An unusual fourth summer full moon rises on Monday, tugging at still-warm seas and signaling the start of royal tide season in South Florida.
While meteorological summer effectively ends on August 31, astronomical summer ends with Wednesday’s autumnal equinox – the day the Northern Hemisphere begins to move away from the sun and into the embrace of autumn.
It is also a time when a collision of factors causes what is known as a “sunny day” or “nuisance flood”.
High tides are amplified during full moons as a solar and lunar alignment sandwiches the Earth, creating greater gravitational pull. In the fall, seasonal elements contribute to high tide flooding as water warmed in summer expands and the Gulf Stream current slows, accumulating more water along the Florida coast. Sea level rise exacerbates annual flooding at high tide.
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The City of West Palm Beach sent a memo to residents this month warning of possible tidal flooding as the Intracoastal Waterway bubbles through storm drains and overflows aging levees, but the first event — the start of a new moon on September 6 – came and went with little notice.
National Weather Service meteorologists in Miami said Monday’s full harvest moon could cause tide heights to rise above average, but without pressure from incoming swells or onshore winds, they are unlikely to reach a flood threshold.
“When we go through October and November, those are really the peak times of the main tide,” said meteorologist Larry Kelly. “As we come to each new full moon phase, this will likely become more of an issue.”
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The only warning is if the disturbance monitored off North Carolina’s Outer Banks by the National Hurricane Center starts sending waves south to Florida. The burgeoning low pressure area had a strong chance of becoming a depression or tropical storm on Friday afternoon, but is expected to leave South Florida beaches alone. There is a low likelihood of rip currents throughout the weekend for Palm Beach County.
“We posted Tropical Cyclone Nicholas in Louisiana and the hurricane center is monitoring three other areas, but everything has eluded us so far this season,” Kelly said.
A tide monitor from the National Oceanic and Atmospheric Administration lists the Oct. 6 new moon as the next date when royal tides can occur. It also highlights the days of November 4-8 as potential flood days at high tide.
Brian McNoldy, principal investigator 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 reached any of the highest expected peaks yet, and this coming full moon should have water levels comparable to the last new moon, barring any weather influences,” he said. declared. “Keep in mind that NOAA tidal forecasts are only astronomical and climatological in nature and do not take into account current ocean or atmospheric conditions.”
While this full moon may not bring flood tides, a July NOAA report predicts high tide flooding will occur 10 to 55 days a year by 2050 – a steep range which takes into account greenhouse gas emissions mitigation efforts at lower and higher levels.
NOAA’s July report follows the release of a NASA-funded study that found a “rapid increase” in the frequency of high-tide flooding could occur in the mid-2030s due to rising sea levels. sea level and a natural “wobble” of the moon’s orbit.
William Sweet, a NOAA oceanographer, said forecast estimates for southern Florida are likely low given a higher-than-expected number of flood days in recent years.
“All clues point to more flooding here and elsewhere,” Sweet said in a July interview.
Most often, the seasons have three full moons, but Monday’s is the fourth in the astronomical summer of 2021.
It is followed by the autumnal equinox on Wednesday – a promise of cooler days with less humidity.
But not yet for South Florida.
In West Palm Beach, normal daytime highs remain in the mid to high 80s through October, with overnight lows not cooling to 70 through October 23.
It will be Thanksgiving before the average day peaks at 80 degrees and nights dip into the mid-60s.
The forecast for this weekend is for typical summer weather with days near 90 degrees and a 60% chance of showers and thunderstorms on Saturday and Sunday.
Localized flooding from slow-moving storms — not tides — is a concern through the weekend, Kelly said.
Kimberly Miller is a veteran reporter for the Palm Beach Post, part of Florida’s USA Today network. She covers weather, climate and the environment and has a certificate in weather forecasting from Penn State. Contact Kim at [email protected]