Almost Big Rain Days

My old philosophy was …

It either rained or it didn’t.

Calendar of daily rain across south Florida, 1992 to present

That changed when a local meteorologist introduced me to the idea of Big Rain Days (BRDs). Basically, a BRD is any day when an inch or more of rain, on average, falls across all of south Florida. If that doesn’t sound like a lot, that’s because you’re thinking “locally.” Regionally for all of south Florida, a day that produces over an inch of rain is a big event. The result: The water table usually jumps up an octave or two, using the guitar scale, and from cypress to pinelands if we’re talking specifically in the Big Cypress Swamp. On average (as shown above by the big black drops) south Florida averages 5 BRDs per year. This year is trending on the low side with just 3.

But back to my point: It’s now been many a year that I’ve classified south Florida’s rain into three tiers: (1) no rain, (2) moderate rain and (3) BRDs. Then struck my brainstorm. What about the days that just fall short, but over a consecutive 2-day period meet the “one or more inch” mark?

That’s where the new “almost BRDs (ABRDs)” come in handy. They’re shown on the graph above as the larger blue dots with the dark-blue outline. In my book, they’re as good as a low-order single-day BRD.

The new fourth category also adds an important wrinkle of detail to the above chart. Water’s peaked in September thanks to a short string of ABRDs followed immediately by a good week of no rain at all, thus leading us to the believe the summer rainy season was done. That changed in mid October with a return of summer humidity and another short string of “almost BRDs.”

That goes to show: Like a game of horseshoes, “almost” counts in hydrology, too.

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