The Florida Rain Machine

I have heard many people say that they can set their watch by the time it rains every summer afternoon in Florida.  While this is an exaggeration, the daily and seasonal rainfall in east central Florida do have a somewhat rhythmical nature.

The timing and distribution of daily showers and lightning storms during the summer months is most closely tied to the north/south migration of the "Bermuda" high pressure ridge.  The position of this ridge controls the low level wind flow and dictates where the Gulf and Atlantic sea breezes collide each day.  This collision often produces widespread storms with frequent lightning and heavy rainfall.  I have heard this dynamic feature referred to as the "Florida Rain Machine."  The regularity with which it occurs is also the main reason that the inland area between Tampa and Cape Canaveral is often called the "Lightning Capital of the United States."

While you can usually count on the afternoon cloud buildups during the summer months, it's a different story during the cooler season in east central Florida.  Rainfall is much less frequent, sometimes only occurring a few times during a whole month.  This seasonal change is really quite simple to explain.  Ask any resident of the state and they will certainly know that the humidity is much lower during the cool season.  This lack of moisture is reflected in the virtual absence of the "Florida Rain Machine."  Rainfall during this time of the year is primarily associated with cold and warm fronts that oscillate across the area.

The Rainy Season.
So when does this "Rainy Season" begin?  Everyone who has lived here or visited here can tell you that it is hot and humid during the summer with frequent showers and lightning storms.  Many will be able to tell you that June through September are the wettest months.

In a classic start to the rainy season, there would be a noticeable return of humidity during the latter part of May.  I can remember a few of these years when my first reaction to going outdoors was "yuck."  Once this higher humidity (dew points 70 degrees or higher) arrived, there would usually be an increase of showers and lightning storms within a few days.

Of course there are years when the rainy season onset occurs in early May, or holds off until well into the month of June.  The highly changeable year of 1998 saw the onset of the true rainy season during the first week of July.  When you eliminate the extremes and concentrate on trying to find a "typical" year, the third or fourth week of May stands out as the most common start for the rainy season.

The Dry Season.
When does the "Dry (or cooler) Season begin?  First of all, in order for the frequency of rainfall to decrease, the humidity must decrease.   This occurs when a cold front passes through and brings with it some drier air.  Many people will recognize this as the first time that they can turn off their air conditioner and open the windows.  In most years, this happens by the middle of October.

However, just as the onset of the Rainy Season can be quite fickle, the same can be said for the start of our Dry Season.  In many years, the first cool fronts will be short-lived and moisture will return after several dry days.  This might lead to a temporary restart of the "Florida Rain Machine."  Tropical systems can also result in significant amounts of rainfall throughout October and into November.

So if we get a nice refreshing air mass into the area on the first of October, be wary, because you will usually need those air conditioners again.   However, just as winter cold snaps usually last only a few days in the Florida peninsula, the return of humidity during the Dry Season is intermittent and not long lasting.

What the Data Shows.
Looking at more than 50 years of temperature and rainfall data from the Daytona Beach and Orlando airports, the median date for the start of the Wet and Dry Seasons was determined.  Note that the "median" is simply "the middle of the road."  Half of the data is before and half of it is after.

The median date for the beginning of the Rainy Season in Daytona Beach is May 27, while in Orlando it is May 24.

The median date for the beginning of the Dry Season is October 15 for both Daytona Beach and Orlando.

There was not enough data available for this study to determine the onset of the wet and dry seasons in Melbourne, Vero Beach, Fort Pierce and Stuart.  However a study done by the National Weather Service Office in Miami found that the average rainy season in southeast Florida extends from May 21 to October 16.   This allows for an estimation of the start of the wet and dry seasons over the southern half of east central Florida.

The approximate start of the wet season in Melbourne and Vero Beach is May 23, while the start of the dry season is October 15.

The approximate start of the wet season in Fort Pierce and Stuart is May 22, while the start of the dry season is October 16.

More detailed information about this study can be found at:  www.srh.noaa.gov/mlb/?n=wetdryseason

Randy Lascody
Senior Forecaster
Randy.Lascody@noaa.gov
 
posted May, 2002 


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