BREAKING: Rainy season start, end dates now fixed

The onset of South Florida’s rainy season used to be a cyclical anticipationunique in the U.S., a water cooler guessing game of when the tropics will open wide with daily showers to quench winter’s thirst.

But the annual mystery is over.

The National Weather Service in Miami has designated May 15 through Oct. 15 as the permanent dates for the rainy season, fixing the days similar to the set time frame given for hurricane season.

Robert Molleda, the warning coordination meteorologist with the NWS in Miami, announced the change Thursday, saying it will help increase awareness of what can be the most dangerous time of year for weather in South Florida.

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“We get most of our rainfall and all the associated hazards — lightning, flooding, tornadoes, severe thunderstorms — during this time,” he said. “I think we can use it as a way to get everyone ready for the rainy season similar to the way we get ready for hurricane season.”

In the past, the rainy season was determined by looking at dew point temperatures, sea surface temperatures, and an established pattern of rainfall typical to the rainy season — at least three consecutive days.

“Many of these factors occur independently of each other and don’t have clear beginning and end dates, but instead occur mainly during transition periods which can last anywhere from 2-4 weeks in length,” Molleda said.

The decision on May 15 to Oct. 15 was made after examining records dating to the 1960s on rainy-season start dates. South Florida gets an average of 70 percent of its rain during the wet season.

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Florida climatologist David Zierden said the reasoning for setting rainy season dates is “sound.”

“Having fixed dates certainly simplifies things for messaging and simple comparisons,” Zierden said. “For scientific studies, a more meteorological-based definition that accounts for year to year variability would still be better.”

In addition to hurricane season, which runs June 1 to Nov. 30, other fixed weather dates include the calendrical beginning of summer, fall, winter and spring. But even those differ depending on whether they are meteorological or astronomically-based. Meteorological seasons run in succinct blocks. For example, winter is Dec. 1 to March 1.

Astronomical seasons begin on the solstice and equinox.

While many areas have seasonal shifts other than the traditional four — such as the southwest’s monsoon season — South Florida’s seasons are unique in that they are influenced not just by latitude, but also by being surrounded by water.

Molleda said he doesn’t want to discourage people from debating when the rainy season will kick in, or end, and plans to keep an in-house record of rainy-season start dates that may differ from the official calendar for climatological reference.

The set rainy-season dates will cover the seven counties overseen by the Miami office of the NWS — Miami-Dade, Monroe, Broward, Collier, Palm Beach, Glades and Hendry.

Treasure Coast counties, which are overseen by the Melbourne office, will not have set rainy season dates, said meteorologist Scott Kelly.

“Our onset is more variable,” Kelly said. “It’s not very different than Miami’s but we are not comfortable setting a beginning and end date because it is more variable up our way.”

Late season cool fronts that let dry air reach Central Florida, but don’t push through South Florida, can delay the rainy season in areas north of Palm Beach County.

While having set dates may increase awareness of rainy-season hazards, others say it’s inconsequential.

“I think having an official announcement date of when the rainy season stops or starts is not of that much value to locals,” said Tequesta resident Tom Knapp, Jr. “People who live down here know about when the heavy rains and thunderstorms start to either ruin our afternoon plans, or bring us a welcome relief from the heat.”

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A pedestrian shelters beneath a large umbrella as heavy rain falls in downtown Lake Worth Monday morning, January 8, 2018. (Bruce R. Bennett / The Palm Beach Post)

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