How To: Read a Rain Chart
I get it, charts can be boring (without narration)

Not that I’m a wildly dynamic speaker …

Nor are rain charts especially charismatic.

Bob has a one-on-one conversation with a rain chart

But combine the two together and I think you get, well — I think you’ll see the result. At the heart of the issue is what I’ve been told so many times: “Bob, you make a splendid rain chart, but most people don’t know how to read them.” And so my journey began, hours after hours, years upon years, in the quest to make the perfect rain chart. My conclusion: I think the only way to give a rain chart its due is to allow it to talk, and speak for itself. Okay, I’ll admit. I had to add the voice. And yes, I had to juice up the charts a bit (some would say with too many colors). Just don’t say I didn’t try.

Comparison of dry season rainfall, from 1970 to present. Cool color-coded bars indicate wet winters and warm color-coded bars indicate drier than normal winter.

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#Overheard: South Florida’s water year starts on May 1st, but the wet season doesn’t officially kick in until around May 20th.

Click “Read More” to see all the hydrographs!

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Weather Drop

SFWMD Weather Forecast
Courtesy of SFWMD's Meteorology Team

South Florida Water Management District Weather
10:08AM Thursday, May 16, 2022 (mrn)

Synopsis: An upper-air trough of low pressure extending from the southeast U.S. coast to the eastern Gulf of Mexico will gradually move eastward across Florida today and lie east of the SFMWD by Tuesday. A good supply of moisture and instability immediately ahead of the trough and sufficient daytime heating should cause showers and thunderstorm to form, with southwesterly steering winds favoring the rains over the eastern part of the SFWMD from around Lake Okeechobee southward and eastward through the eastern metropolitan areas. The median areal average rainfall across this region ranges from a quarter to half of an inch, with the 90th percentile or reasonable worst-case scenario in excess of half of an inch. As the trough passes across western areas and the Kissimmee Valley by this afternoon, subsidence (sinking air) and a subsequent drying with low instability should result in essentially no rainfall, or at least little of note. On Tuesday and Wednesday, large-scale subsidence in the wake of the trough is forecast to greatly suppress typical shower and thunderstorm development. However, there should be enough low-level moisture to cause isolated rains to develop along both east and west coast sea breezes on Tuesday and then isolated or widely scattered rains on Wednesday over the interior and the west when the steering winds become easterly to southeasterly. The 10% exceedance on either Tuesday or Wednesday should be no more than about a tenth of an inch. Next, a tropical wave located over the central Caribbean Sea on Tuesday should move into the western Caribbean Thursday and Friday, with its moisture feed beginning to stream northward through the Florida Keys and far southern part of the SFWMD overnight Thursday. The wave’s moisture will also be accompanied by an influx of instability, both ingredient of which should support a large increase of rainfall and rain coverage SFWMD-wide Friday and Saturday. Given the favorable large-scale conditions accompanying the wave passage south of the area, there is likely to be an enhanced risk of locally significant rainfall area wide but especially over the interior of the SFWMD. Although the most best moisture/instability seems as if will have passed by Sunday, southwesterly steering winds and daytime surface heating should still result in a good coverage of rain probably focused over the interior and the east due to southwesterly steering winds ahead of a cold front dropping southward into north Florida by Monday morning next week. For the week ending next Monday morning, total SFWMD rainfall is forecast to be at least normal and probably above normal. Monday: Very warm over parts of the SFWMD> A few showers and isolated thunderstorms developing in the east or southeast before noon. Then scattered to locally numerous afternoon showers and thunderstorms developing south and east of Lake Okeechobee through the southeastern metropolitan areas, some of which could be heavy. Rains could continue to around or after sunset before dissipating over these areas. Then quiet overnight as drier air moves across the entire area. SW to W winds 5 to 15 mph, except S near the east coast. Winds W to WSW 5 to 10 mph overnight. Tuesday: Very warm over the interior. A general lack of much rainfall area wide and far below the daily climatological average. Whatever rains do form should occur during the afternoon to around sunset inland of the east coast along the east-coast sea breeze and inland of the west-coast along the west coast sea breeze through the Kissimmee Valley. Rains diminishing by or during the early evening. Mainly W to WNW winds of 5 to 15 mph, except for E to SE winds developing by the afternoon to early evening along and near the east coast. Wednesday: A continued reduced total amount and rain coverage compared to climatology. Afternoon rains developing over parts of the interior and then over the interior and the west by late afternoon to around sunset. Isolated rains over the far west could produce heavier totals of rain. Rain chances diminishing in the east by afternoon. Mainly E to SE winds of 5 to 15 mph in the east but with W winds of 5 to 15 mph over the west during the afternoon. E to SE winds 5 to 10 mph south of Lake Okeechobee but SW winds 5 to 10 mph north and west of Lake Okeechobee overnight. Thursday: Quiet with essentially no rain in the morning. Then isolated or widely scattered afternoon rains across the SFWMD, with a few embedded heavier rain areas. However, an increase of rains should begin rising from the south into the far southern part of the SFWMD during the afternoon, followed by a good or widespread coverage of rain overnight through the Florida Keys. SSE to S winds 5 to 15 mph during the morning but then E to SE over the eastern part of the SFWMD during the afternoon and W to SW over western areas. SE to S winds area wide overnight. Friday: A large increase of rain area wide, with total rainfall likely well above the daily climatological average. Scattered to numerous showers and thunderstorms producing a widespread coverage of rainfall, some of which could be heavy. Heaviest rains probably over the interior and the west from the afternoon to early evening. An enhanced risk of locally significant rainfall accumulations. Rains diminish late in the evening and then mostly quiet overnight. SE to S winds mainly 5 to 15 mph. Saturday: Total rainfall again probably well above the daily climatological average. A good or widespread coverage of moderately heavy or heavy rains over the interior of the SFWMD but especially around and north of Lake Okeechobee from the afternoon to early evening. An elevated risk of locally significant rainfall over these areas. After evening rains diminish, mainly quiet conditions likely overnight. SE to S winds 5 to 15 mph, except S to SW north of Lake Okeechobee (0.36″). Sunday: Low-confidence forecast. Little rain in the morning, but scattered to locally numerous showers and thunderstorms forming over the interior and the east during the afternoon. Rains diminish during the early evening. SE to S wind 5 to 15 mph, except SW north of Lake Okeechobee, with winds becoming W to NW area wide overnight (0.27″).

