Seaworthy - July 2010

Predicting Storm Surge

Forget Saffir Simpson,
Here’s What You REALLY Need To Know!

By Steve Letro
Meteorologist in Charge, National Weather Service, Jacksonville, FL

Meteorologists and emergency managers had worried about it for years. It seemed like such a great way to convey storm surge information…easy to understand, with specific numbers that people could relate to and make decisions. Yet for all its simplicity, there was an elephant in the room that would not go away.

The “it” being referred to was the Saffir Simpson Hurricane scale, and the elephant was how that scale dealt with storm surge. First devised by Dr. Herb Saffir as a pure wind-versus-damage scale, it was modified by former National Hurricane Center director Dr. Robert Simpson to include storm surge impacts before being used publicly starting in the 1970’s. While it seemed to work fairly successfully for over 20 years, the fact was that those 20 years coincided with an era of generally unfavorable oceanic and atmospheric conditions for Atlantic hurricane development with very few strong hurricanes actually reaching land. Unfortunately, that pattern switched in the mid 1990s, becoming far more active. As more hurricanes began making landfall

in the U.S., some of the inherent “warts” in the application of the Saffir Simpson scale to storm surge began to become apparent.

The problem is that the Saffir Simpson scale related storm surge directly and solely to a storm’s maximum winds. In reality, storm surge in any particular location is dependent on many factors, including the physical size of the storm, speed of forward motion, and your location with respect to where the center makes landfall. Furthermore, important local factors such as the depth of coastal waters, coastal elevations and astronomical tides also play a key role. Since these parameters can vary greatly not only from location to location, but also from one storm to the next, it becomes virtually impossible to come up with a “one size fits all” storm surge scale that will work equally well at every location or for every storm. Unfortunately, that is exactly what the Saffir Simpson scale was being used for, with increasingly questionable results.

This issue began to come into focus when
Hurricane Charley made landfall along the
southwest Florida coast in August, 2004.
Wind speeds in Charley were Category
4—145 mph— which should have produced
a storm surge of between 13 and 18 feet.
Instead, because of the storm’s small size
and fast forward motion, the surge was
only in the five- to seven-foot range—
more reflective of the surge that would
be expected from a Category 2 hurricane.
While surge inconsistencies such as
Charley, as well as some of the storms
in the infamous 2005 season, including
Katrina, raised suspicions, the problem
really came home to roost with Hurricane
Ike in 2008. Ike’s maximum wind speed as
it approached the Texas coast was in the
Category 2 range—110 mph—but by now
forecasters recognized that Ike’s massive
size was likely to produce a much greater
storm surge, as much as 15 to 20 feet. This
prompted statements that went so far as
to warn of “certain death” for those who
did not evacuate. Most people heeded the
warning, but not all.