Florida's Rainy Season

Florida's waterways, big and small are quickly shrinking as the relentless dry heat of May 2021 drags on. So how long will it last? The short answer is that Rainy Season usually starts in Florida the end of May. But in 2021 it will be delayed by at least a week or two of intense dry heat as relentless high pressure sets up over the Florida peninsula. So expect more weeks of scenes like the one above on Deltona's Lake Theresa and below on Volusia County's Lake George.

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Floridian's often wonder when or if we'll get a rainy season as even huge lakes like Lake George (above) start to run dry. For years the rainy seasons have been unpredictable or absent. 2021's spring has been mostly variable. While it is very hot and very dry in late May the National Weather Service is already warning of the potential for summer flooding.

Much drier than normal conditions during January and March across central Florida were offset by increases in rainfall in February and April. May has been completely dry and in late May is turning hot and dry. This has led to overall near to slightly below normal rainfall so far for the year across the region.

The latest three month outlook from the Climate Prediction Center indicates a 33 to 40% chance of above normal precipitation for the months of June, July and August. May will end as it started, hot and dry.

Beginning of the Wet Season for Orlando

Using data for the period 1949-2001 to look for the median date (half earlier, half later) when minimum temperatures were in the 67-70° range (19-21° C) and dew points were in the same range one can estimate the start of Orlando's Rainy Season. These dates almost always coincide with the onset of the Rainy Season. This combination of high nighttime temperatures and high humidities (dew points) usually occurs a few days before the start of frequent showers and thunderstorms.

Based on those data the start of the Rainy Season

for Orlando is: May 27

Beginning of the Rainy Season for Daytona Beach

For Daytona Beach data from 1935-2001 showed that there were more frequent intrusions of drier air after the apparent beginning of the Wet Season. Nevertheless,

Based on those data the start of the wet season

for Daytona Beach is also May 27.

It must be noted that a purely objective analysis is not possible because the exact onset of the Wet Season is difficult to determine in some years. There were classic years when dew points and minimum temperatures rose to around 70° in mid/late May, a rainy period ensued shortly thereafter and continued through the summer.

Some years saw the start of showers/storms in late May, followed by several weeks of little/no rain, and then the onset of frequent rains once again in late June. Notably in 1998 neither occurred until late into July. It was in 1998 that much of Central Florida burned in wildfires.

This year dew points and nighttime temperatures are already high but a thermal inversion and semi-permanent high pressure have precluded any rainfall. A series of shortwave troughs are forecast to move through Florida the last week of May which should bring beneficial rains but not what we would consider rainy season rains (from sea breeze thunderstorms).

Attempts to objectively pick the date when dew points/minimum temperatures remained above 70° degrees failed, since many years had brief periods of readings in the 60°s through the month of June. This would have resulted in the median date for the onset of the Wet Season not correlating with a reasonable person's perception, and being much too late in the season.

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Precipitable Water and Precipitation Rate

Can Determine the Start of the Rainy Season

Another method for determining the start of the Wet Season is inspection of two variables, Precipitable Water and Precipitation Rate. Using the 1960-2000 time period the areal average of these parameters was attained for a one degree of latitude by one degree area that covered Cape Canaveral, Orlando and Daytona Beach. There are approximately 69 miles in 1° of latitude.

A sharp upward trend in precipitable water starts historically about May 20. Precipitation Rates rise significantly beginning about May 22. This corresponds rather well with the median dates determined by looking at minimum temperatures and dew points. The small differences in the dates determined by the two methods is likely due to the different time periods examined.

This will not happen in 2021 as a dome of high pressure builds over the Florida peninsula. Temperatures will soar into the high 90°s and skies will remain clear into the first week of June 2021.

East central Florida experiences seasons that differ from most of the remainder of the country. Rather than the four seasons of winter, spring, summer and fall, east central Florida exhibits a distinct Wet (warm) Season and Dry (cooler) Season. This duality of seasons is similar to the Monsoon or Wet-Dry climates that other regions of the world experience.

The Wet Season

The Wet Season is typically considered to begin in the latter part of May and resembles "summer" across much of the remainder of the country. Warm temperatures (mid to upper 80s) begin earlier, but usually do not coincide with the beginning of frequent summer-like rains. The primary difference between summers in east central Florida and those at higher latitudes is that the heat and humidity are relentless (i.e. there are no synoptic scale fronts that bring significant cooling and drying). Though it does not rain every day during the summer, the frequency of rainfall usually begins to increase in late May. The start of the Wet Season is occasionally delayed until June and in rare cases, even as late as early July (e.g. 1998).

The Dry Season

The Dry Season usually begins in October as the first synoptic scale cold front brings drier and slightly cooler air into the area. This first front sometimes results in a significant rain event. Tropical systems, additional fronts and gale centers can bring periods of heavy rain through November, but the frequency of rain almost always decreases after the first significant frontal passage.

