Jul 11, 2019

Developing hurricane threatens New Orleans levees

Computer model simulation of water vapor and winds in developing Hurricane Barry on July 12. Credit: Earth Simulator

A developing storm in the north-central Gulf of Mexico, which is expected to become a hurricane by Saturday, threatens to unleash one of the worst flood events in New Orleans' history this weekend.

The big picture: Because the storm, to be named Barry, is lumbering off the coast, it's pulling copious amounts of moisture from the warm waters of the Gulf of Mexico and is poised to direct it like a firehose at Louisiana and Mississippi. The Mississippi River is already so high in the New Orleans area that heavy rain plus a storm surge could overtop the cities' river levees for the first time, flooding large parts of the city.

How it works: The National Hurricane Center is predicting that Barry will become a tropical storm by Friday, and intensify into a Category 1 hurricane by Saturday, before making landfall along the coast of Louisiana.

  • The storm's winds are not the main threat, however.
  • Up to 2 feet of rain is likely to fall in parts of Louisiana as the storm wanders onshore like a car inching forward at rush hour. This rain could overwhelm New Orleans' pumping systems, causing a repeat of Wednesday morning's flash flood emergency.
  • The rain, combined with a storm surge of 3–5 feet from the incoming storm, could cause the Mississippi River to rise to near or above 20 feet in New Orleans.
  • The river levees protecting the city are 20 feet high, and haven't been tested in this way. (Hurricane Katrina in 2005 caused failures in the city's storm surge protection system.)

Between the lines: The National Weather Service has issued a rare "High Risk" excessive rainfall outlook for coastal Louisiana, including New Orleans, three days in advance. Since this product became available in 2007, such day 3 outlooks have only been issued twice.

  • One such instance was for Hurricane Florence, which caused billions in flood damage in North Carolina in 2018, and the other was Hurricane Harvey, which unleashed the heaviest rainfall event the U.S. has ever experienced.

Why now? Ordinarily, a storm of this magnitude would not risk overtopping New Orleans' levee system.

  • However, the massive amount of rain that has fallen across the Plains, Midwest and Mississippi River Valley has created an exceptional antecedent condition that sets up a potentially unprecedented disaster.
  • The Mississippi River at Baton Rouge has been at flood stage since late February, an unprecedented stretch.

Climate change may be playing some role in this as well. While a tropical storm or hurricane in the Gulf of Mexico during mid-July is relatively common, scientific studies show that as air and ocean temperatures increase, more rain is falling in heavy bursts, including from tropical cyclones.

  • Modeling studies show that future tropical cyclones will carry even more water vapor with them, and bring more flooding to the areas they hit. They may also have stronger winds, overall, than typical storms today.
  • Other research suggests that tropical storms and hurricanes are, in general, slowing their forward movement as the climate changes, which also favors major floods.
  • In addition, studies show that climate change can set in motion a series of unprecedented events, similar to a domino effect.
  • A study published in 2018, for example, projected greater instances of simultaneous extreme events with wide-ranging repercussions as global warming continues during the next several decades.

The bottom line: Residents of New Orleans, and indeed much of the central Gulf Coast, need to prepare now for a significant, dangerous storm.

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