Calendar-date onset of dry season begins …
SFL Weekly Watersheds Summary: Oct 30 – Nov 5

RAINFALL.

Rainfall in south Florida occurs during relatively distinct wet and dry seasons. The late Spring to early Fall wet season accounts for about 75 percent of the annual total, after which relatively rainless conditions prevail through the late Fall, Winter, and early Spring months. Unlike the seasons that are carved up by calendar date, determining the exact start and end of the wet and dry seasons are a little more loosely defined, … or at least there’s a couple of different ways to look at it depending on one’s perspective.
Meteorologically, the dividing line has become fairly precise in recent times with the the dew point (which coorelates roughly with the night-time low) being used as a guide. The start and end of the wet season coincides with occurrence of the night-time low consistently rising above and falling below 70 degrees for a few consecutive days. This year the wet season started in late May and ended in early October using that criteria.

Tropically speaking, the hurricane season spans a 6-month period, from Jun 1 to Nov 30. This means that the wet season started a few weeks before the official start of the wet season and that the meteorologically-determined wet season ended around seven weeks before the official end of the hurricane season. According to Wiseberg (2003), the earliest hurricane to hit Florida (the panhandle) occurred on June 9, 1966 and the latest to hit occurred on December 2, 1925 in Sarasota. Both were weak storms, since the most powerful storms are generated when off-shore water temperatures are highest in the later part of the summer and early fall, … but any given storm can bring with it bountiful rains, independent of strength. That’s what happened in early November 1998 when a slow moving and weakly organized Mitch passed across south Florida and dropped over a foot of rain in the preserve. There had been only 35 inches of wet season rain in the preserve up to that point, which was a low wet season total. So Mitch filled up the swamps at the end of a dry summer.

This is where we run into some discrepancies with a consistent calendar-year division of wet and dry seasons. I divide the year into equal 6 month long wet and dry season; with the wet season spanning from May 1 to Oct 31 and the dry season spanning from Nov 1 to Apr 30. … You can see where Mitch got included as a dry season rainfall event in this calendar-year categorization. … But from a number crunching perspective this simplifies the math, and also makes for more consistent historical comparisons. For most of the KOE system, May and October are transition months that I call “shoulder” wet season months. When you look at them on a monthly graph, both May and October “bookend” the onset and demise of the wet season with about half as much rainfall as the core summer months (which average aroun 8 inches per month). In comparison, 6 dry season months from November to April average about 2 inches per month.

Suffice it to say that there is a definitive wet and dry season in south Florida, … from a calendar book-keeping perspective this year’s dry season started a week ago, from a meteorological perspective it started over 4 weeks ago, and in terms of the official hurricane season we still have another 3 weeks before its done. The good news on the latter is that the burgeoning El Nino condition put an early end to this year’s tropical activity, … but it also brings with it a chance that we’ll see above-average winter rainfall as a result of more frontal storms. … So maybe this year’s dry wet season will be followed by a wet dry season*.

* Interestingly, going back to the year of Mitch in 1998, over 20 inches of rain fell in the preserve during the previous-year’s dry season (6 months of November 1997 to April 1998) as a result of a high El Nino signal that winter (much higher than today’s El Nino). To put that in perspective, the Lake and Kissimmee Valley recorded only around 25 inches of rain during this year’s wet season. … So sometimes a dry wet season can be the same as a wet dry season, … but that’s definitely the exception, not the rule.

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