The tide can be pushed higher than usual when winds out of the northeast shoot more ocean water to the coast like a boat wake.
The tide can also be pulled higher when the moon is full or when the moon is at the point in its orbit closest to Earth — exerting the strongest gravitational force.
All three happened over the weekend. And areas around Charleston got soaked from the ground up.
Winds hitting 30 mph, along with a perigean full moon, created a perfect storm of extreme tidal flooding of streets and properties. The severity caught people off guard, including forecasters at the National Weather Service office in Charleston.
"The tide wasn't a complete surprise," said Weather Service meteorologist Rebecca Davidson. "We knew it would be high Saturday. Just not that high."
It will happen again, and maybe soon. The moon will be full in another two months, on Jan. 20. The next day it reaches perigee, that point where it's closest to Earth. Better keep an eye on the wind.
This flood tide wasn't caused by heavy rain or sea rise. But rising seas are expected to bring flooding tides four times more often by 2045, according to the fourth National Climate Assessment issued by the National Oceanic and Atmospheric Administration released last week.
The NOAA report said the current 38 days per year of tidal flooding will become 180 days per year by 2045. The report reinforced earlier studies that make similar predictions based on warming air and seas and what's happening in front of our eyes.
"These floods can cause problems ranging from inconvenient to life-threatening," the report said, listing road closures, failing storm water drains and damage to infrastructure such as roads and rails.
In Charleston, the morning flood tide is expected to settle — finally — below flood stage at noon Tuesday, largely because of a change in the winds. Weather Service forecasters called for a 6.5 feet high tide.
The tidal level called king tide, or astronomical high tide, is 6.6 feet or more. That's at least a foot above the usual tidal range. The higher the tide reaches, the more flooding and damage it does. Tides at 8 feet can undermine properties as well as exacerbate erosion.
Weather Service forecasters expected the Friday morning tide of 8.14 feet to be the worst. Then Saturday swept in, with the winds already having pushed up the seas and still blowing harder than 20 mph. The tide rose to 8.76, the sixth all-time highest tide.
Everything went under that was low-lying near a waterway or marsh, and the flooding persisted farther inland than usual. People were driven out of bottom-floor apartments on Folly Beach. Brookside Drive on Turkey Creek in Hanahan was impassable.
The winds were strengthened by what meteorologists call a pressure gradient, a steep shift from high pressure to low pressure in the atmosphere.
"On top of it and making it worse, we had the full moon on Friday, then the perigean moon on Sunday," said Weather Service meteorologist James Carpenter.
The tides were higher in the morning than the evening because in the yearly cycle of tides, Charleston is at a time when the morning tides generally are higher, according to the Weather Service.