South Florida strolls into the dry season this year plump on an October rainfall largesse, but wary of a still simmering tropics.
About 8 inches of rain has fallen at Palm Beach International Airport this month in the runup to the official Oct. 15 start date of the dry season for counties covered by the National Weather Service in Miami.
That’s nearly 5 inches above normal for West Palm Beach, but not enough to catch up a widespread year-to-date rain deficit that is 7.5 inches deep at the airport and varies from about 2 inches on the southwest coast to as much as 15.7 inches in Melbourne, according to the Southeast Regional Climate Center.
Vero Beach has an 11-inch deficit, with Fort Pierce gauges showing it is down 7 inches.
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“Normally by late October we are saying, don’t worry about the tropics,” said South Florida Water Management District senior meteorologist Todd Kimberlain. “But this year, we'll have to be on the lookout a little longer than normal."
Kimberlain said a visit from the globe-circling Madden-Julian Oscillation, or MJO, is expected toward the end of the month into early November. The traveling pulse of energy circles the globe every 30 to 60 days and is known to provoke whatever prowls the tropics.
It’s not unprecedented for a tropical cyclone to form after a dry season was already underway. Kimberlain points to 1994’s Gordon, which made landfall as a tropical storm near Fort Myers Beach on Nov. 16. Four years later, Mitch made landfall north of Naples as a tropical storm on Nov. 5.
The official end date of hurricane season is Nov. 30.
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“I really don’t know exactly when the rainy season will end, but it’s going to go past mid month,” Kimberlain said.
How is the end of South Florida's wet season determined?
The South Florida Water Management District determines the start and end dates of the rainy season after the fact, looking at details such as dew point temperatures, sea surface temperatures and an established patter of rainfall typical to the rainy season.
Last year, with hurricanes still pumping well into November, the district didn’t declare the end of the rainy season until Nov. 18.
It was in 2018 that the National Weather Service in Miami decided to give the wet season specific calendar dates of May 15 to Oct. 15. The decision was made, in part, to create awareness of what can be a dangerous time of year for weather in South Florida and is similar to the dates given to hurricane season.
The set rainy-season dates cover the seven counties overseen by the Miami office of the NWS – Miami-Dade, Monroe, Broward, Collier, Palm Beach, Glades and Hendry. South Florida gets an average of 70% of its rain during the wet season.
Through Oct. 13, West Palm Beach’s rainy season had tallied 36.8 inches, which is near the normal rainy season amount of 36.3 inches based on the 1991-2020 averages.
In the past decade, the wettest rainy season for West Palm Beach was 2012 when 56.7 inches of rain was measured. But that's topped by 68.8 inches during the rainy season of 1938.
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“Like the astronomical seasons, it doesn’t mean there’s an automatic switch from one to the other on a specific day,” said Robert Molleda, the warning coordination meteorologist for the NWS in Miami. “October is a transition month that can vary from really dry periods to really wet periods.”
This year, the National Weather Service office in Tampa joined Miami in setting fixed rainy season dates. For the southwest coast, including Fort Myers, the dates are the same as Miami – May 15 to Oct. 15.
But for the Tampa area, the dates are May 25 to Oct. 10, meaning Tampa has officially already entered the dry season.
“The moisture this year has definitely decreased, and our weather pattern is looking fairly dry,” said Tampa-based meteorologist Austen Flannery. “That being said there have still been a few storms around.”
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Melbourne’s NWS, which covers the Treasure Coast and Central Florida, still calculates the rainy season the old-fashioned way by looking at the weather patterns and determining after the fact the official start and end dates.
But on average, the rainy season runs May 23 to Oct. 23 for Stuart, May 28 to Oct. 17 for Melbourne and May 27 to Oct. 15 for Orlando.
“We had a week without rain, then we had rain again, so we haven’t declared anything yet for east Central Florida,” said meteorologist Krizia Negron.
Wet season rainfall through Wednesday in the 16-county region monitored by the South Florida Water Management District was about normal.
But the district's oversight area – which includes Orlando to the Keys – is down 5.6 inches for the year, with the east being the driest. Coastal Palm Beach County shows an 11-inch deficit Jan. 1 – Oct. 12.
Jan.1 - Oct. 13 rainfall totals in inches / compared to normal
Coastal Palm Beach County: 40 / -11
Martin and St. Lucie counties: 40 / -6.3
Coastal Broward County: 43.5 / -7
Coastal Miami-Dade County: 41 / -9
Southwest coast: 46.5 / -5
June 2 to Oct. 13 rainfall totals in inches / compared to normal
Coastal Palm Beach County: 32 / -1.6
Martin and St. Lucie counties: 32 / +.37
Coastal Broward County: 34 / +.09
Coastal Miami-Dade County: 32 / -3.6
Southwest coast: 40 / +1.4
Source: South Florida Water Management District
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 firstname.lastname@example.org
Source : https://www.palmbeachpost.com/story/weather/2021/10/15/south-florida-dry-season-october-tropics-la-nina-weather-play/6048653001/2120