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| Kayaking into the sun on Deltona's Lake Theresa |
Follow Phillip
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| Lake George, one of Florida's largest lakes, is currently very difficult to navigate due to decreasing water levels. |
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.
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| The Climate Prediction Center if forecasting a wetter than normal summer for much of the country east of the Mississippi River, including Florida (green colors indicate increased chances of precipitation forecast). |
Formula for Finding the Beginning
of Rainy Season
Warm Nighttime Temperatures (67-70° F)
+
High Humidities (dew points in the 70°s)
=
Rainy Season
Normally around June 1
Rainy Season Normally ends around October 15
(see more on this below)
Water level and flow meters at the Astor station on the St. Johns River currently report that the depth is .48 feet away from the channel. The flow at Astor is virtually non-existent. At the other end of Lake George there are tidal influences from the Atlantic Ocean pushing brackish water south toward the lake twice a day.
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.
A very low Wekiva River at Katie's Landing
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.
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| A lot of underwater hazards start to appear as lake levels drop very low. |
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).
Me and my shadow, on another blazing hot May day on the shores of Lake George, Florida's 2nd largest lake.
Follow Phillip
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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| Lake George is only knee deep out hundreds of feet |
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.
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| My butterfly garden is stunted by the heat and dry in 2021. I irrigate by hand, and after dark to keep these flowers alive. |
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.
Onset of the Wet Season at Daytona Beach and Orlando, Florida(Updated through 2010)
Daytona Beach Median Date = May 29
Orlando Median Date = May 27
RAINY SEASON DATA
UPDATED THROUGH 2010 REVEAL A
SLIGHTLY LATER START DATE
FOR DAYTONA'S WET SEASON
ORLANDO'S WET SEASON BEGINS MAY 27
Orlando Median Date = May 27
2004
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6/1
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6/3
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2005
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5/30
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5/30
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2006
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6/11
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6/11
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2007
|
6/6
|
6/6
|
2008
|
6/10
|
6/9
|
2009
|
5/17
|
5/17
|
2010
|
5/30
|
5/30
|
We've Read:
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.
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| In Oakalawa, fields of coreopsis make a last stand as the long hot dry of May takes its toll. |
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.
Butterflies and skippers will return with the rains. We are currently between flights of both except on the coasts where there are plenty of coastal sunflowers in bloom.
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| A Skipper butterfly (Hesperiidae) on a Mexican Sunflower (Tithonia rotundifolia) |
Florida's Two Seasons
Florida does not experience the four seasons of Spring, Summer, Fall, and Winter as northern latitudes. There is generally an agreement that the seasons in Florida can be divided into two; the first extending from May through October, and the other from November through April. These seasons are sometimes referred to as warm and cool seasons or wet and dry seasons.
Successive years of below average rainfall have left our 700 acre lake mostly dry (below). Above looking northeast toward New Smyrna Beach in July of 2009 and below, the same image, same spot this week. Despite the storm clouds pictured here, rains have been much below average in 2016 and our lake is looking more like a forest than a wetland. It has been many years since I've had my jon boat out from its spot at the once-high water line.
Part of the problem of classifying Florida's seasons is from Florida's geographic location, which puts it squarely in a climatic transition zone. Climatology text books generally put south Florida and the Keys in the "wet and dry tropics" category and north Florida in the "humid subtropical" category, which also includes much of the southeast United States.
Central Florida Straddles Climate Regimes
Between 29° and 31° N. wet-dry seasons are more variable
As you move from extreme south Florida to extreme north Florida you transition from classic tropical wet and dry seasons to what is more aptly described as cool and warm seasons. This is because extreme north Florida is subject to more extratropical cyclone passages in the northern winter months, receiving more rainfall during that time while south Florida is experiencing its true "dry season." In addition, northern Florida's temperature difference between seasons is more noticeable compared to southern Florida. Central Florida is the transition zone between those two climate regimes.
In the image below a green lynx spider (Peucetia viridans) hangs onto sunflower top right of image.
Click on this or any other photo for a larger view.
The Florida peninsula south of about 30° north latitude clearly experiences wet and dry seasons on a consistent basis, while north of 30° experiences more variability in seasonal types from year to year. In north Florida, at the southern limit of the humid subtropical climatic region, some years could be more aptly described by wet and dry seasons and others by cool and warm seasons.
Formula for Finding the Beginning
of Rainy Season
Warm Nighttime Temperatures (67-70° F)
+
High Humidities (dew points in the 70°s)
=
Rainy Season
Normally around June 1
This year's rains have been sporadic, at best.
Read more about calculating beginning of
Florida's Rainy Season at
El Niño and Florida Rains
or lack thereof
The relationship between ENSO and rainfall is perhaps the most straightforward and statistically significant of all weather parameters leading to wet and dry season fruition in Florida. Daily thunderstorms on the sea breezes fueled by tropical moisture during Florida's wet season gradually end as the transition to the Florida dry season takes place in late October and November. This year, as a strong El Niño wanes in the Pacific Ocean, Central-South Florida, south of 29° N is experiencing predominantly hot and dry conditions when it should be wet. But maybe things are changing as the long term forecast is for wetter than normal conditions.
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| A Lubber Grasshopper (Romalea microptera) hangs onto some bamboo stakes in the garden. The lubbers are now grown and have shed their nymph bodies and black colorings. |
Occasionally a late season tropical system will bring excessive rainfall to Florida in November such as was the case with Tropical Storm Gordon in mid-November 1994. But generally, during the Florida dry season, substantial rainfall is only provided by passing extratropical disturbances in the westerlies or stalled frontal boundaries. See discussions on storminess and seasonal aspects of Florida climate for more details.
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| The current El Niño is now considered strong and may become historically strong by the time northern hemisphere winter arrives, when the El Niño should have its largest impact on Florida. |
These patterns are upended by strong El Niños. During strong EL Niños such as in 1982-83 and 1997-98, storminess is greatly increased over Florida and the Gulf of Mexico leading to well above average rainfall in winter and perhaps widespread flooding while El Niño summers can be extremely hot and dry.
The map below shows rainfall greatly exceeding normal (in the blue areas) over Florida and California for the month of February 1998 during one of the last strong El Niño events.
So based on past experience discounting climate change and its impacts Florida could expect a potentially stormy and wet dry season 2015-2016, but how much more heat and dry will we have to endure before then? It is anyone's guess until more data are available.
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