Relentless High Water Won’t Let Miami Go

November 20, 2019, 7:07 PM EST

Above: An aerial view from a drone shows a car as it drives through a street flooded with ocean water on October 22, 2019 in Key Largo, Florida. King-tide level waters combined with earlier storms and other factors has forced water onto the streets in parts of the Florida Keys, which will likely see increased flooding as sea levels continue to rise. Image credit: Joe Raedle/Getty Images.

This is the autumn of Miami’s discontent. King tides driven by astronomical factors are a regular part of life on the Southeast coast in autumn, but in the last few months—and especially over the last few days—the high water keeps coming back, again and again, especially on the southeast coast of Florida. A slowly evolving nor’easter that followed a long period of onshore flow late last week led to flooding that was largely minor, but widespread and prolonged, in the Miami area and along other parts of the Southeast coast. Eight solid days of continuous minor flooding ended on Wednesday in Charleston, South Carolina.

Brian McNoldy (University of Miami) has kept a close watch on the relentlessly high tides. He’s pored over data on the water levels at Virginia Key, on Biscayne Bay about a mile east of downtown Miami, at 6-minute intervals going back to 1994. “There's a difference in impact if the water level just touches a threshold for a single observation or if it sits well above it for several hours (i.e., a flooded road),” said McNoldy.”

For the year to date through 8 am EST Wednesday morning, November 20, McNoldy found that Virginia Key has seen 2262 hours in which water levels were above mean higher high water, or MHHW (that’s the average daily high tide as calculated for the period 1983–2001). In other words, the water has been above the level one might see for a brief time each day almost a third of all the hours in this entire year (or 29% of the 7780 hours from January 1 through 8 am Wednesday).

Graphic explaining tidal functions
Figure 1. A graphic (left) and graph (right) showing mean higher high water and other tidal features. Image credit: UCAR (left) and NOAA (right), from “The Alphabet Soup of Vertical Datums” a helpful explainer by Robbie Berg, National Hurricane Center.

What’s more, five of the twelve months this year—March, July, August, September, and October—have set records for the highest water on record at Virginia Key, in data going back to 1994.

Monthly tidal records at Virginia Key, FL
Figure 2. Monthly records for water levels at Virginia Key, Florida, just east of downtown Miami. Image credit: Courtesy Brian McNoldy.

“We are still coming out of ‘king tide’ season, when tides are naturally at their highest of the year (August-November),” said McNoldy. “When the water levels are already elevated, it's easy for weather systems to push them well above the climatological predictions and cause flooding of low-lying areas.”

McNoldy stressed that almost all of this year’s high water at Virginia Key has stayed below the level classified as “minor flooding” by the National Weather Service, which is 1.74 feet above MHHW. In fact, only 10.6 of the 2262 hours above MHHW this year have exceeded the minor-flooding threshold. The upshot is that this region has been dealing with day after day of water that isn’t quite high enough to qualify as flooding but is still making a difference in the lives of many. The water has been highest when astronomical forces team up with onshore flow, as is common during king-tide season. Yet even seemingly minor events—such as the nor’easter now far off the East Coast—have proven enough to push water into some streets and send tides more than a foot above the predictions based purely on astronomical factors.

From nuisance to norm

What’s transpired this fall in Miami Beach is right out of the playbook of impacts from our human-warmed climate. When thinking about sea level rise, the focus tends to be on long-term inundation—the doomsday scenario of cities eventually going completely under water. However, the effects are already playing out consistently in places where minor but troublesome “nuisance” or “sunny day flooding” at high tide is fast becoming the norm.

Subsidence is accentuating sea level rise from global warming in some locations, such as the mid-Atlantic coast. This isn’t a factor in Southeast Florida, which is underlaid by a limestone substrate. “It’s quite firm and quite stable—but unfortunately it’s quite porous, so it has quite a few conduits for ocean water to make its way through by infiltration,” said NOAA oceanographer William Sweet, who specializes in nuisance-flood issues. The porous nature of the substrate makes seawalls an impractical long-term solution for sea level rise in Southeast Florida, since seawater would still be able to push its way through the substrate below any seawall and bubble up into streets and sidewalks, much as it’s already doing.

