JACKSONVILLE, Fla. – High water continues to be a problem along the St. Johns River, where nuisance flooding has been showing up during periods of high tide. Unfortunately, relief isn’t coming quickly. Both weather and astronomical factors are working against the river’s ability to drain.
Water levels at Jacksonville’s Southbank have been running about a foot above normal for more than a week. The culprit? The last local nor’easter, which shoved water into the river system. When this happens, the St. Johns behaves like a bathtub—water sloshes in and becomes coastally trapped.
Unlike open coasts, where tidal water can freely ebb and flow, the St. Johns is a long, northward-flowing estuary that empties slowly into the Atlantic. When nor’easters push ocean water landward, the tides essentially get “stacked” on top of each other, and the river can take days to weeks to fully flush out.
This process of coastally trapped tides means that even when winds relax, the water doesn’t immediately drain. Instead, it remains elevated until tidal cycles and ocean levels align in a way that allows the excess water to bleed back into the sea.
Unfortunately, the timing now is about as bad as it gets. The full Harvest Moon arrives Saturday at 2:10 p.m. (Sept. 7), which will drive astronomically higher tides. Forecast guidance indicates that water levels will run 1.5 to 2 feet above Mean Higher High Water (MHHW) during high tide cycles over the weekend. Coastal Flood Advisories may expand to cover much of the St. Johns River basin, and elevated tide levels are possible along the Atlantic coast as onshore winds increase.
Looking ahead, the weather pattern only adds more fuel to the problem. A frontal boundary will approach Sunday and stall into early next week. As strong high pressure builds over New England, the stage is set for another nor’easter Tuesday into Wednesday. This one looks stronger, with onshore winds of 20–25 mph along the coast and seas building to 6–8 feet offshore.
The worst news? That wind-driven surge will coincide with the month’s highest astronomical tides, peaking near 6.5 feet above mean sea level during the high tide cycles.
That combination nearly guarantees another round of widespread nuisance flooding for communities along the river as water levels will run 1.5 to 2 feet above Mean Higher High Water (MHHW) during high tide cycles.