Under most circumstances, when we think about surf forecasting we try to find a region on a surface weather map that has some significant area of winds blowing over the ocean in the general direction of the area we are trying to forecast for. These winds will create ocean waves that travel in the direction of that area and, if unobstructed by any other landmass, will produce rideable surf as the waves transition into shallow water. South Florida forgot to read that memo.
While some may claim that surf forecasting for the East Coast is easier than anywhere in the world because it's "always flat", it's actually a tad more complicated -- and nowhere exemplifies this better than South Florida. First of all, South Florida has an extremely narrow swell window. The Bahamas block almost any swell of significance from ENE to SE direction. Hatteras can block some energy from the North, and the fact that Palm Beach juts out to the east a bit also prevents swell energy from the N/NE to reach a lot of beaches to the south of Palm Beach.
And it gets worse.
After every fall in South Florida just about every local surfer starts talking about when the first winter "bounce" swell is going to hit. What the heck is a bounce swell? Ask any two people in Florida and you can be about 100% sure to get conflicting answers.
Here's the story: In the winter time Florida occasionally gets strong cold fronts that clear the entire state. Behind these fronts, if the rebuilding high pressure area is strong enough and in the right position, Florida and the Southeastern United States will get a significant surge of cold, dry NW winds. A significant surge of NW winds means the associated swell will be pointed away from the SE Coast and Florida, correct? Again, South Florida forgot to read that memo. In fact, under the right circumstances the described weather pattern above can produce clean 4-to 6-foot-plus faces at scattered South Florida breaks, even South Beach, Miami! So what happens, how does South Florida get swell with an offshore component in the wind?
The first argument is that Palm Beach and the surrounding areas to the north are in fact exposed to steep NNW wind swell. As the NNW wind swells begin to "feel" the bottom near Palm Beach, they will bend (refract) back towards the SW and "wrap" into beaches south of Palm Beach. However, with short period wind swells, refraction of this kind is not likely to be significant. Since Palm Beach sticks out like it does, I can understand how Palm Beach and the areas north can get steep, sideshore NNW wind swell of some significance, but to me this doesn't really explain the phenomenon that occurs further south.
Second we have the possibility that the swells actually bounce (reflect) off the west side of the Bahaman Islands and return to South Florida producing rideable swell. Maybe, but its probable that most of the swell energy would be dissipated after breaking upon the Bahamas shorelines, and any reflected swell energy would be minimal. But until someone has inspected every west facing shoreline on the Bahaman Islands for steepness, you can't count this out. The steeper the shoreline the more swell energy will be reflected back.
The last two theories deal with the ocean waves interacting with the Gulf Stream, which runs south to north just east of South Florida. Hypothesis #3 is that the Gulf Stream acts as a wall because of the sharp temperature (or density) contrast with its surrounding waters. As NW or possibly even WNW swells hit this "wall", they will be reflected back towards South Florida. Finally the last argument, and perhaps the most likely, is that the waves bend, or refract, off the Gulf Stream back towards South Florida. This is known as "refraction off a horizontally sheared cross current".
Regardless of what actually happens, it does happen. Until more solid research is done regarding this subject, it will be very difficult to determine the exact cause of these swell events. The key in forecasting these mysterious swell events for South Florida is to recognize the weather patterns that cause them. In general, it takes a WNW to NNW wind of 15-20kt+ for a duration of 12+ hours off the Florida and Southeast US Coast. Something like that seems pretty easy to recognize, and it is... but with so many other factors involved - South Florida gets my vote as one of the most difficult areas in the world to forecast surf for. -- Mark Willis
