Timing is everything.
Hurricane Dorian is forecast to approach the Florida coast as a major Category 4 storm while hitting the brakes and possibly stalling near the coast for a time. This will maximize the storm’s potential storm-surge flood in a state that is already ground zero for risks related to a rise in sea level. Storm surge, which is the storm-driven rise in water above normally dry land at the coast, rides atop background tide levels.
Storm-surge flooding can engulf communities, as ocean water pours into homes and businesses and over roads, sometimes rising 10 or more feet high.
Adding to the storm surge, background tides will be running at some of their highest levels of the year this weekend into early next week because of the alignment among Earth, the sun and the moon, and the resulting gravitational “pull” on the ocean.
“King tide” lacks a scientific definition but, rather, is a term used to describe exceptionally high tides. As sea levels rise because of human-caused global warming and land subsidence, coastal areas have become more vulnerable to flooding during king tides. In Florida, low-lying areas of Miami, for example, regularly flood during exceptionally high tides.
This summer, Miami set daily high-tide records for more than a week straight for the period bridging late July and early August. Neighborhoods were flooded, and some roads turned into rivers.
A hurricane moving from east to west into Florida would cause a life-threatening storm surge at any time of year, but given the King Tides, this storm presents an extreme risk from a historically deadly threat in its arsenal.
For example, in the Florida cities of Miami, Melbourne (Port Canaveral) and Jacksonville, as well as Savannah, Ga., and Charleston, S.C. — each of which could experience storm surge flooding from Dorian — tides will be elevated above normally dry ground through Tuesday or Wednesday. In Miami, for example, the afternoon high tide on Monday looks to bring 0.55 feet of water above normally dry ground even without any influence from the hurricane at all. Having water elevated above normally dry ground doesn’t equal flooding, however.
Miami starts flooding at 16 inches above normally dry ground, for instance. Given sea-level rise during the period, there has been a 3.2-fold increase in how often Miami sees nuisance flooding. That flooding occurs without any storm around, including on sunny days with light winds.
Add a hurricane to the mix, and it’s a recipe for a potential calamity.
In Port Canaveral, the Tuesday morning high tide is forecast to be 0.67 feet above normally dry ground, without counting any influence from the storm, which depending on its track, could add 10 to 15 feet or more of water on top of that background tide level if the storm were to hit at high tide. In Savannah, moderate coastal flooding begins at 2.13 feet above normally dry ground, and the king tides could get the ocean part of the way there by the time Dorian moves that way.
Coastal flood warnings were up for coastal Georgia and South Carolina on Friday night because of the king tides, and high-tide flooding has already occurred in recent days.
Sea-level rise along with king tides make it easier for storm surge to cause more damage, much as a basketball game played on a court in which the floor is gradually rising would result in far more slam dunks than usual.
Sea-level rise is forcing Miami and other coastal communities in Florida — which have spent millions on infrastructure projects — to reckon with increased flooding on a routine basis, and it makes the city particularly susceptible to storm surge flooding.
“In the Miami area, sea level has risen about 5 inches since the mid-90s, and while that may not sound like much to people, it’s a big deal in a flat city that exists just a few feet above sea level,” says Capital Weather Gang tropical weather expect Brian McNoldy. “The frequency of ‘sunny day’ flooding has increased, and when it comes to hurricanes, it raises the baseline for storm surge flooding. Places that might have avoided water damage a few decades ago will now have water on their street or in their house, and places that would have flooded before will flood even worse now.”
Since 1900, the city has seen cumulative sea level rise of 0.92 feet, according to data from Climate Central, a research and journalism group that has extensively mapped coastal vulnerability to sea level rise.
In Charleston, which could also be in the path of Hurricane Dorian, the combination of land elevation change and sea-level rise has yielded a 1.26-foot increase in sea level since 1900.
“Because mean sea level serves as a platform, even a small amount of sea-level rise can significantly increase the frequency and magnitude of flooding from tides and storm surges," said Maya Buchanan, a sea-level-rise scientist with Climate Central.
As of Friday, many computer models were depicting a worst-case scenario for storm-surge flooding along Florida’s east coast, particularly along the Space Coast, with a slow-moving major hurricane approaching the shore at a perpendicular angle, which would drive a surge from east to west north of the storm center, before turning the storm northward, directing onshore winds up the coast and into neighboring states.
This would bring a lengthy period of onshore winds to the coast in the Southeast, particularly into northern Florida and coastal Georgia and South Carolina, which will yield progressively higher flooding levels as the event wears on.