Weather 101 - Hurricane Storm Surge

Anyone who has experienced a hurricane (tropical low pressure system with sustained winds of at least 74 miles per hour) knows that storm surge (the onshore push of water by wind) is as much a part of the hurricane as are hurricane winds. Wind-driven waves and high tides add to the main storm surge (Figure 1).

Typically the storm surge (the hurricane's greatest danger) is portrayed as being pushed onshore by a storm moving directly toward a coastline. The storm's leading front right quadrant (with its onshore counterclockwise winds) becomes the most dangerous part of the storm. In the left front quadrant, offshore winds blow water away from shore, creating lower than average water levels.

However, the surge is NOT a "wall of water" (such as an earthquake-driven tsunami would be), but rather the continuing onslaught of waves that are not allowed to drain back to the sea. Hence, water levels build over time. Various factors can lead to a quick rise in water levels or a more gradual one.

The rise in water is similar to what many of us did in the bathtub while growing up. I recall that I kept trying to push all the water to one end of the bathtub but the water kept flowing back to fill in the depression I created. I guess that many of us were youthful hurricane researchers without knowing it.

Similarly, tides can affect surge-related water levels, but the surge is itself NOT a tide.

High surges are often the result of many of the following factors in combination: higher winds, a strong fetch (or long length of the wind's path over water), persistent onshore winds and several factors relating to the character of the coastal underwater bathymetry (water depth) and coastal topography. Surges tend to be greatest when water is forced into a v-shaped bay or coastal river valley (a so-called funneling effect). Camille's onslaught into St. Louis Bay (Mississippi) in 1969 is an outstanding example of worst-case coastal factors. The height of the storm surge there reached 25 feet.

The many faces of storm surge

Meteorologists are now recognizing that the effects of storm surge can take on as many variations as the hurricanes themselves. For example, as Hurricane Elena moved toward the lower Louisiana coast in 1985, offshore winds from the north just ahead of the center pushed water out into the Gulf of Mexico. As the center passed by, winds shifted to the south (onshore) and quickly pushed water back onshore. The swing in surge levels was almost 11 feet in a very short period of time (Figure 2).

As both Frances and Jeanne moved northwest across the Florida peninsula (September 2004), winds in the leading front LEFT quadrant of the storm blew water offshore (as expected), but it was the winds in the rear LEFT quadrant that blew water onto the shore (Figure 3). This was not what coastal residents had come to expect.

And as Dennis moved northwest through the Gulf of Mexico (2005), his surge continued along Florida's west coast even after the storm started moving way.

To add further confusion, the classical Saffir-Simpson Hurricane Scale links wind to storm surge height. Ike's arrival on the upper Texas coast in 2008 showed that this relationship was far from being a standard; Ike, a category 2 storm, carried with it a category 4 surge. As a result, starting with the 2009 hurricane season, NOAA's (National Oceanic and Atmospheric Administration) National Hurricane Center will only be using a newly-defined Saffir-Simpson Scale to classify the winds in a hurricane. You will have to listen to the content of hurricane advisories to get storm-specific surge information.

And let's not forget how forecast errors, no matter how small, can affect surge forecasts. Tiny Hurricane Charley made a sharp turn to the right as he closely paralleled Florida's southwest coast in 2004. Whereas areas from Tampa southward were in the expected high storm surge region, many of these places actually experienced offshore winds and minimal to negative surges (Figure 4).

Being aware and being ready

Whatever the hurricane situation (movement, winds, storm size and expected surge), coastal residents need to take surge forecasts seriously and recognize that there can be many local variations in surge height and timing. They should know their home's elevation and / or flood zone rating and be prepared to evacuate from low-lying areas whenever high water is expected. If one is not in a high water zone, they should consider staying put (a.k.a., "hunker down") instead of fleeing, unless their home is poorly constructed or a mobile or modular variety, or unless category 3 or higher winds are expected.

Check with your county officials, look for flood zone maps online (Figure 5) and/or just look in the local phone directory. These maps will also tell you if you are at high risk from flooding and need to purchase flood insurance. Note that there is a 30-day waiting period before newly purchased flood insurance takes effect.

Before hurricane season gets into high gear, also, try to learn a bit more about how these storms operate and how they do their handiwork. There is a wealth of information at the NOAA Hurricane Research Division's web site. TV stations and newspapers in coastal areas from New England to Texas will also likely have information at their web sites or in special printed hurricane season information guides.