Monthly rainfall for Big Cypress Nat’l Preserve. Can you see the lull?
The reason?
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.
Why then have discharges down the Caloosahatchee River’s S-79 into the Estuary suddenly spiked to over 5,000 cfs, which in layman terms is around 34 Fenway Parks filled to the top of the 37.5 foot high Green Monster every day? Answer: Blame all the rain in the Caloosahatchee Watershed, not the Lake. The S-77 which controls Lake flows into the C-43 are closed, and is therefore recording no flow.
Tidbit: At 13 feet above sea level, Lake Okeechobee is about 8 feet below the level it naturally overflowed its southern shore into the Everglades prior to drainage.
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.
Confused by Florida’s convoluted hurricane history?
If so, this calendar graph may come in handy.
When hurricane strength storms made Florida landfall
There’s more to this graph than meets the eye:
The dots show the year, month, and strength of each storm which made Florida landfall over the past 110 years. Clicking on each dot transports you to a detailed write-up of each storm. The “outer” dot plots each storm’s intensity at peak strength and the “inner” dot at the time it made landfall.
Major patterns?
September is Florida’s big month. Late August and October also light up the chart. And three is the most hurricane-strength storms that have made landfall in the peninsular state in any one year. We almost had four in 2004 but the eye of Ivan made its way into land on Alabama shores.
When and where storms form by month
That leads me to one caveat, and its a big one:
The graph only includes “hurricane strength” storms that made “Florida” landfall. That rules out many a monumental rainmaker (i.e., Fay) and any storms whose eye crossed just across state lines.
We cover them all: The Districts, the estuaries, the aquifers and the watersheds. Also the rain, and the dew. Plus the humidity. Did I mention evaporation? The list goes on.
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