Monthly rainfall for Big Cypress Nat’l Preserve. Can you see the lull?
Someone once described it to me as the lull that sets in between the last of the continent’s spring time fronts and before the tropical waves really kick in. Over the long term that’s resulted in consistently (slightly) less rain in July than the other four core wet season months (i.e. June, August and September).
When will the wet season get its groove back?
Answer: Probably with a tropical storm. August, September and October are south Florida’s peak storm 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.
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).
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!
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.
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.