Fronts continue to push through the area during the traditional "meteorological winter" months of December, January and February. Cold frontal passages during this time period will sometimes be preceded by a line of showers and thunderstorms, but the occurrence of rainfall is much less frequent than the summer. The greatest coverage of rainfall during the winter months often occurs when one of these fronts moves back northward as a warm front. Some winters have more frequent frontal passages, which can result in much above normal rainfall. The most recent occurrence was the 1997-1998 winter when a strong El Nino resulted in significant severe weather episodes and flooding across the Florida peninsula, followed by severe drought the summer of 1998.

For east central Florida, late February through March might be the time period that most closely resembles typical Spring weather in the higher latitudes. Large swings in temperatures often occur along with occasional severe weather episodes, but rainfall is usually infrequent. April is often the driest month of the year as fronts become weaker and yield less rainfall, yet manage to pass through the area and reinforce the dry and stable air mass. Temperatures warm through May with average maximum readings reaching the upper 80s by the end of the month. Rainfall frequency increases compared to April, with the most notable increase usually beginning late in the month.

Climate Classification of East Central Florida?

Hard to Determine. . .but. . .

Cw = best fit

The Glossary of Meteorology defines a monsoon climate as one in which the wind flow reverses itself during the course of the year and there is a distinct winter dry season. Data indicate that there is a duality of seasons for east central Florida, with the cooler season being significantly drier than summer. Additionally, the mean wind during most of the east central Florida Wet Season is southeast (tropical) while the mean wind during much of the Dry Season is northwest to north (continental).

The Wet Season is marked by a daily inland progression of the Atlantic and Gulf of Mexico sea breezes as a low pressure trough develops across the peninsula due to surface heating. Showers and thunderstorms occur most days along the sea breezes and become numerous when these boundaries collide. While the low pressure trough that produces this sea breeze circulation is not on the same scale as the large heat lows that drive monsoon circulations elsewhere in the world, there is enough regularity to suggest some similarity. Some climatologists refer to the daily sea breeze circulation as a "diurnal monsoon."

The classic monsoon climate, of which India is the best example, has a "hot season" prior to the onset of the rains. Though this does not occur every year in east central Florida (and hence is not climatological), the hottest temperatures of the season sometimes occur in May or June, prior to when rainfall frequency increases. In fact, the hottest month on record in Melbourne occurred in June 1998 when the area was in the midst of a prolonged dry spell. India also has a secondary maximum in temperature right after the Rainy Season. This does not occur in east central Florida since there is a strong marine influence, and temperatures are modified in September/October as the prevailing wind flow becomes more easterly.

The Koppen Climate Classification System designates north/central Florida as a Subtropical Humid climate (Cf) with a year-round distribution of rainfall. This places east central Florida in the same climate classification as Memphis, Tennessee. Clearly, Memphis and Melbourne should not have the same climate classification.

The Cw classification is a Subtropical Humid Wet-Dry climate with a monsoonal influence (dry winter). This classification requires the climatologically wettest month to have ten times more rainfall than the driest month. East central Florida does not meet this requirement since the driest month averages around 2 inches, and there is no location in the state which averages 20 inches in a month! An alternate to the 10:1 ratio is that 70% of the yearly rainfall must occur during the six warm season months. Rainfall for Daytona Beach during the months of May-October is about 64% of the yearly total, while Orlando and Melbourne were 68%, and Vero Beach was 66%.

So it can be seen that east central Florida does not meet the strict definition of a Cw climate. However, during any given year, there is most always a month that is ten times wetter than another in east central Florida, since rainfall less than one inch is quite common during at least one of the Dry Season months. In fact, examination of Orlando rainfall from 1927-2001 showed that there were only 9 years when the 10:1 ratio was not met. This is the nature of statistics in which the average or "normal" conditions mask the extremes.

The Cs classification (Mediterranean Climate) occurs where there are dry summers and the wettest winter month has at least 3 times as much rain as the driest summer month. The climatologically wettest month in east central Florida has a little greater than 3 times the rainfall of the driest month. Therefore, east central Florida also does not meet the strict definition of a Cf climate.

The southern tip of Florida is an Aw (Tropical Wet-Dry) climate. The requirement for this designation is that the coolest month averages above 64.4 degrees Fahrenheit. East central Florida does not meet this requirement. However it is interesting to note that the rainfall requirements to receive a Tropical Wet-Dry designation are far less stringent when compared to the 10:1 ratio for the Cw (Subtropical Humid Wet-Dry) classification. The requirements are only that there must be a "marked seasonal rhythm" of rainfall and at least one month must have less than 2.4 inches. Both of these rainfall requirements are met for most of east central Florida.

A rainfall formula is used within the Tropical (A) classification to determine whether the climate is Monsoon (Am) or Wet-Dry (Aw). According to this formula, rainfall in east central Florida does not meet the Monsoon classification, but it does fit that of a Tropical Wet Dry (Aw) climate. As noted above, rainfall requirements for Humid Subtropical Wet-Dry (Cw) are not achieved in east central Florida. Therefore, the Wet-Dry sub-categories of the A and C climate classifications appear to have a discrepancy with regard to the rainfall requirements.

So while it cannot be technically stated that this area has a Monsoon or a Wet-Dry climate, most years exhibit a monsoonal influence with distinct wet and dry seasons. The east central Florida climate fits more closely with the Cw classification than it does with Cf.