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Summer rainy season in southwest Florida set to begin this week

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South Florida residents have been waiting for the start of the summer 2014 rainy season. Well, it’s coming this week, albeit a few days behind schedule!

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According to Robert Molleda, The Warning Coordination Meteorologist at the National Weather Service (NWS) Office in Miami, FL, the median start date for the rainy season is May 20 (southeast coast) and a week later for inland and west coast locales. The season (which brings roughly 70 percent of south Florida’s annual rainfall) lasts for about 5 months (150 days or so).

Thanks to a late season cold front (which dragged in some drier air at middle altitudes), it has been hard for thunderstorm-like clouds to develop in recent days. On May 24, a few small towers popped up on the west coast sea breeze front, well inland. They were weak and short-lived (Fig. 1). Today, there were a few stronger showers and thunderstorms and the building cloud towers were noticeably taller and more developed (Fig. 2).

As the upcoming week unfolds, an upper level low-pressure system is expected to develop and park itself across the Sunshine State (Fig. 3), bringing not so sunny weather. Given growing amounts of low-level moisture, this upper level low should be the catalyst for the onset of the rainy season across south Florida. It is likely that some areas of the state will receive far more than the 2 inches of rainfall officially predicted for the week.

As is always the case with summertime showery and tropical cyclone rainfall, look for large daily, monthly, seasonal and event variations in rainfall amounts. Fig. 4 shows rainfall associated with Tropical Storm Isaac in 2012. Even though Isaac passed close to the southwest Florida coast, the heaviest rainfall was concentrated within outer feeder bands and their interaction with Florida’s east coast sea breeze front. Fig. 5 shows a 4-month summer-time rainfall variability that occurred across south Florida in 2013.

The above scenario begs the question, “what makes for a start (or end) or a south Florida rainy season?”

Actually, most everything is locally driven, just as it is in other places that have seasonal rainfall patterns (e.g., Arizona, India).

In south Florida, there are three weather-related conditions that need to be met in order for the NWS to declare the onset of the rainy season. These conditions are:

• near ground level dew point temperatures (a measure of absolute atmospheric water vapor) that remain above 70 degrees for several consecutive days.
• the onset and almost daily persistence of showers and thunderstorms over the south Florida peninsula.
• an evolution or abrupt transition to a more easterly low-level wind flow pattern (rather than one in which wind directions transition a lot from day-to-day).

As of this afternoon, May 25, 2014, the first condition has been met; the second is imminent; and the third is evolving.

So, look for a declaration that the south Florida rainy season has officially begun on or before mid-week.

© 2014 H. Michael Mogil

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