Dry season gets drier

Dry season enters “heart of dryness”

Rain Or Shine Report for May 7
Dry season numbers are in.

Palm Beach County recorded the most dry season rain since November.

Mark them down for nearly 18 inches of rain, with 10 inches of it falling in March and April.

At first glance, that would appear to be “El Nino-esque.”

But don’t forget this year was just the opposite: a La Nina, and a wet La Nina at that. It will go down in the record books as one of the wettest La Nina dry seasons on record.

And don’t forget that last year’s dry season (2007) was under the influence of El Nino that also defied expectations. It was one of the driest El Nino dry seasons on record.

So it’s been two consecutive dry seasons where the actual conditions that unfolded countered the common wisdom of what we expected. (See previous post)

That’s what keeps us water cycle aficionados coming back for more.

But back to those Palm Beach numbers.

As it turns out, 18 inches isn’t as wet as it sounds.

Over the long-term, Palm Beach tops all other basins with regard to dry season rainfall. It has averaged around 15 inches of rain during the past 10 dry seasons. Compare that to only ~10.5 inches for Lake Okeechobee and Water Conservation Area 3, ~11 inches for Big Cypress National Preserve and the southwest Coast (Naples and Ft Myers), and 12 inches for Miami-Dade, Kissimmee Basin, and Loxahatchee.

And when I say “dry season,” I’m talking about the 6-month period from the start of November to the end of April. By that metric, we’ve already left the “dry season”, and any rain that falls in May is counted toward the 6-month “wet season,” which I tabulate from the start of May through the end of October.

But that’s an imperfect division: it’s convenient (and consistent) for number crunching.

The meteorological reality is that right now as we speak, the “dry season” is not only “alive and kicking,” it’s actually ramping up into high gear.
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The final weeks of April and much of May, especially when rainless conditions prevail, is the precarious time when we enter the dry season’s “heart of dryness”: the heat index is cranking up, the humidity index is low, and the evapotranspiration is wicking the ground dry – and ramping up the threat of wildfires, which start kindling to life across south Florida’s landscape, not to mention the bottom falling out of the shallow aquifer.

People always ask me if Lake Okeechobee waters flow into Big Cypress National Preserve.

The answer is no – Big Cypress is predominantly a rain-driven watershed. But with the ~5,000 acres of wildfires raging inside the Lake’s levee, and the winds turning to the southwest, they’re saying this morning on the radio that we can expect some smoke from the Lake today across much of southwest Florida.

That’s another example of how our watersheds are interconnected.
In closing, I thought it would be fun to look back into deep time: how did this dry season rank historically?

District-wide, this year’s dry season tally (13 inches) chimed in right around the long-term average, extending back into the 1930s, which is around 13.5 inches.

That’s a step up from the previous two dry seasons (2006, 2007), both of which were under 9 inches. The wettest dry season of the new millennium is Nov 2002 to Apr 2003 when ~17 inches fell district wide. But the real chart toppers were 1983, 1995, and 1998 – when an El Nino condition were in play – when dry season rains tallied 25, 22, and 25 inches of rain.

But what made 1995 the real chart topper was the 50 inches of District-wide wet season rain that followed. That made for a potent 1-2 punch soaking.

What does this upcoming wet season bode?

That’s a good question, but first thing first: that means getting out of meteorologic dry season and into our summer storm pattern.

More on that as it develops.

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