NewsSeptember 17, 2012

Hurricane Isaac Spurs Design of Storm Surge Warnings

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Andrew Freedman

By Andrew Freedman

When they strike land, hurricanes bring a slew of threats ashore, from strong winds to flooding rains. The deadliest weapon in a hurricane’s arsenal is its storm surge, the wall of ocean water that the storm’s winds and very low air pressure push ashore. It was the storm surge from Hurricane Katrina, not strong winds, which overwhelmed inadequate levees in New Orleans in 2005, killing about 1,800 people. History is full of examples of catastrophic storm surge events, such as Hurricane Camille in 1969, which killed 259 people and flattened the Mississippi coastline under a wall of water nearly 30 feet high.

Satellite view of Hurricane Isaac approaching Louisiana on August 28, 2012. 
Credit: NASA.

Yet despite the fact that storm surge is the leading killer during hurricanes — and may become even more of a concern as seas rise in response to global warming — the National Weather Service (NWS) has never issued a weather warning specifically for this threat. Instead, storm surge has long been addressed as part of the broader hurricane warning system.

The glaring absence of a storm surge-warning category was already recognized within the National Hurricane Center (NHC) prior to Hurricane Isaac, which was a Category 1 storm on the Saffir Simpson Hurricane Wind Scale when it made landfall in coastal Louisiana on August 28. But in the wake of that storm, which brought a significant storm surge that seemed to surprise thousands of low-lying coastal residents, there have been more calls to reform the warning system in order to more clearly communicate the storm surge threat to coastal residents.

The low rating of Isaac presented a major challenge for NHC forecasters in Miami, local NWS offices in the hurricane-warning zone, as well as the media and emergency management officials. Many Louisiana residents simply looked at the Saffir Simpson rating and based their preparations on that, perhaps not realizing that, since 2010, the Saffir Simpson rating has not taken into account storm surge or the effects of heavy rainfall, which were the two biggest threats that Isaac posed.

Despite its relatively weak sustained winds, Isaac was an unusually broad storm, and had a very low central pressure that was more characteristic of a strong Category 2 hurricane. Making the situation worse, it moved ashore at a snail’s pace, ensuring that flooding would occur during multiple high tides. Hurricane Isaac brought a storm surge of 6 to 12 feet or more above ground level to the Louisiana coast, a surge that was accurately forecasted by the NHC.

Hurricane Isaac's winds and storm surge caused the Mississippi River to flow backwards for 24 hours on August 28, 2012, as data from this USGS gauge at Belle Chasse, La. shows. Click on the image for a larger version.
Credit: USGS.

Despite the NHC forecast, it seemed that many people were surprised that a Category 1 storm could push so much Gulf of Mexico water ashore. There were numerous rescues during the storm in low-lying LaPlace, St. Tammany, and Plaquemines Parishes, closest to where Isaac first roared ashore.

Jamie Rhome, the storm surge lead for NHC, said the surge forecasts for Isaac were accurate, yet there may have been some breakdowns in the communications process down to the local level. “It’s hard to envision the forecast being any more accurate,” Rhome said.

Hurricane Isaac, along with other recent storms, has helped advance work that was already underway at the NHC to improve public understanding of storm threats by devising new warnings and graphical communications tools. A storm-surge warning is among the measures that are currently being considered, along with more precise and engaging graphics showing anticipated inundation from storm surge.

In addition, some meteorologists have been advocating for a new scale — or for modifications to the Saffir Simpson scale — that would incorporate the threat of flooding rains and storm surge. However, hurricane forecasters at the NHC are largely opposed to this idea.

“Tropical cyclones cannot be easily categorized by storm surge because the surge is not a characteristic of the storm alone, being also dependent on the shape and bathymetry of the affected coastline, the storm’s forward motion, angle of approach, and so on,” the NHC said in a statement released in early September.

Diagram of a hurricane storm surge.

“[The] NHC believes that the clearest way to communicate each of the hurricane hazards is to do so directly and distinctly, and not conflate them as the proposed integrated scales do,” the NHC said.

In addition, the Hurricane Center said that combined scales can do more to confuse the public, since they don’t provide much information about which hurricane-related threat is going to affect them the most, and different threats require different responses. For example, for areas that will mainly face very strong winds, the Hurricane Center usually advises people to shelter in place, yet for storm surge, the recommended course of action is usually to evacuate.

Regarding storm surge and rainfall, NHC Hurricane Specialist Robbie Berg said, “We have a scale for those. It’s called ‘feet’ and ’inches.’ “

Instead of devising a new scale for rating tropical storms and hurricanes, Berg and his NHC colleagues favor improving the forecast products they issue. To do this, they have tapped into the expertise of social scientists, who are helping forecasters understand how people interpret graphics and process information. “This is a new thing,” Berg said. “Most of the time the National Weather Service has done things by themselves.”

Rhome said that the old way of doing things was all based on physical sciences, and the results were often lackluster communications’ products that confused people. Now, however, the goal of clear communication is the main driver behind developing new forecast products, he said. “At least from my perspective, it is a bit of a paradigm shift.

A NOAA storm surge threat map issued as Hurricane Isaac approached the Louisiana coastline on August 28.
Credit: NOAA/NWS.

“The scientific community has been trained to design graphics that are inherently complicated,” said Gina Eosco, a risk communications expert at Cornell University who is working with the Hurricane Center to rethink the way the Hurricane Center communicates the storm surge threat.

Eosco said a major challenge is ascertaining how to convey to people that each storm packs a unique punch. “Every hurricane is like a fingerprint; every one is different,” she said.

Some changes to storm forecasts have already been made. For example, text-based storm-surge information is now expressed as “depth of water above ground,” rather than “feet of water above normal tide levels,” which is how it used to be communicated.

The old standard required coastal residents to look up the normal tide levels on the day a storm was making landfall and then add in the expected surge to arrive at the expected inundation level.

The next step is the inundation graphic, which will represent a shift away from text-based warning information. “It’s very hard to envision storm surge with text,” Rhome said.

As for the timeline on these new products, Rhome and other experts interviewed said the inundation graphics could be rolled out as soon as next year, while the warnings will have to go through an experimental phase before officially being added to the roster of NWS watches and warnings.

Meanwhile, the unusually active Atlantic Hurricane Season continues, with forecasters scanning the horizon for the next big storm to affect the U.S.