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Intro - Sunshine or Rain State?

By Robert V. Sobczak

South Florida is a bipolar ...

When it comes to it's annual rain cycle.

The reason?

Four fifths of its five feet of rain fall in the six month summer season. Five feet of rain sounds like a lot, until you consider that half the year only averages 2 inches or less per month. Or in other words: Meteorologic drought is hardwired into the annual cycle.

Listen to the Audio Introduction

One day of big rain can be a game changer that lasts for many months.

In the same way, multiple months of no rain can dry everything out. At the risk of being Captain Obvious, it's at the times of deepest drought we most regret shunting the wet season bounty of rain to tide.

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Recent Blog Posts

wet season

Holiday summer rain guide
Why months don't matter, or maybe they do

Often we think of summer rainfall …

In terms of a total or by months.

South Florida’s summer has 5 acts: (1) the spring prelude (Apr 1 – May 15), (2) the Early Wet Season (May 15 – Jul 4), (3) Mid Summer Doldrums (Jul 4 – Sep 1), (4) Fall Finale (Sep 1 to Oct 31), and (5) Encore Rains (Nov 1 – Dec 1)

But maybe a better way to frame it is by major holidays. The reason? For one, the rains that come “just before” and “just after” the official wet season (i.e. as defined from June through October) are just as important as the summer rain itself. Timely spring rains can boost the water table just before the summer rain machine turns on in the same way that November Soakers can prolong the summer high water stand. Look no further than 2020’s Eta (in November) as proof. I’m not saying to do away with months (yet), but I do believe holidays for nice mile markers for refining our Water Cycle IQ. BTW: The above chart is for south-Florida wide.

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Minority Opinion: Of course monthly rainfall matters! Here’s a closer look for monthly rains in Big Cypress National Preserve.

The swamp has received 3 months in a row of above average rain
big weather

Florida’s “meteorological” four horsemen
And why they aren't so scary after all

Can you hear the apocalyptic atmospheric stampede?

Not to worry — the world is not coming to an end.  It’s just the sound of one of Florida’s four horsemen galloping across the sky.

Storms roll across the Florida peninsula
like a stampede of wild horses

Who exactly are the Four Horsemen you may ask?

  • The first is our old faithful of the summer: the Enhanced Sea Breeze. I’m not talking your any day old run of the mill sea breeze. This is the one that, with a little help of upper level atmospheric instability and a Gulf flyover of a deep dipping Jet Stream – two factors that puts extra wind behind the sails of the sea breeze, creates our gargantuan Kilimanjaros rising out of the Everglades and the famed morning showers offshore of Miami.
  • The second horseman is the Continental Front. The thunderous squadrons of clouds that they bring, often leaving cold air in their wake, are typically a dry season event. But they’re not unheard of in the early summer season. That’s what makes June such a critical rainfall month for south Florida. Lingering springtime instability up on the continent – both in the upper and lower atmosphere – juices the early part of the rainy season, from Memorial Day to Forth of July. Once July roles around, a more homogeneous air mass takes hold across the southern peninsula. Trade winds blowing due east off the Bermuda High become the prevailing wind pattern.
  • It’s the Bermuda High that paves the path for the third horseman, and the scariest: the Cape Verde. These are the mammoth hurricanes that spawn off the coast of Africa, and head west around the perimeter of the Bermuda High. This one packs the full punch – horizontal rains, instantaneous – if only momentary – sea level rise, and tree-toppling winds. And this is no sucker punch – it broadcasts its potential fury days in advance, but it keeps its exact landfall a secret until the day approaches, and I use the term “day” only in calendar sense, because once the Cape Verde stampedes to shore, it turns daylight into night, other than a brief glimpse of daylight at its eye. That’s its prelude to the second half of its 1-2 punch, more commonly known as its knock out blow.
  • The fourth horseman is the Tropical Tempest from the Gulf and from the Caribbean. Usually not as scary as the Cape Verde, they play a prominent role in the early and late part of the hurricane season. Don’t be overly concerned with the magnitude of these, because even a disorganized wave of tropical moisture can give us the coveted BRD – Big Rain Day, as coined by the District’s Meteorology team. In technical terms, that’s a sFL-wide daily rainfall total of more than 1 inch. Geoff Shaughnessy tells me we need 6 BRDs to keep the annual water coffers filled.

You can hear and see them
coming from miles away

Florida’s four meteorological horsemen are each ominous in their own way, but after a long dry season their hooves, too, are music to water managers’ ears.  Finally, aquifers and wetlands can start to refill.

