Mississippi South Florida Style
The Lake Okeechobee measuring cup

Hydrology numbers get big, confusing …

And obtuse.

Mississippi River’s annual discharged measured in Okeechobees

Millions of gallons per day, acre feet per year, cubic feet per second … the list goes on. The fact is, at some point it all just turns into a jumble of numbers. That’s where Lake Okeechobee comes in handy as a giant measuring spoon. Did you know that each year on average 100 Lake Okeechobee volumes worth of water discharges into the Gulf of Mexico from the Mississippi River. I could have said 400 million acre-feet of water, or some gargantuan number in gallons or cubic feet. But if you’ve ever stood on the levee of Lake Okeechobee and looked out at the expanse, it kind of puts it in perspective: The Mississippi discharges about one hundred Okeechobees into the Gulf each year. That’s one mighty river!

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Tidbit: The Mississippi River discharged over 160 Okeechobees into the Gulf during the flood year of 2000, and just over 60 during the drought years of 2006 and 2012.

animation lake canoe

Caloosahatchee meets Mississippi
And how the two compare

The Caloosahatchee is …

The king of freshwater flows in South Florida.

Comparison of annual freshwater discharge from the Caloosahatchee (red), Apalachicola (dark green) and Mississippi River (light green)

Currently flowing at around 5,000 cubic feet per second (cfs), it’s the single highest flowing point in the Greater Everglades. But not only is that flow rate dwarfed by the 17,000 cfs discharging out the mainstem of north Florida’s Apalachicola River, both are dwarfed by the nearly 700,000 cfs discharging from the Mississippi River into the Gulf. How much is 700,000 cfs … in more relatable lay audience terms? Answer: Every year, on average, the Mississippi River discharges about 100 Lake Okeechobee’s worth of freshwater into the Gulf. Note: The calculation is based on the assumption of a Lake stand of 15 ft above seas level, or the top of the Lake’s interior-levee littoral zone at which time its water volume is around 4 million acre feet. On average, the Caloosahatchee discharges about a quarter Lake Okeechobee volume worth of freshwater into the Gulf per year. Of course in the case of both, it’s just not water quantity — water quality matters, too, if not the most.

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Tidbit: The Mississippi starts its 2,552 mile journey to the Gulf of Mexico at Minnesota’s Lake Itasca, as photographed in 2014.

Lake not the blame (this time)
Closed gate but rising flows downstream?

With The Lake near its annual low …

And the Kissimmee River at a similar ebb:

A semi-complicated hydrograph summarizing some key indices for Lake Okeechobee over the past two years, including rainfall, lake stage, inflows from the Kissimmee River and discharge through the S-79. See the complete cheatsheet for the lake.

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.

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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.

big weather

First Big Rain Day of the Year
One down, four more to go

Well, it’s been over a half year …

But finally we got another Big Rain Day (BRD).

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.

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Did you know? On average, June is south Florida’s rainiest month, and August second.

Handy Hurricane Calendar
Peak storm season is still 3 months away

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.

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Tidbit: In the Atlantic, hurricane season starts June 1, while in the Pacific it starts May 15. Both end on November 30.

It’s not like those winds and waves stop at county lines.