JACKSONVILLE, Fla – The fluid boundary where the Atlantic Ocean ends and the St. Johns River begins has implications on the type of hurricane surge warnings issued when inland water swells. But the imperceptible transition between ocean and river has resulted in past disagreement about how to alert for high river water.
Storm surge warnings from the National Weather Service are issued when onshore winds drive saltwater levels up along the coast three feet or higher. The straightforward warning procedure does not factor in rainfall accumulation.
The ocean reaches far upstream. Tides impact Satsuma in Putnam County and river salinity at the Buckman Bridge can be more than half as high as the ocean when surge pushes into the river as it passes Clay County.
Here the transition from surge to coastal flooding is blurred up the river because weather factors other than wind can influence the swelling of water beyond.
Take for instance when Hurricane Irma sent water five and a half feet beyond the mean higher high water benchmark in San Marco. Rainfall, sloshing river seich action from wind shifts, creek and street runoff all contributed to record river water levels in addition to the ocean surge pushing into the river.
The Jacksonville National Weather Service office takes over responsibility from the National Hurricane Center Storm Surge Unit by issuing Coastal Flood Warnings for areas south of the Buckman Bridge.
The Coastal Flood Warning includes inland areas near water and, while its name differs from a Storm Surge Warning, the dangerous impacts can be just as serious.