a warm Gulf of Mexico very slowly, then parked over a coastal plain full of swamps and bayous.
“It’s just a level of water that keeps going up,” Roth said. “It’s as if Galveston Bay or the Gulf of Mexico is going up.”
Heavy rain also hit the area just two weeks before, on Aug. 7 and 8, with five to 10 inches that triggered flash floods and filled up bayous. It’s when a storm lingers that the worst flooding begins and sometimes, like Harvey, creates water so deep that the land, which would normally sap its strength, becomes a new source of instability.
“All of these examples were due not so much to the wind storm as the tropical moisture in these systems being wrung out over one area for several days,” Dorst said.
Cuba had twice the amount of rainfall as Harvey in part because of the island’s mountains, which can drive up a cyclone’s rain production as moist air is forced up the slopes. It also stalled for twice as long, Roth said.
How much climate change played in Harvey remains to be seen. Scientists have said a warmer planet will likely create more intense hurricanes. But it’s not yet clear if the 1.53 degree Fahrenheit rise since 1880 made the storm’s rain worse.
Harvey started as an average-sized tropical storm. But a day before it made landfall, it encountered wind shear in the Gulf of Mexico that flattened it, broadened the storm’s circulation and extended its reach, Roth said. Most tropical storms have a uniform area of pressure with a radius of about 150 miles. Harvey’s was closer to 200 miles, suggesting the amount of rain was going to be higher than average. Models also predicted the storm would linger, with some leaving it in the same place for so long forecasters assumed the models were mistaken, Roth said.
“That’s what initially scared a lot of us, because if this stalled, it was large and they were going to get a lot of rain,” he said.
Last Monday, forecasters called for about 15 inches of rain — a lot but not out of the ordinary for a tropical system. After repeated model runs kept the storm lingering, they began upping their rain forecast.
“Our model run went from 15 to 20 inches on early Tuesday. By Wednesday, we were at 25,” Roth said. “Keep in mind, neither our color scale or the National Weather Service have colors above 20. We’ve never depicted 25, 35, 40 inches of rain before, so everybody’s color palettes were starting to hit these huge areas with no definition.”
So they added new colors to show 25 inches. And almost immediately, the forecast jumped to 38 inches, he said. As Harvey crept inland, it also started training — or sending the same rain bands over the same spot again and again. It eventually squat over southeast Texas for two and a half days, setting a new record in the continental U.S., Roth said.
Forecasters also released a weather balloon as Harvey made landfall. They wanted to understand just what kind of rain they were dealing with. They can predict rain by calculating a “precipitable water value” in the storm, basically measuring the storm’s water vapor and determining in inches how much water that amounts to on the ground. Usually in the tropics, Roth said, the amount is 1.25 inches. But as Harvey’s western eye wall crossed the coast, they measured 3.26 inches, the highest ever recorded in the U.S., he said.
“You worry over a forecast, that you’re going to cry wolf,” Roth said. “Every forecaster worries about that. But we didn’t this time.”
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