Scientists
say the effects of Hurricane Harvey, which has been stalled over the
Texas Gulf Coast since Friday and dumped more than 20 inches of rain in
some areas, were worsened by a lethal confluence of meteorological
events: warm water in the Gulf of Mexico that intensified the rainfall,
and a lack of winds in the upper atmosphere that could have steered
Harvey away from land.
Exacerbating the situation, said Hal Needham,
a storm surge expert and founder of the private firm Marine Weather
& Climate in Galveston, Tex., was that the storm surge elevated
Galveston Bay, blocking drainage of the rain that pummeled coastal and
inland areas.
“A
two- or three-foot storm surge alone would not have been catastrophic,”
Mr. Needham said. “It was all these ingredients coming together.”
And it’s not over.
Dennis Feltgen, a spokesman for the National Oceanic and Atmospheric Administration’s National Hurricane Center
in Miami, said the driving rains would continue for another two or
three days, pouring an additional 15 to 25 inches over parts of
Southeast Texas. Some areas, he said, could see as much as 50 inches of
rain.
“This is unprecedented,” he said.
Yet it does have some parallels. J. Marshall Shepherd,
director of the atmospheric sciences program at the University of
Georgia, said Harvey is very much like Allison, a tropical storm that
flooded Houston badly in 2001 because it lingered over the city and
dumped prodigious amounts of rain.
Hurricanes are essentially large weather engines fueled by the warm waters of the ocean below.
The
mind-boggling amount of rainfall during Harvey is a function of the
storm sitting by the Gulf of Mexico and continuing to draw moisture
directly from it. Because of the orientation of the storm, Dr. Shepherd
said, “you’ve just got this stream of moisture firehosing into the
Houston region,” as the moisture is constantly replenished by the gulf.
“This could go down as the worst flood disaster in U.S. history.”
Scientists are increasingly able to link some extreme weather events to climate change, but when it comes to hurricanes, many say there remain a number of unknowns.
What is clear, though, is that rising global temperatures warm the
oceans, which causes more water to evaporate into the atmosphere.
Graphic
Maps: Harvey’s Destructive Path Through Texas
Maps and animated satellite imagery show the scale and reach of the storm.
The buildup of moisture in turn contributes to the global increase in extreme rainfall, Kenneth Kunkel, a researcher with the North Carolina Institute for Climate Studies, said.
Even
without climate change as a factor, Dr. Kunkel said, oceans are
normally warm this time of year. But, he pointed out, the Gulf of Mexico
has been warmer than average lately, most likely feeding into the deluge.
Several scientists stressed that while the damage of Hurricane Harvey was unrelenting, it was not unexpected.
Forecasters
were anticipating a very wet storm to park over Houston for an extended
period. “This, honestly, is playing out, unfortunately, exactly as we
thought it would several days ago,” Dr. Shepherd said.
He
said he grew worried when he saw public statements of relief that the
storm had been downgraded from Category 4 strength to a tropical storm.
He said the change in technical terminology may have confused the public
and led local officials to lose focus on the greater threat, a multiday
rain event.
“There
was always a one-two punch with this particular storm, but we were
always more concerned about the ‘two,’ the rainfall,” Dr. Shepherd said.
“Once that subsided, people like me said, ‘O.K., we’re just getting
started.’”
Different models predict different aspects of storms. Rick Luettich
is director of the University of North Carolina’s Institute of Marine
Sciences and an internationally recognized expert on storm surge, and a
principal developer of the ADCIRC
computer programs, which can be used to predict storm surge. For Harvey,
Friday’s calculations suggested a surge of eight to 10 feet in the
coastal areas, with as much as 12 feet in some of the shallow coastal
estuaries.
The
water threat that is getting the most attention with Harvey, however,
is the intense rainfall predicted as the storm lingers over the Houston
area. With storms growing wetter thanks to climate change, Mr. Luettich
and his collaborators are trying to add rainfall calculations to the
coastal surge forecasting model.
“People don’t care, if they got wet or got drowned, whether their water was salty or fresh,” he said.
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