Hurricane Storm Surge Scales – What Does the National Hurricane Center Think?


Link: NHC Views on Storm Surge Scales
Released September 10, 2010

There are scales for tornadoes, hurricanes, earthquakes, and other natural hazards.  In the aftermath of Hurricane Isaac, there were numerous calls for the National Hurricane Center to add back in a storm surge scale into the hurricane scale.  In an August 31st article from the New York Times, “Climatologists like Kerry Emanuel of the Massachusetts Institute of Technology have said that any classification should include both wind speed and surge. Otherwise, he argues, coastal residents can be easily misled.”

In 2010, the National Hurricane Center removed verbage in the Saffir-Simpson Scale that referred to storm surge in hurricanes.  In a one-page document posted on the National Hurricane Center (NHC) website earlier this week, the NHC Public Affairs staff shared the reason why storm surge was removed from that scale.  Additionally, they elaborated on why:

“the National Hurricane Center (NHC) does not believe that combined or integrated hurricane scales help local emergency managers or members of the public make informed decisions about their particular vulnerabilities.”

Quoting more from their release,

“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. A hurricane striking the Gulf coast of Florida, for example, would cause a much greater surge than an identical storm striking Florida’s Atlantic coast. This is why storm surge was formally removed from the original Saffir-Simpson Hurricane Scale in 2010.”

The article in the New York Times mentioned that Isaac (a Category 1) caused 14′ storm surge flooding in Plaquemines Parish while Katrina (a Category 3) caused 15.5′ storm surge flooding.

When comparing experiences in natural disasters, we tend to say, “I went through Katrina” or “I went through Ike”, or “I went through an EF-4 tornado, or “I went through that earthquake,” when in fact most people only went through the periphery of the event, and the “worst of the impacts” occurred elsewhere.  But in this instance, Katrina had a path to the east of Plaquemines Parish, while Isaac tracked over the parish.  Because of this, while Katrina was a stronger storm, the winds pushed storm surge into the parish before landfall, but quickly pushed waters back out to sea after passing by.  Isaac on the other hand had the parish in its sights, keeping the parish on the right front quadrant of the storm.  This location means that the worst of the winds pushed storm surge into the parish, and that the parish actually had the worst of the winds.

However, one fact is often left unmentioned about the comparison between Katrina and Isaac.  Had Katrina tracked along the same path as Isaac, then the storm surge in the parish would have been even higher than what was actually experienced in either storm.

The National Hurricane Center is experimenting with a number of products including storm surge warnings and graphical displays of relative risk for storm surge impacts.  The probabilistic storm surge product has been used experimentally for a number of years, but let’s take a look at how this product fared in both Katrina and Isaac.

That area in yellow is the image on the top (near Braithwaite, LA) shows where storm surge flooding overtopped a levee.  Now, compare that same area to the image at the bottom depicting Katrina’s storm surge product.  The areas are nearly identical.. but ONLY at that one location.  The other areas were CLEARLY worse off from Katrina.

To quote further from the National Hurricane Center release,

“The new approaches to surge are being designed to reinforce instructions from local emergency managers. We cannot overstate the importance of following evacuation orders and other instructions from local officials, regardless of the category or strength of a tropical storm or hurricane. Ignoring evacuation orders risks not only the lives of those who stay behind, but also the lives of first responders who may be called upon to rescue them.”

Landfalling hurricanes bring a number of hazards, but the role of the National Weather Service is to provide high quality products that can be used by decision makers to recommend protective action.  In the case of Louisiana, the location where so much flooding occurred in the Braithwaite area was actually under evacuation orders.

The next time that we face a landfalling hurricane in the United States, it will be imperative to listen to warnings from local National Weather Service Offices as well as evacuation orders from local officials.  They really are professionals making decisions that are in the best interests of their communities.  We always joke that “the weatherman is wrong again”, but in this case, their products for storm surge really hit the nail on the head.  When it’s time to go and officials order evacuations for storm surge flooding, it’s best to heed their warnings and seek safer shelter.  It’s your choice ultimately whether to stay or to go, but in this case, the experts and local officials had it right and did their job to evacuate the area.  For those of you who live elsewhere in hurricane prone country, you missed the storm this time… but next time it could be you.  Will you listen to the warnings and evacuate?  Or will you stay around and live the rest of your life saying – “they were right – I should have left”?

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