Florida conjures images of pristine beaches and endless sunshine. But beneath this sunny facade lies another, more dramatic side: the summertime afternoon thunderstorm. These frequent storms are a hallmark of the Florida summer, bringing a short-lived break from the heat while showcasing the state's raw natural beauty.
A Collision of Air Masses
Florida's sea-breeze thunderstorms are born from a daily dance between the state's relentless sunshine and its surrounding bodies of water. As the sun heats the land throughout the morning, the ground warms much faster than the ocean. This temperature difference creates a pressure imbalance, with lower pressure over the hot land and higher pressure over the cooler ocean.
This pressure difference sets the stage for the sea breeze. Air from the ocean rushes inland to fill the low-pressure zone. As this air rises, it cools and condenses, to form clouds. These clouds, under the right conditions,, further grow into towering cumulonimbus clouds, the signature feature of a thunderstorm. Just where the "right conditions" setup from day to day depends heavily on the position of the subtropical ridge, a semi-permanent area of high pressure over the Atlantic Ocean that influences the direction and strength of the prevailing wind flow across the state of Florida. This area of high pressure evolves in strength and exact position through the summer season, determining where and when the summer storms are most likely to be impactful.
Beauty and the Beast
For Floridians and visitors alike, these
storms can be a source of both fascination and frustration. They frequently disrupt outdoor plans, but also provide a dramatic spectacle of nature's power. The downpours are essential to the state's water-balance, replenishing freshwater supplies that are vital for the ecosystem. The disruption to daily life and their importance to the ecological balance makes seasonal prediction of these storms paramount to decision makers and within the government and public alike.
A Look Back at Summer 2023
During the summer months of 2023, large-scale troughing frequented the eastern portions of the United States, which helped suppress the subtropical ridge to climatologically southern position much of the time. The mean position of the ridge axis (for the 3-month period of June, July, and August) resided across the southern half of the Florida peninsula.
The clockwise flow around this feature provided much of the Florida peninsula with a prevailing west to southwest wind flow regime. When it comes to who and where sees the greatest coverage of storms during the summer, this wind flow regime is the most important factor. Sea-breeze boundaries that develop and slowly migrate inland will either resist this large-scale flow and enhance the convergence along the sea-breeze, or compliment the large scale flow, effectively weakening and making more diffuse the sea-breeze boundary convergence. To put it simply, thunderstorms and coverage are enhanced where the sea-breeze's motion is opposed by the large-scale flow, and suppressed where these motions are in the same direction. A typical wind and thunderstorm pattern for the ridge position seen in much of summer 2023 is shown graphically in images 4 & 5 below.
 Image 4) Subtropical Ridge Axis over south Florida or the Florida Straits |  Image 5) Generalized Daily Precipitation Pattern over the Florida Peninsula with a more southern positioned subtropical ridge axis. |
Summer 2023: Month by Month
June 2023: The mean subtropical ridge position set up far to the south over the Florida Straits (Image 6). This position produced a typical precipitation pattern for the resulting west-southwest large scale flow across the peninsula. The east coast sea-breeze experienced enhanced convergence (more storms), while the west coast breeze migrated with the flow, easing the convergence and suppressing thunderstorms potential. Wetter areas of the peninsula were found down the spine of the peninsula and over to the I-95 corridor. Significant rainfall was also seen in the "Big Bend" region of the state, between Tallahassee and Gainesville (I-10 corridor). This secondary region of enhanced thunderstorm development comes about due to the vast displacement of the ridge axis from northern portions of the state. This displacement results in an synoptic flow over the north that is much weaker, allowing various sea-breezes and convective outflow boundaries to interact and drive convection in a less systematic/organized way..
 Image 6) June 2023 Composite MSLP |  Image 7) June 2023: Rain Departure from Normal |
July 2023: The middle of the sea-breeze storm season saw the subtropical ridge shift ever so slightly northward, but still hold a southerly potion (Image 8). The position would continue the trend of providing the majority of the Florida Peninsula with W-SW synoptic flow.
