It was impressive to see chest high clean swell on our beaches Thursday. An offshore gale kicked seas up to 23' off the mid Atlantic and some of it sneaked into our breaks. It is rare to get powerful waves with offshore winds in Jacksonville. I thought the surf would of been much smaller but the swell was big enough to resist attenuation by the west winds that gusted over 30 mph. Sometimes offshore winds will knock the surf flat. In retrospect, a look at the models shows why the surf stayed up. Above the WW3 period chart clearly show an impressive 15-16 second swell hitting Florida and the Bahamas in yellow. The blue is a separate 5-6 second windswell moving northeast due to the WSW winds. The 2 dimensional graphics masks out the underlying groundswell.
Above is a look at only the new windswell period. It would of been totally flat here if it weren't for the groundswell running against the northeast moving windswell.
A graphical version of actual wave observations at the St. Augustine buoy confirms two separate swells. You can see the graph of wave energy with the period at the top and energy measured at the left. This information shows two wave groups. The largest peak is the new windswell at 6.5 seconds and the groundswell is indicated at 15 seconds. When looking at wave models they only show swells with the most energy. In this example it would be the windswell since its energy is higher. A forecaster could totally miss predicting the groundswell without confirming this spectral density data. Unfortunately this happens often since there are so few buoys off our coast.
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