Storm Surge: The Beginnings...

The first stage beyond a tropical disturbance is a tropical depression. A tropical depression has winds of less than 39 mph and a defined, closed circulation with wind spiraling in from all directions to the center. The air pressure is not as low as a tropical storm or hurricane. A tropical depression has no name, it is just called tropical depression #1, #2 and so forth. There is typically heavy rain associated with a depression but no storm surge to speak of.

Moving up the scale, we get to a tropical storm. At this point the system gets a name, let's call our model storm "LISA". Lisa will be our model for examining how a tropical cyclone brings storm surge ashore and the devastating effects that come along with it. A tropical storm has winds from 39 mph to 73 mph and a better defined wind field and overall circulation pattern. There may be minor storm surge flooding in a strong tropical storm in the areas just to the right of where the center comes ashore. This is especially so if the storm has a fast forward speed. We'll look at why that is later on.

Lisa is now a minimal hurricane. With winds of 75 mph, Lisa does not quite have an eye feature yet, but that will come soon. The air pressure has dropped to 985 millibars placing Lisa in the category 1 area of the . Storm surge will be from 4 to 5 feet depending upon the exact track and location of landfall of a category 1 hurricane. Our hurricane, Lisa, will continue to strengthen as we jump up to a category 3 hurricane.

Now Lisa is a category 3 hurricane. Lisa has winds of 120 miles per hour, a little stronger than Fran did in 1996. She is out in the Atlantic, northeast of the northern Leeward Islands. Lisa is about 5 days from making landfall in the Southeast U.S.

The first sign of Lisa's impending threat are the massive swells that are generated in all directions from the hurricane's center. They move outward and arrive on our shores days after being generated by the powerful winds of our hurricane. As the swells encounter the coastline, dangerous rip-tides often result. So even though Lisa is still 9 days away, she is already creating a problem on the coast as the swells, or the forerunner waves, come rolling in. These waves typically add a foot or two to the overall height of the ocean at the beachfront. If Lisa were to pass harmlessly by and never get much closer, then these forerunner waves are all that we'd experience. Unfortunately, Lisa is going to come closer to shore. Let's move ahead a few days.

As Lisa draws ever closer to the Southeast Coast, the swells intensify, heavy surf advisories are often issued and there is a looming sense that a hurricane lurks a few hundred miles off shore. Indeed, Lisa is now 36 hours away from making a direct hit somewhere in Southwest North Carolina. With the seas building and a constant onshore wind, small craft (and indeed all marine vessels) are urged to remain in port and prepare for the hurricane. At this point a Hurricane Watch has been issued. It means that hurricane conditions are possible within the watch area with 36 hours or less. It is time to get ready for hurricane Lisa and the destrcutive storm surge that she will bring.

Now we are a mere 24 hours prior to landfall of what will be a destructive hurricane. This one will rival Fran in strength and in terms of dollars lost to destruction. As evacuation takes place along the barrier islands and low-lying areas, weather forecasters across the country are focusing on the overall intensity of Lisa and where she might decide to end up. Let's take a look again at the stats regarding hurricane Lisa:

• Hurricane Lisa
• Wind: 120 mph
• Pressure: 955 millibars
• Movement: North at 16 mph
• Let's assume that Lisa comes ashore at a normal high tide between New and Full Moon phases.