But come high water the same horsemen are cause for concern.

That’s the thing about the four horsemen:

They are a wild breed.  Yes, you can tame the landscape upon which they roam with levees and canals only so much.  The horsemen in their full fury have a reputation of running roughshod over civilization’s carefully laid plans.

In 1990 Lely Development Corporation commissioned
five 1 1/4 life sized running horses for the entrance to their luxury country club community in Naples, Florida.

But mostly the four horseman are fun to watch (and hear) from a distance.

Just be sure to take good cover when they run near!

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Caveat: Of course the predominant direction that all storms arrive is “downward” from up in the sky!

Big Saharan Swamp?
And during soggy season no less

Umbrella be darned,

It’s almost impossible not to get caught in a rainstorm come summer time in the swamp, and getting thoroughly drench.

Available on Apple Podcasts and Podbean

But a dust storm? And from the Saharan Desert, no less? And during the soggy summer season in south Florida? Now that’s something if I were to warn you about, you would find hard to believe, and equally uninterested in preparing for. The good news: The plumes of Saharan-desiccated dust blowing in on the wings of the Trade winds are not on the same scale as an Oklahoman sand storm during the Dust Bowl. But it does give one pause for thought: The weather is south Florida’s famed summer rain machine isn’t as isolated as we think. External forces can both stoke and stymie its might, and sometimes completely shut it down as often happens when a hurricane passes off shore up Florida’s east coast (i.e. pumping down drier northern air).

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Tidbit: The Sahara Desert measure 3,000 miles from east to west (about the distance between New York City and San Francisco) and about 1,000 miles from north to south (the distance between Chicago and Houston). Now that’s one big desert!

Summer’s plateau?
And why the swamp isn't as low as it seems

Starting in late May …

South Florida’s air temperatures plateau.

This hydrograph compares air temperatures in Naples and Gainesville, Florida. Each graph shows the normal (light gray) and record (dark gray) statistics in the background. Despite Gainesville getting colder during the winter, both places lock into the “summer plateau” mode come the end of May. How long will it last? Answer: Into September for Gainesville, and through most of October for south Florida.

By plateau, I mean they flatline, or stay steady, at an elevated height. That height is expressed in two numbers: A daytime high in the high 80s and the nighttime low in the low seventies. So remember that the next time you’re standing knee deep in the summer swamp looking in the distance at a giant mountainous cloud (or range of mountainous clouds) rising up and approaching.

Driving into the cloud on a plateau-like levee called the Tamiami Trail

The swamp isn’t as low as it seems, but rather a summer plateau that gives us expansive views of the cumulonimbus clouds as they rise.

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Swamp History: 10,000 years ago, south Florida was in fact a peninsula, both high and dry and perched 350 feet above sea level.

big weather

First Big Rain Day of the Year
One down, four more to go

Well, it’s been over a half year …

But finally we got another Big Rain Day (BRD).

Historic calendar of Big Rain Days (BRDs) in south Florida. Black drops show the BRDS. Orange circles show the “little to no rain” days. Baby blue and blue-ringed circles are somewhere in between.

And I know what you’re thinking — what exactly is a BRD? According to the official Go Hydrology Dictionary, a Big Rain Day (BRD) is any day that an average of an inch or greater is recorded across all of south Florida.  For that to happen requires a lot of rain to fall everywhere. The weekend storm makes it the first BRD of both the new water year (starting in May) and the calendar (starting in January) year, too.

Annual number of BRDs per year, 1990 to present

On average, south Florida averages about five BRDs per year. The most in recent history was 2005 with ten BRDs followed by the least in 2006 when we only got two.

In terms of when they occur, this is where it gets interesting. June leads the way with 18 percent, accounting for about one in every five every BRD. The lowest are February and July. Okay, February makes sense — It’s the heart of the dry season. But July? Isn’t that the smack dab in the middle of the rainy summer months?

Distribution of BRDs by month

Answer: Correct, it is. Unlike the June that is still juiced with upper-level instability from continental fronts, come July that instability dissipates with the full maturation of the Trade Winds out of the east. The result: July reliably brings us the “bread and butter” summer storms, but not the big rainmakers that define the start and end of the wet season.

In summary, it feels good to get the first BRD of the year. Now, on average, four more to go.

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Did you know? On average, June is south Florida’s rainiest month, and August second.