WEATHER

Summer's final full moon heralds autumn and South Florida's king tide season

Palm Beach Post

An unusual fourth summer full moon rises Monday, tugging at still sizzling warm seas and signaling the beginning of king tide season in South Florida.

While meteorological summer wraps up efficiently on Aug. 31, astronomical summer ends with Wednesday’s fall equinox – the day the Northern Hemisphere begins its lean away from the sun and into autumn’s embrace.

It’s also a time when a collision of factors cause so-called “sunny day” or “nuisance flooding.”

High tides are magnified during full moons as a sun and lunar alignment sandwiches the Earth, creating a greater gravitational pull. Come fall, seasonal elements contribute to high tide flooding as summer-warmed water expands and the Gulf Stream current slows, piling more water along Florida's coast. Sea level rise exacerbates the annual high tide flooding.

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The City of West Palm Beach sent a note to residents this month warning of possible tidal flooding as the Intracoastal waterway bubbles up through storm drains and overflows aging sea walls, but the first event – the debut of a new moon Sept. 6 – came and went with little notice.

Water from the Intracoastal Waterway floods Lake Trail at Sunset Avenue in Palm Beach at high tide Sunday morning, Oct. 18, 2020. The flooding is the result of the yearly King Tides, which began in September. The annual higher tides occur because of the influence of the moon and the Gulf Stream, but sea level rise due to climate change has increased the height and flooding.

National Weather Service meteorologists in Miami said Monday’s full harvest moon may lift tide heights above average, but with no pressure from incoming swell or onshore winds, they are not expected to reach a flooding threshold.

“When we move into October and November, those are really the king tide peak times,” said meteorologist Larry Kelly. “As we get to each new and full moon phase, it will probably become more of an issue.”

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The only caveat is if the disturbance being watched off the Outer Banks of North Carolina by the National Hurricane Center begins to send waves to South Florida. The burgeoning area of low pressure was given high chances of becoming a depression or tropical storm Friday afternoon, but it should leave South Florida beaches alone. There is a low chance of rip currents through the weekend for Palm Beach County.

“We have post Tropical Cyclone Nicholas in Louisiana and the hurricane center monitoring three other areas, but everything has been avoiding us so far this season,” Kelly said.

A National Oceanic and Atmospheric Administration tide monitor lists the new moon of Oct. 6 as the next date when king tides may occur. It also highlights the days of Nov. 4-8 as potential high tide flooding days.

Brian McNoldy, a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, notes that a perigee moon will occur on Oct. 8, near the new moon. Perigee is when the moon is closest to Earth in its orbit.

McNoldy said the highest predicted tide this season is Nov. 5, a day after that month’s new moon.

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“We haven't hit any of the higher expected peaks yet, and this upcoming full moon should have water levels comparable to the last new moon, barring any weather influences,” he said. “Keep in mind that the NOAA tide predictions are only astronomical and climatological in nature and don’t account for current ocean or atmospheric conditions.”

While this full moon may not bring flooding tides, a NOAA report released in July forecasts high tide flooding to occur 10 to 55 days per year by 2050 - a steep range that takes into consideration greenhouse gas emission mitigation efforts at lower and higher levels.

NOAA's July report followed the release of a NASA-funded study that said a "rapid increase" of high tide flooding frequency could occur during the mid-2030s because of sea level rise and a natural "wobble" in the moon's orbit.

William Sweet, a NOAA oceanographer, said the forecast estimates for South Florida are likely low given a higher than expected number of flood days experienced the past few years.

"All the clues are pointing to more flooding here and elsewhere," Sweet said in a July interview.

Most often, seasons have three full moons, but Monday’s is the fourth for 2021’s astronomical summer.

It is followed by Wednesday’s autumn equinox – a promise of cooler days with less humidity.

But not just yet for South Florida.

In West Palm Beach, normal daytime highs remain in the mid to upper 80s well into October, with the nighttime lows not cooling to 70 until Oct. 23.

It will be Thanksgiving before the average day tops out at 80 degrees and the nights dip into the mid-60s.

This weekend's forecast is for typical summer weather with near 90-degree days and a 60% chance of showers and thunderstorms Saturday and Sunday.

Localized flooding from slow-moving storms - not the tides - is a concern through the weekend, Kelly said.

Kimberly Miller is a veteran journalist for The Palm Beach Post, part of the USA Today Network of Florida. She covers weather, climate and the environment and has a certificate in Weather Forecasting from Penn State. Contact Kim at kmiller@pbpost.com