Day 1

Go to SFWMD’s Weather Page

Thank you to the Meteorology Team at South Florida Water Management District for this forecast. I always learn something new when I read it, whether it be an atmospheric process or a new word. And more than any other forecast, it really lets you get the big picture of when and where rain may fall. The one problem is it’s buried on their website, thus my inspiration to feature it on Go Hydrology.

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Proverb: “All droughts end in flood.”

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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?

animation switch short

Follow the rainbow
A pot of gold awaits

Sometimes in south Florida …

You simply have to ignore the month.

Looking north towards I-75 Alligator Alley

Technically speaking, we should be bearing down (and scarfing up) in preparation for a deep polar freeze (or two). Well, at least not yet. So far this December, the weather has been closer to the hot and humid summer pattern of pop-up showers and copious morning fog. How thick is that fog? Thick enough to make pea soup seem transparently thin. The commute ride into work, for those that have been doing it, has been a white knuckle ride with 300 ft visibility. On the good side, there’s the mid afternoon rainbow as seen above. At 1,000 feet above the ground, the air temperature is also delightfully cool.

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.

rain charts

Summer fizzle?
Or will more rain showers save the day?

The good news:

We still have 3-4 weeks of wet season to go.

About 43 inches of rain fall in Big Cypress National Preserve every summer, as tabulated for the 6-month period from May to October.

By wet season,

I’m talking meteorologically, and specifically the regular pattern of afternoon rain showers.

Yes, we may get tropical weather in October and November (think Wilma and Eta), and yes the swamp will remain soggy through the calendar year and winter cold months.

But by mid October the rain machine usually shuts down.

Transition between the Everglades and Big Cypress Swamp, looking west

By my counting, we still have some filling up to do.