During the rainy season in SWFL, it is all about the timing of the storms that determines how you plan your outdoor activities. As weather forecasters, we look at a number of things to determine the timing and severity of these storms that form each day during the summer. Everything from total atmospheric moisture, to the direction of the upper-level winds to the temperatures of the atmosphere several thousand feet up...all of it is important in figuring out when storms will occur.
The most important factor is the direction of the upper-level winds. We use the winds between the mean wind between 700 mb and 500 mb pressure levels in the atmosphere or between 10,000 and 15,000 ft. to determine this. The winds at these levels are what steers the storms in any given direction. A westerly wind flow will move the storms from the coast to inland areas by afternoon. The opposite is true with an easterly wind, where storms will start on the east coast of Florida and work their way westward during the afternoon. The speed of the wind along with the direction at the surface is also particularly important. A stronger wind will tend to either keep the sea breeze from moving inland or will push it well inland fairly quickly. There are some days where the wind flow is strong enough to keep storms offshore, leaving the entire area dry.
The feature that controls the wind direction is the position of the Bermuda High which is centered offshore in the western Atlantic. Depending on where the high resides, determines what direction our winds flow across the peninsula. A high pressure position farther north over Bermuda will tend to bring us east to east-southeast wind flow. In contrast, if a cold front and trough of low pressure dives down into the eastern US and pushes the high south into the Caribbean, the clockwise flow around the high will lead to onshore westerly winds in South Florida. The digging trough will also provide the westerly winds aloft.
There is a lot that goes into forecasting daily sea breeze storms. It isn't as simple in the summertime as saying, "It will be partly cloudy with a few afternoon and evening storms". Sometimes those storms will occur in the morning, and sometimes most of them will be in the afternoon. It is the upper-level wind flow that ultimately determines this.
FOX 4 CHIEF METEOROLOGIST DEREK BEASLEY
