May is the microcosm of the rain year.
SFL Weekly Watersheds Summary: Apr 30 – May 6

WEATHER. The start of May is a momentus time in south Florida. It marks the beginning of a new rain year! From a historical book-keeping point of view, I have found it convenient to divide the rain year into a 6-month rainy season (May 1 to Oct 31) and a 6-month dry season (Nov 1 to Apr 30). To be sure, there are other ways of defining the wet season’s start and end. For example, the official meteorologic start of the wet-season doesn’t occur until the later part of May when the dew point (night-time low) consistently breaks above 70°. That’s a tried-and-true sign that there’s finally enough moisture in the atmosphere to fuel afternoon thunderstorm activity. The rainy season doesn’t officially end until the dew point consistently drops back below 70° in the early Fall. That occurred last year on October 6th. But for the sake of simplifying the math, its easiest just to include all of May and all of October as wet season months.

Here’s a quick run-down of the numbers for the “May 2006 to Apr 2007” rain year. Big Cypress NP chimed in at 52 inches of rain for that 365-day period, 45 inches for Water Conservation Area 3, 51 inches for Miami-Dade, 50 inches for the SW Coast, 28 inches for Lake O, and 34 inches for the Kissimmee Valley. Just looking at our most recent 6-month dry season, Big Cypress NP received 9 inches (about 3 inches below its long-term dry season average), Miami-Dade received a whopping 14.5 inches of dry season rain (~2 inches above average), the SW Coast received 6 inches (~6 inches below average), Lake O received 5.5 inches (~6 inches below average), and the Kissimmee Valley received 9 inches (~4 inches below average).

May is interesting not only because it marks the start of a new rain year, but more importantly because its 31 days straddle the meteorological transition from dry to wet season conditions. At the tail end of the 6-month dry season (make it 7-months for this year because of a dry October), the start of May typically coincides with the driest part of the year, when our watersheds are bottoming out at their low-water marks and searing daytime heating dessicates the already parched landscape of our dense wetland vegetation. That’s the tinderbox that fuels the wild fires. These dry conditions can persist for the entire month of May and even push into June, as occurred in 2000 and 2004, when a “dry-season-esque” less than 2 inches of rain fell during May of those years. In May 2003 a bumper crop of 8.5 inches fell in May for the Big Cypress — which set the stage for what turned out to be a wet rainy season for that year.

The long-term average for May is 4 inches of rain for Big Cypress National Preserve, and 3.4 inches District-wide. The first half of the month averages only 30 percent of the monthly total, with 70 percent falling in the final half of the month. In that way, May is sort of like the rain year in miniature — the first half representing the dry season (~30 percent of yearly rainfall), and the final half the wet season (~70 percent of yearly rainfall).

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