Sunday, May 27, 2012

Tide Graphs for Southeast U.S.


This image is a tide graph for Fort Pulaski, Georgia. The red line shows actual water levels, the blue line shows predicted water levels, and the green line is the observed minus predicted level. The green line reveals the storm surge level, which is sometimes called the residual.

Note that the next high tide is predicted early this afternoon. This means that water levels would normally rise almost six feet above the level at low tide. During a storm surge event, water levels will exceed predicted values, making flooding most dangerous at the time of high tide.

Tide graphs for areas impacted by Beryl are posted to a special NOAA Tides and Currents page at: Water levels are updated frequently, so check back often for the latest updates.

Subtropical Storm Beryl Elevates Water Levels Along Southeast U.S. Coast

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 Subtropical Storm Beryl is centered off the Georgia Coast this morning. Water levels at Fort Pulaski are already more than two feet above normal. Winds at this location are form the north at 15 knots as of 7:12 EDT.

Note that water levels are elevated, although winds along the Georgia Coast are blowing perpendicular to shore. A process called Ekman Transport pushes water at a right angle to the wind direction, enabling water to pile up along the GA coast during a north wind event. A similar process can flood coastal Texas and Louisiana during a strong east wind. However, in many of these cases, water levels increase even higher when winds shift onshore.

The National Weather Service is forecasting north winds today and tonight, shifting to east winds and increasing in intensity tomorrow. The National Hurricane Center predicts possible surge inundation of 1-3 feet above ground level along the FL, GA and SC Coasts. This could get particularly interesting at the time of high tide. High tide at Fort Pulaski is predicted at 6.3 feet, which should peak at 12:24PM EDT.

Although surge potential from a tropical or subtropical storm may seem minimal, several factors in this event may elevate water levels higher than might be expected. One factor is the slow movement of Beryl, which is moving to the west-southwest at only 10 miles per hour. The National Hurricane Center is predicting a decrease in speed tomorrow, which may prolong strong winds. Also, the combination of north winds, followed by east winds may pile up water along the coast in this area. Finally, concave-shaped coastlines tend to trap water, enhancing surge levels. The Georgia Coast has a nice profile to enhance surge levels in this way. More can be read about the physical processes that enhance surge levels at: This is a chapter in Recent Hurricane Research that I co-authored with Dr. Barry Keim.