National Weather Service United States Department of Commerce
Rainy Season dates graphic

West Central and Southwest Florida is located in what is referred to as the Subtropics, located between the Temperate zone to the north and the Tropical zone just to the south. During the late spring and summer months of June through September the tropical climate shifts north into our area and this combined with the oceans surrounding the Florida peninsula and daily sea breezes leads to our Thunderstorm Season.

The National Weather Service in Tampa Bay (Ruskin) evaluated local thunderstorm science and climatology to define the rainy season in our area and to increase public awareness of the associated hazards. The rainy season in West Central Florida runs from May 25 to October 10, while across Southwest Florida (Charlotte and Lee Counties) it is from May 15 to October 15. Overall for the NWS Tampa Bay county warning area we can see in the graph below how the rainfall coverage quickly increases in June and continues into early October.
Median Daily Rainfall Coverage for NWS Tampa Bay County Warning Area

The rainy season can begin abruptly in some years and the onset can take weeks to develop in other years. As described in the image below, there are a few parameters that need to come together and setup. Therefore, the beginning of the rainy season is usually a transition period rather than a single date. The actual start of the rainy season can be before May 25, similar to the way hurricanes can form before the official start of Hurricane Season on June 1.
 
Florida Thunderstorm Season Timeline


In general the rainy season is characterized by warm, humid conditions with frequent showers and thunderstorms. The start date of the rainy season varies from year to year and is largely determined by the onset of almost daily showers and thunderstorms over the Florida peninsula, as well as late night and morning showers and thunderstorms over the waters of the eastern Gulf of Mexico.

As described in the graphic above this is typically accompanied by an increase in humidity with persistent dew point values above 70°F, daily low temperatures in the 70s to around 80°, and high temperatures in the upper 80s to middle 90s. As far as moisture aloft, we look at the precipitable water (PW) values, which in general terms is all of the moisture one could squeeze out in a vertical column of the atmosphere up to about 30,000 feet over a given location, then the amount or depth of water that you would get is the precipitable water. PW is measured in millimeters or inches and is usually calculated from the upper air soundings which are available twice a day at numerous locations across the United States and other parts of the world. The values of PW range from 0 to about 76 millimeters (or up to around 3 inches), but can vary greatly depending on the season and location. For West Central and Southwest Florida, typical values during our thunderstorm season range from around 43 to 48 mm (1.7 to 1.9 inches). When they fall to around 38 mm (1.5 inches) or less we tend to see very limited coverage.

The rainy season usually has three phases:
  1. Late May through early July is the "stormiest" part of the season. Severe weather impacts include damaging winds, waterspouts, tornadoes, excessive lightning, hail, and flooding rain.
  2. Early July through early September remains hot, humid, and wet
  3. Mid-September through early October tends to have higher rainfall variability due to potential tropical systems and early-fall cold fronts.

During the summer Thunderstorm Season, the locations and timing for favored thunderstorm development on any given day changes based on the position and strength of the subtropical ridge (mentioned above). The National Weather Service in Ruskin, FL uses 8 varying subtropical ridge wind “regimes" to aid in daily thunderstorm forecasting. For more information on the differences we see in our daily thunderstorms associated with these differing positions of the subtropical ridge (Regimes 1 - 8) visit https://www.weather.gov/tbw/ThunderstormClimatology. Here you will find the thunderstorm climatology for your Florida city, county, region, or media market.
 

Some other helpful links are listed below:

SPC Observed Soundings:
https://www.spc.noaa.gov/exper/soundings/

SPC Sounding Climatology:
https://www.spc.noaa.gov/exper/soundingclimo/

SPC Observed Surface and Upper Air Maps:
https://www.spc.noaa.gov/obswx/maps/

Sea Surface Temperature Maps:
https://www.ospo.noaa.gov/Products/ocean/sst/contour/index.html
https://weather.msfc.nasa.gov/cgi-bin/sportPublishData.pl?dataset=sst&product=sport_gomex1