Sweet is the author of a blockbuster 2018 report (see our post) on projections of increased tidal flooding at locations along the U.S. Atlantic, Gulf, and Pacific coasts. The report found that disruptive tidal flooding that now affects the U.S. Gulf and Atlantic coastlines on 3 to 6 days per year will strike as often as 80 to 180 days a year by the 2040s, and almost every day by the 2090s, under an intermediate projection of sea level rise.

The report notes: “By definition, ‘every other day’ high tide flooding would bring to fruition the saying championed by NOAA’s (late) Margaret Davidson: “Today’s flood will become tomorrow’s high tide."

Here are some of the projected number of days per year when high-tide floods can be expected in 2041-2050 and 2091-2100, based on the new report:

Projected changes in tidal flooding in 2040s and 2090s

Sweet noted that even though Miami has only seen minor flooding, as officially defined, on eight days this year, “the tremendous amount of hours or days spent at a lesser threshold that could still be consequential is staggering from a citizen’s perspective—not so much from a hydrologist’s perspective….It may seem startling as a resident, but as a data scientist looking at the data and projecting the data, this is largely what we are projecting to occur, and unfortunately it is occurring.”

When will tidal norms be updated?

One reason that waters have exceeded the MHHW level so often lately in Miami is that long-term sea level rise is making the current MHHW value increasingly dated. Sea level in Miami has risen about 6 inches in the last 25 years, according to Sweet. That’s roughly twice the global average for sea level rise since 1993 of about 3 inches as calculated by the University of Colorado Boulder—not because of subsidence, but because of shifting ocean-and-atmosphere patterns that are pushing more water into the coast.

NOAA updates its atmospheric normals every 10 years on an overlapping schedule; the 1981-2010 norms will soon be replaced by 1991-2020. Updates to tidal data have been less frequent. NOAA’s calculations of MHHW and other tidal functions for all coastal locations are based on the period 1983–2001, as noted above. This “datum” will soon be replaced by a new set of numbers reflecting the period 2002–2020. These 19-year periods are designed to roughly correspond with an 18.6-year cycle in lunar tides. The data are modified every five years in some locations that are experiencing relatively rapid subsidence (parts of the Gulf Coast) or rebounding from the glaciers of the last ice age (parts of Alaska).

With concerns about sea level rise on the upswing, NOAA is now looking at the possibility of increasing the tempo of tidal-data updates. The update “will likely occur more frequently in the future to account for potentially accelerating sea level rise in many locations,” said oceanographer Greg Dusek on Twitter.

Another option being considered, according to Sweet, is calculations to assess how flood frequency will increase at fixed numerical thresholds, such as every half foot, rather than at the current NOAA flood categories. “We recognize at NOAA that one size doesn’t fit all in terms of thresholds,” Sweet said.

John Morales, a longtime broadcast meteorologist in the Miami area, has been tweeting examples of where this autumn’s relentless high water is affecting residents. “With sea level up a half foot locally just since the 1990s, these saltwater sunny-day floods are happening not just on the highest of the king tides, say once or twice a year. We’re now seeing them in all kinds of other circumstances, with an onshore wind flow, with a higher ocean swell, with a full moon, with a new moon, with a slower Gulf Stream, with warmer waters — the list is growing longer. The flooding, boosted by sea level rise, has transitioned from novelty and nuisance to a lifestyle and, soon, pocketbook issue.”

Sweet put it this way: “The future is unfolding before our eyes in terms of the impacts in the South Florida region.”

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Bob Henson

WU meteorologist Bob Henson, co-editor of Category 6, is the author of "Meteorology Today" and "The Thinking Person's Guide to Climate Change." Before joining WU, he was a longtime writer and editor at the University Corporation for Atmospheric Research in Boulder, CO.

bob.henson@weather.com

@bhensonweather

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