dry season

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Intro - Dry season paradox?

Winter rains are the exception, not the rule

By Robert V. Sobczak

Believe it or not,

It rains during the winter in south Florida.

South Florida skies are reliably sunny in winter and spring

Even more quizzical, some of those individual rainfall events can be quite large, gulley washers even. The big difference with dry season rains is that they simply don’t add up over a monthly scale. One two inch rain event (however impressive) plus 29 zeros add up to a whopping two inches of rain. Compare that to the typically 7-9 inch totals of the core summer wet season months of June, July, August and September.

The water table reliably and steadily drops during the dry season, starting in November and lasting into May, as a result of the lack of steady daily rains. The caveat is when a big frontal storms pass through. Unlike summer storms that tend to be more local in nature, a winter cold front can dump water across the entire southern peninsula.

Another factor is the cooler temperatures. Winter storms don’t lose as much of their water to evapotranspiration back into the sky. The result is that every drop of winter rain counts as two, and also has a longer “staying power” on the landscape.

That changes in the latter half of the dry season somewhere around the vernal equinox when daylight hours start to grow, the cypress trees green out and air temperatures start to rise. Without rains in March, April and May, the swamp nosedives into a deep drought.

Cycles of flood and drought

How deep and how long will the spring drought last?

Usually not too far into May and rarely into June. It only takes a few weeks (sometimes less) for the summer rains to lift south Florida out of drought.

Isummary: Yes, dry seasons are wet, and that’s not paradoxical. Just don’t count on them too much.

Recent blog posts

dry season

Dry season review
A holiday guide to the dry season

How did a relatively normal dry season …

appear to be so darn wet?

Months or holidays: Which interval do you prefer for comparing rain? The advantage of months is that they are equal units. The advantage of the holidays is it allows us to partition the dry season into its various acts (i.e. opening gate, cool season, green out, spring ebb, etc.). We were headed for a “dry” dry season until the April unexpectedly kicked in.

Answer: It’s not how much but when the rain fell. And I’m not talking summer rains or fall hurricanes, which together give us about 43 inches per year. And I’m not even talking the thirteen inches of dry season we recorded this year for the 6-month span between the start of November to the end of April, which for the record was about 1-2 inches above the normal dry season total. The big difference maker when it comes to the swamp batting back the descent into deep spring drought is April rainfall. No April rain means deep drought in the swamp. This year, as indicated by the yellow bar above, the swamp recorded a solid 6 inches of rain from the spring solstice (March 22) until now. That’s twice as much as the year before (2021) and three times as much as the year before that (2020) and just the right amount of rain necessary to keep standing water in the cypress domes and strands.

Did I mention we had a subpar summer? It didn’t matter thanks to the timely April rains!

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Question: What’s your favorite dry season holiday?

dry season

Nothing lasts for ever
But November rains last a while

When the rain falls …

Is just as important as how much.

Holiday guide to the winter dry season

And there’s no better example than November rains. Last year it was Eta. This year it was the frontal storm that caught us by surprise. Or at least it caught me by surprise (as much as it was impossible to ignore as it slowly slogged through). Usually November is the gateway to the dry season: the start of the slow but steady descent into a state of less drenchedness (new word). In my book, November rains are as much wet season rains as they are dry season events for the reason that they have the effect of bumping the water table back up to a peak water state. November rains also have “staying power” because the regional evaporation machine is shutting down.

About the chart above. It’s plotted by replacing months into five major milestones (or inflection points) of winter dry season. They include (1) opening gate rains (Nov 1 – Nov 30), (2) cool season rains (Dec 1 – Feb 2), (3) green out rains (Feb 3 – Mar 22), (4) spring ebb rains (Mar 23 – May 15) and (5) start of wet season rains (May 16 – May 31).

Why those units and not months? Months are boring! And they also obscure the sub-seasons within the larger winter dry season whole. Dry season totals are not as important as when it falls. Last year’s Eta was the case in point. That November bounty (light blue bar) had staying power, but it didn’t save the swamp from descending into a spring drought. The taller green and yellow bars are the true drought-killing rains.

dry season

Globe-trotting La Niña
Why south Florida is a top destination spot

Where does La Niña travel to …

When it really needs to get away?

Winter rainfall in south Florida is strongly correlated to the ENSO cycle

The Everglades and Big Cypress may be its favorite choice. The reason? The formation of cooler waters off the coast of Peru are highly correlated with drier-than-normal winters in south Florida. Not that we don’t get fronts of cold air — we do. But unlike during the El Niño phase of the El Niño Southern Oscillation (ENSO) cycle, those fronts are usually devoid of moisture.

La Nina
Global response to El Niño and La Niña events

The above being said, the prevailing La Niña was also supposed to cause a higher frequency of hurricane-strength storms during this summer and fall. Knowing that didn’t materialize has me wondering: Could we be in store for a similar dry season contradiction to the normal winter La Niña effect? Last year’s Tropical Storm Eta comes to mind. All it takes is one storm.

Dry season wrap up?
And why it's not over yet

The dry season isn’t over …

But it’s entering its final weeks.

Rainfall over the past 7, 30 and 90 days across south Florida

When does the rainy season start?

Afternoon showers can start sputtering in April and early May, but it isn’t until later in May and early June that the water table typically rebounds.

Whatever the case, the verdict is in:

The 6-month dry season was was wetter than average across the entire southern peninsula with the exception of the Upper Kissimmee and with the lower east coast leading the way. District-wide, south Florida averages around 13 inches of rain compared to the 15 inches recorded this year.

Basin by basin comparison rainfall since the start of the dry season (blue) and the start of the calendar year (red). The hollow black bar shows the dry season average.

But May is a pivotal rain month.

Typically drought extends and deepens in its early half before the afternoon showers kick in or we get a big regional storm.

One more week, and Water Year 2022 is here!

As usual, it’s a wait and see.

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