NOAA says South Florida is in for more high tide flooding in 2021 — again

South Florida can expect even more days this year where the high tide keeps rising, causing flooding even on sunny days.

This prediction, while unsurprising for anyone familiar with sea level rise’s expected impact on the coastal region, is the conclusion of NOAA’s annual high tide flooding outlook, released Wednesday.

Last year, NOAA predicted that the Virginia Key tide gauge would record three to six flood days from May 2020 to April 2021. It recorded six.

For May 2021 to April 2022, NOAA is raising its prediction to four to seven days, but that’s likely an under-count for many South Florida residents.

What counts as a flood is individual for every community, William Sweet, an oceanographer at NOAA’s Center for Operational Oceanographic Products and Services, said on a press call. But NOAA’s annual report is based on a single threshold for flooding that allows researchers to better make an “apples to apples” comparison across the country.

“Those thresholds are a national calibration,” he said. “One size doesn’t fit all.”

The national standard for flooding makes it easier for researchers to view national trends, but it tends to under-count flood days in some cities. A great example is the Keys. NOAA reported zero high tide flood days at either the Key West or Vaca Key tide gauge station last year and predicts zero this year.

That would be news for residents in the Keys, who’ve dealt with extreme flooding on an annual basis for years, including some neighborhoods that see flooded streets for months at a time.

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But experts like Sweet say it’s clear that sea level rise is likely to push those numbers up, no matter how high the flooding threshold is. By 2050, NOAA predicted that Virginia Key could record anywhere from 10 to 55 high tide flood days a year, with the Keys seeing up to 65.

However, the numbers take a curious dip for 2030 — a new prediction threshold NOAA included this year. Despite predictions for up to seven flood days this year, NOAA officially calls for only two to five flood days in Virginia Key by 2030.

Sweet said that’s likely because NOAA is under-predicting future flood levels in some communities like Miami in the mid-term. Short-term predictions, like what Miami may experience this year, have a lot more data behind them, while mid-to-long-term predictions rely more on models and formulas.

“What we’ve experienced in terms of South Florida is that sea levels have been increasing more so than the expecting trends,” he said. “Likely what we’re going to need to do is revise our 2030 numbers, but we’d like to wait a few more years to see if this trend holds.”

“All the telltales are pointing to more flooding here and elsewhere,” he said.

In addition to human-caused global warming, new research by NASA shows that a wobble in the moon’s orbit is set to increase flooding in coastal cities in the mid-2030s.

The moon’s wobble takes 18.6 years to occur, a natural phenomenon scientists have recorded for hundreds of years. For half of that time, its gravity suppresses tides and makes them a bit lower. For the other half, the gravitational pull increases tides.

Right now, we’re in the phase of the wobble where the moon is suppressing tides the most, said Brian McNoldy, a research scientist at the University of Miami who’s previously written about the topic. The height of the tide increase effect will occur around 2035, and combined with sea level rise, McNoldy said it could lead to double the rate of relative sea rise in places like Miami.

“When it was at its last, most upward part back in 2012 it’s not exactly the same as its most upward part in 2031 because sea levels will have risen half a foot,” he said. “The oscillation is doing the same thing it always has done, it’s just that we’re raising the baseline.”