Big barn door to the sky

Rain or Shine Report for February 12

Big Barn Door to the Sky is Impossible to Close

Around 2.3 million acre feet of water escapes from Lake Okeechobee over the course of a year. The area of a baseball field within the foul lines (~2 acres) is a covenient way to visualize that volume as a daily rate, or during the course of a 3-hour span of a 9-inning game.

Hydrology is the science of water – its flow, its depth, its quantity, its quality, its distribution, among other things – but the essence of hydrology, or at least making sense of it, lies in converting it – in all its forms – into language and metrics that people can wrap their mind around.

I’ve often thought that weather gets a bad rap as the topic of last resort that, the one that we nervously find ourselves broaching when, in the midst of an unexpected encounter in the hallway, and with not much else to say (at least at the tip of our tongue), we find ourselves defaulting to the routinely posed “how about the weather?” which is usually responded to with something equally scripted, such as “we really need the rain.”

But in south Florida, not only does water loom large in all its forms and in many of our minds. It’s fun to try to track it as it unfolds – through the numbers. And we are fortunate to live in a land where so much attention, time, and resources are dedicated to trying to better understand it.

Hopefully, when the day is done, The South Florida Watershed Journal contributes to enhancing those conversations, and gives the interested reader a place to go to stay in tune, or to dig deeper and find out more.

We have a rainmaker moving in from the north in the form of a front.

Forecasters are calling for up to 2 inches of rain — with torrential downpours and up to 25-50 mph wind gusts — for later today and Wednesday. That could bump up rain totals for February.

Fronts are an underappreciate rainmaker in south Florida.

So often its the summer rain machine and the tropical storms that make the headline news as the primary one-two punch in south Florida rain story.
The summer rain machine is our old faithful.
It’s reliable year in and year out; and there seems to be a point every summer — even if its just for a few weeks — that we can practically set our watches to the arrival of the afternoon showers. That’s about as close as we get to an old faithful in Florida.

Our old faithful produces 8-10 inches of rain per month during the four core summer Months of June, July, August, and September; but as we saw this year, those amounts are not iron clad, nor are they spread uniformly through the region. Over the past 10 years, Lake Okeechobee has average 26 inches of rain over those 4 core months. The Southwest Coast (Naples to Miami) has averaged 39 inches in comparison. That’s over a foot more rain. But this year, the Southwest Coast only mustered around 30 inches of rain for those 4 months.

And then there are the storms from the Tropics that can bring big buckets of rain, and equal doses of wind and storm surges, but are hit and miss; and not something that can be reliably predicted or counted on.
Water mangers were thinking ahead when they proactively released water from Lake Okeechobee during the early part of 2006, in anticipation for a repeat of the active hurricane seasons of 2003, 2004, and 2005 — but the 2006 hurricane season fizzled at its midpoint due to upper altitude wind-sheer from a burgeoning El Nino, and we’ve been hit with a consecutive string of dry weather since then.

Not only are they unpredictable, but its often the poorly formed tropical systems, not the big ones, that end up being the bigger rain makers.

Think Mitch back to 1998. It had already deteriorated energy wise, but brought about a foot of rain when it passed over south Florida in early November.

Or 1999 when Hurricane Floyd made transfixed all eyes to the east as it slowly churned across the Atlantic, seemingly headed straight for Miami, but — as predicted by forecasters — curved to the north about 100 miles off-shore, bringing narry a drop of rain to south Florida. The coveted near miss! But then a poorly formed storm named Harvey sprouted out and came onshore from the Gulf and dropped around a half a foot of rain in Naples.

Everyone remembers the near-miss Floyd, but it was Harvey that delivered the rain.

The dry season fronts are similar in that way — we sort of discount them, but they can be big rain makers, just when we need the rain the most in the winter and spring.

And don’t forget their role in setting the stage and complementing the summer rainy season. That’s what happened during the record-setting +20 inches of rain that fell in June 2005. That bonanza of rain was driven by spring-time frontal storms blowing in off the continent, and not from the peninsula-fueled summer rain machine.

And fronts bring not just rain, but also energy — which can often rival the fury of the tropics. Nobody any time soon will forget the Spring-time superstorm that wallopped Florida’s Big Bend area back in 1991. That was a continental-derived low pressure system that collided in the Gulf with a cold front from the north and tropical air from the south, resulting in hurricane intensity winds

Rain-making fronts can rewind, or at least momentarily stall, the dry season clock.

That’s what happended in February 2006, about 2 years ago to the date, when a frontal storm passed through and dropped around 3 inches of rain across south Florida.

The wetting front in Big Cypress National Preserve was dropping down into the tall cypress, but that February 2006 storm pushed the wetting front back up into the wet prairies, where it stayed for the remainder of February, and prolonged the wetting fronts disappearance down in the aquifer until late March. In comparison, last year the wetting front disappeared in early March, and this year we are on track to drop back into the aquifer in by middle February.

Lake Okeechobee has officially dropped below the 10 ft milestone.

Two years ago to the date, when that 3 inches of rain fell in early February 2006, the big lake stood at over 15 ft. A year ago today the Lake was at around 11.5 ft. Back during the drought year of 2001, Lake O was at 10.8 ft in early February. A big difference-maker during the 2001 drought year was the 4-inches of rain that fell in March 2001. The Lake eventually dropped to around 9 ft by June 2001, but that rain slowed down the dry season recession, just when the recession typically starts accelerating. No similar dry season rains materialized last year.

Even when all the gates are closed on the Lake, the big giant barn door to the sky remains wide open.

Evapotranspiration accounts for around 60 percent of the lakes annual outflow — around 2.3 million annual acre feet. That’s about half the total volume from the lake, which when completely full (+17 ft) holds around 5 million acre feet of water.

The apples and oranges of hydrologic mixed units are notoriously hard to grasp for the professional and lay person. Acre-feet, cubic feet per second, and gallons per minute are routinely used in unison, but so often defy comprehension in a true comparative sense.

The 2.3 million acre feet averages out to around 6000 acre feet per day. An acre foot is the volume of a one foot depth of water across a flat acre field, or 43,560 cubic feet, or 326,000 gallons.

A good way of thinking of evaporation, as a daily flow rate, is to imagine being in a major league baseball stadium.
Between foul lines it holds about 2 acres of area. That’s about the same size as the base of the Empire State Building. Over the course of a typical day, the atmosphere pulls enough moisture from the lake to fill a hypothetical column between the 2 acre area of the baseball field just over 3,000 ft high. That’s a little over 2 times the height of the Empire State Building (including the spire = 1,472 ft tall) on a daily basis.
Over the course of the 3 hours you’d be in the stadium stands watching a 9-inning game, that computes to around a 400 ft column of water. That’s taller than the tallest of the Redwood Giants in northern California, or about 4 times the height of the tallest of the old-growth cypress trees right here at home at Corkscrew Swamp.
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