temperature check

Shades of warmth
Gainesville vs Naples air temperatures

Northerners think of Florida as being …

A land where eternal summer prevails all year round.

Temperature chart comparison: Naples vs Gainesville, FL. The background light and dark gray show the typical and historic range. Blue bars show the daily high and low. Red dots show the average daily temperature.

But really you have to travel south of Lake Okeechobee to get the full dose of year round Florida sun. The reason? Two come to mind. (1) First, north of the Lake and including the entire panhandle, continental fronts are much more common and packed with cooler air. Many a weak front makes it to Orlando but peters out south of the Lake. That leaves Naples and Miami basking in warm air as the upper half of the peninsula plunges into cooler and drier air. (2) Second is the issue of the clouds. Sunny blue skies prevail south of the Lake as the northern climes of Florida are more frequently shaded under continental frontal clouds.

Not that all of Florida isn’t warm by Minnesota standards all winter long. Just don’t expect to wear shorts in January and March in Gainesville. In fact, you’ll probably need a good fleece and scarf. At least that’s my opinion looking at the weather chart chart from the Naples point of view.

Other notes: (1) Notice how Gainesville is hotter than Naples during the peak summer weeks. (2) Summer starts about a month later in Gainesville (more June-ish) compared to Naples (by May).

How to: Predict a cold front
Fine art of forecasting good weather

We’re still waiting for our first official cold front …

But no reason to get worried quite yet.

This histogram shows the frequency that the first official cold front (i.e. two consecutive nightly lows below 60° F) arrives to Naples Florida, as based on the historical record from 1941 to present.

By “official” cold front, I’m going by Morton D. Winsberg’s definition in his seminal book Florida Weather. It’s a book that I’ve read over and over again. I call it a rereadable. Keep in mind Winsberg’s definition doesn’t count just drier air, an end to the summer rains or slightly cooler morning and evening temperatures. For the cold front to be “official” it needs to be a true blue slug of continental air that sends nighttime lows plunging under 60° F for two days in a row. Or in other words, it will inspire you to wear long sleeves, if not a scarf. And yes, if you’re a year-rounder who’s endured a full summer, it will have you celebrating, too.

As for when they arrive, it varies from year to year. Some years we get what I call an abnormally early teaser front (i.e. 2000) in early October, but most years (as shown by the distribution curve above) our first real dose of cold air doesn’t arrive until a week or two on either side of Halloween. Then there’s the year’s we have to wallow all the way to the end of November to get our official dose (i.e. 2013, 2020).

Thus my prediction: I’m going to be conservative and say by Thanksgiving it will have arrived (unless it’s another 1986 – see chart above).

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.

animation switch short

Final tally is (almost) in
Counting continues until Halloween

South Florida has one wet season …

But the final tallies vary geographically.

Comparison of wet season rain by basin

For example, Lake Okeechobee usually gets the lowest amount of wet season rain, around 33 inches. Compare that to the Lower East Coast (Miami), Big Cypress and Southwest Coast that averages 43 inches of wet season rain. South Florida Wide, the number falls somewhere in between at around 38 inches. For water drop counting purposes, we compute wet season rain for the six-month period from the start of May to the end of October. Thus, it’s too early to call the final tally yet, but we are pretty close so it’s worth taking a look. As stands, we’re a little below the typical average. That could still yet change, as the clouds have until Halloween to get their final drops in.

BTW: October is better understood as a transition month between the wet and dry season, but we lump its entirety into the wet season rainfall tally for book keeping purposes, and to be consistent from year to year.

animation switch

Trick or treat?
And some years right in between, but rarely

October is a “hit or miss” rainfall month.

It alternates between bountiful and barely any rain at all.

The reason? Blame it on tropical moisture. With the rain machine shutting down in the first week or so of the month, the remainder depends on the last gasp of whatever the tropics have left. Over the long-haul, October falls right in the middle with four inches of rain. That’s half as much as a core summer month and twice as much as a typical 30-day dry season span. But don’t let the long-term average confuse you. It’s an imaginary number. Yes, some years we get about 4 inches on the nose. But usually October picks a side. All it takes is one big rain to give the wet season new life. Or a run of dry weeks to slide the dry season into an early start.

So will this October be a trick or a treat? Answer: We won’t know for sure until October 31st, Halloween.

dry season

Summer sputters to end
Who keeps turning on and off the switch?

It’s a long summer in south Florida …

And then suddenly like a flip of a switch the rains stop.

The regular pattern of afternoon rains stops in early October

At least that’s how it seemed a week ago.

The late September slug of air had us convinced the summer rain machine had shut down for the year. Ten days later more humid air has returned, and the rain machine has even shown some signs of life. But the bigger picture is it’s starting to sputter off. Usually by Columbus Day (early-mid October), the winter dry season has begun even if from a monthly book-keeping sense we wait until November 1st to start the official dry season clock.

Bar chart dynamics
How to read a monthly rain chart

South Florida has two distinct meteorologic seasons:

A 6-month wet season and a 6-month dry season.

How to read a rain chart

Things you should know: (1) The water year begins anew the start of May each year. But it’s not an exact science (i.e. precise point) when they start and end from one year to the next. For example, we classify October as a wet season month even though the afternoon rain showers usually end in early October. And the start of May is probably the swamp’s driest time, yet it’s also the same month, as it approaches June, that the summer rainfall pattern begins. (2) Most of my rainfall charts show background gray coding. That’s the historical statistics as counted from 1983 to present. Why 1983? It was a good year, and most of the SFWMD’s record by basin reaches back that far. Looking at the chart above, the dark gray band is the average range for each month (i.e. between the 25th and 75th %tile) and the light gray is the historic rang (i.e. between the max and min). The white bar in the middle is the normal or median monthly rain. (3) My charts are based on basin-wide rainfall, not local rainfall.

Newspapers calculated rainfall by calendar year. (They are wrong. How dare them!) They also calculate rainfall for an individual gage in Ft. Myers and Naples (The shame!).

In summary, numbers mean more if you can frame them against the expected values and ranges that came before. And its by water year, not calendar year, that we tally rainfall totals in south Florida.

fall

Change in the air?
Not scarf weather, but definitely fall-ish

Although not an official cold front …

And it’s still possible to overheat in the midday sun:

The cooler morning and evening temperatures are a welcome relief. Daytime highs and still ramping up into the high 80s and nighttime lows are staying above 70, but with daylight hours on the wane and last week’s dose of a drier air front, its as cool an early October as I can remember.

Fall is definitely in the air.

Did summer just end?
Just when it was getting interesting

It was shaping up to be a subpar summer …

And then September kicked into high gear.

The swamp finally peaks, but for how long?

Back to back weeks pushed the swamp to its annual peak.

Then came the recent front of dry air?

Overnight the rain machine shut down.

Or is there still time for it to rev back up?

A flooded marl prairie with periphyton

I‘m never one to complain about the start of fall, but seriously – summer was finally starting to get interesting. It’s good to see the swamp’s sheet of water spreading out.