(Bloomberg) -- Harvey came on the Texas shore as a Category 4 hurricane evoking comparisons to Katrina, Ike and Sandy. But it’s proven to be a very different beast.
While the public often ranks hurricanes by wind speed, thanks to a traditional scoring system known as the Saffir-Simpson scale, a tropical cyclone has more than one way to kill. Two of the deadliest can’t be measured in terms of miles-per-hour wind speeds, and are all but invisible to that classification.
Katrina, Ike and Sandy were known for their initial surge, and each slammed into high population areas with devastating results. But they traveled further than Harvey as hurricanes, pushing massive walls of water before them. The reconstituted Harvey only reached Category 4 strength shortly before landfall, lacking the time to build a larger surge. In the end, though, Harvey is likely to secure its own spot in hurricane history for its effect on Houston.
“Every hurricane is known for one of three things: wind, surge or inland rain,” said Phil Klotzbach, a hurricane researcher with Colorado State University. “This is the inland rain variety. Its winds are weakening but the rain is not. There could be another four or five days of this.”
As much as 25 inches of rain has already fallen in the Houston area, and another two feet is possible, the National Hurricane Center has said.
“Houston has another 100 hours of this,” said Todd Crawford, chief meteorologist at The Weather Company in Andover, Massachusetts. “Words really can’t express the impacts this will have when all is said and done. There is no historical comparison. It is simply a tragedy of epic proportions.”
The rain has largely been concentrated in just one part of Texas because the storm has been basically motionless for more than day, and is shifting very slowly back into the Gulf. That’s because it’s been pinned between two high pressure systems -- one anchored in California, causing a heat wave in that state, and the other in the U.S. northeast, where it is bringing fair weather.
The dueling high pressure systems have cut off strong currents across the central U.S. that are needed to pull or push away hurricanes, which can’t move under their own power, according to Klotzbach. “It is a pebble in a stagnant stream," he said. “There is no flow to make it move.”
Harvey has been parked to the northwest of Victoria, Texas for a day and because tropical systems rotate counter-clockwise, it has just been pulling moisture off the Gulf of Mexico, called a feeder band, and dragging it through Houston where more than 25 inches (64 centimeters) has already fallen and a flooding disaster is mounting.
“That feeder band is going to remain in the general vicinity of Houston perhaps through Monday or even beyond,” said Dan Pydynowski, a meteorologist with AccuWeather Inc. in State College, Pennsylvania.
A Houston flood has been the fear of meteorologists for almost a week. On Tuesday, the U.S. Weather Prediction Center warned Houston and the Texas coast would get heavy rain because of Harvey. The forecast was made before Harvey even managed to regenerate into a hurricane and was nothing but a ragged collection of thunderstorms over the Yucatan Peninsula in Mexico.
While Harvey was lumped initially with Katrina, Ike and Sandy, that link was more a product of phrasing and tradition than anything else.
The Saffir-Simpson scale is based primarily on wind speed, according to Dennis Feltgen, a spokesman for the National Hurricane Center in Miami. But that measurement doesn’t take into consideration what’s referred to as "storm surge," which accounts for nearly half of all hurricane deaths and is one of the most destructive forces a tropical cyclone can unleash.
Katrina was the most costly U.S. weather disaster, killing at least 1,800 people and causing $160 billion in damage, adjusted for inflation. It came ashore as a Category 3 system. Sandy, the second most expensive, was only Category 1 before being downgraded to a hybrid as it came to shore in New Jersey. Finally, Ike, fourth on the damage list, was a Category 2 when it erased the Bolivar Peninsula in Texas.
The damage figures point out a flaw in the Saffir-Simpson scale, Klotzbach said. Many meteorologists would like to get rid of it, or replace it with something better, but it is “deeply engrained with the public.”