The ridge itself did become a bit more dominant (stronger pressures), however, the overall result of this slight evolution was to keep the main wet (east coast) and dry (west coast) areas across the Florida peninsula in place. The stronger ridge and slightly northward evolution did produce a setup more conducive for impactful W-SW flow up into the Florida Big Bend region. The sea-breeze boundaries up in this region now migrated steadily with the synoptic flow, suppressing overall convergence and producing a vastly drier month of July along the Gulf coast of Apalachee Bay.
 Image 8) July 2023 Composite MSLP |  Image 9) July 2023: Rain Departure from Normal |
August 2023: The subtropical ridge continued a very slow northward evolution, while also showing a weakening trend. The position of the ridge over the central to even north-central peninsula, flipped the synoptic flow for many spots, resulting in new areas from previous months now primed for the diurnal storms. The southwest Florida coast saw local sea-breeze boundaries directly opposing a now synoptic SE flow regime, and thunderstorms activity was above average for the first time that summer. Much of the Florida east coast now saw the synoptic flow either switch to match their local sea-breezes inland propagation, or flow light enough as to not have a significant influence on convergence. Either way, we saw a much drier August along much of the I-95 corridor compared to the earlier months of the season.
 Image 10) August 2023 Composite MSLP |  Image 11) August 2023: Rain Departure from Normal |
A Look Forward to Summer 2024
Utilizing an ensemble of the last 15 days of mean-sea-level forecasts from the Climate Forecast System (CFS), a fairly consistent pattern emerges for the coming months, that tends to be reversed from the prevalent pattern during the 2023 season.
The forecast for this upcoming June reveals a strong subtropical ridge, with the ridge axis set up along, and even north, of the I-10 corridor. This ridge position places the entire peninsula within a consistent east to southeast wind flow regime. Representations of this ridge position/flow and the expected favored zone for diurnal convection resulting from the synoptic flow / sea-breeze interactions are shown in image 13 and 14 respectively.
 Image 13) Subtropical Ridge Axis over south Florida or the Florida Straits |  Image 14) Generalized Daily Precipitation Pattern over the Florida Peninsula with a northern positioned subtropical ridge axis. |
Looking ahead to July, the ensemble forecast produces a ridge axis a tad further south, but still squarely on average over the north-central Florida peninsula (Image 15). While the overall southeast flow would be weaker under this pattern for the central and southern peninsula, the synoptics would continue to favor an enhanced corridor of sea-breeze convection along the west coast of the peninsula from the general Tampa Bay area and southward. Areas across the northern peninsula would see a very light synoptic flow on average, resulting in daily convection along both the east and west coast sea-breeze boundaries,. However, favored locations for convection would tend to migrate inland toward the spine of the peninsula, where convergence is maximizes, resulting from a collision of the two main sea-breezes during their opposite treks inland across the state. Representations of this ridge position/flow and the expected favored zones for diurnal convection resulting from the synoptic flow / sea-breeze interactions are shown in image 17 and 18 respectively.
 Image 15) July 2024 Composite MSLP Forecast |  Image 16) August 2024 Composite MSLP Forecast |
Finally looking ahead to August, the ensemble forecasts again migrate the main ridge axis north of the Florida peninsula (Image 16). As was the case in June, this pattern favors the entire west-coast of the Florida peninsula for enhanced thunderstorm activity, as predominant synoptic southeast/easterly flow encompasses the entire peninsula (Images 13 & 14).
 Image 17) Subtropical Ridge Axis over south Florida or the Florida Straits |  Image 18) Generalized Daily Precipitation Pattern over the Florida Peninsula with the subtropical ridge axis centered over the north-central Florida peninsula. |
A long Summer Thunderstorm Season Ahead:
The Florida summer thunderstorm season can show signs of beginning as early as May, but historically does not begin in earnest until early-mid June. Long-range forecasts, such as those presented in this blog, are broad brushstrokes, painting a general picture of expected areas favored for these thunderstorms weeks or months ahead. These predictions consider researched climatological regional weather patterns. While the forecasts represent the most likely scenario given the data presently on hand, individual daily weather can, and likely will, deviate due to unpredictable atmospheric fluctuations. One of those fluctuations will always be the influence of tropical cyclones on the region during the upcoming hurricane season. However, that complex subject is for an entirely different blog...
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