Has dry season begun?

Driest week since May: Is the Dry Season here?
Week of Oct 8 – 14

From a meterological standpoint, we’re at the fuzzy in-between ground between the final gasps of the Wet Season and an early-out-of-the-gate head start into the Dry Season.

This past week was definitely dry. South Florida wide, it was the driest week we’ve had since May. By that metric then — yes, we’ve entered Dry Season territory — but we still have Wet Season to go on the calendar in front of us.

The S12s deliver water into downstream Everglades National Park. S12A is the westernmost of the S12 gates, at the confluence of the Big Cypress and Everglades watersheds. This year S12A has been closed for most of the summer, but is currently discharging at 15 cfs in comparison to 400 cfs last October and +700 cfs in October 2005. Click here to view historical calendar of discharge through the S12A. Photos by Paul Murphy and Robert Sobczak.

Calendar-wise, I’ve always lumped October as part of the rainy half of the year — from May 1 to October 31. Over the past 5 years, South Florida has received 75 percent of its rain (41 inches) during that 6 month period. The other 6 months, from Nov 1 to Apr 30 account for the other 12 inches, for a yearly total of around 53 inches.

From a meterological standpoint — as the wind blows — we tend think of May and October as more Dry Season than Wet Season. Most of May is a continuation of spring’s cloudless skies, but the Summer rain machine typically starts revving up towards the end of May.

And we typically associate October with arrival of lower humidity, and weather gods willing, our first cold front. But besides the Summer Rain machine making its last stand before packing its bags for the Dry Season, we can also get a surprise thunderstorm from a continental cold front, and of course there is always the looming threat of a curveball from the tropics.

October accounts for 20 percent of historical hurricanes to make a Florida landfall. These events, and tropical storms of lesser magnitude just as much, can be significant rain makers, but also makes October a hit-or-miss rainfall month. Octobers of 2005, 1999, and 1995 were hits — all three averaged 7-10 inches for the month. Last year’s October was a miss — with only an inch of rain — as were 2004, 2003, and 2001.

Both months of May and October average around half the rain of the core months of the Summer rainy season (Jun – Sep), but also receive around twice as much rain as the core winter months. That makes May and October bookend months, and also straddle months — each with one foot in the Dry Season and the other in the Wet.

Where does this October stand? So far its leaning on its Dry Season foot. But we’re still only at the month’s midpoint, and that can change with one big rainfall event. (District rainfall overview)

Bottom line: the tropics don’t mothball their operation until Nov 30th. A disorganized Mitch dropped a foot of rain on Big Cypress National Preserve in November 1998. Over the past 100 years, only 4 hurricane strength storms have made a Florida landfall after Nov 1. The most recent being Kate at Port St Joe as a Category 1 in 1985.

Any discussion of the upcoming Dry Season would be remiss without brief mention of the state of the ENSO index. For the past few months its been swinging toward the La Nina phase — the twin sister of El Nino that signals cooling of the shallow coastal waters off the Peruvian Coasts, and portends higher potential for a dryer dry season than normal for south Florida.

ENSO’s swing in the opposite direction — to the El Nino phase — amplifies the potential for above-average winter rainfall. That’s what happened in 1983 and 1998 when El Nino-stoked winter rains pushed Lake Okeechobee to its annual high-water mark in March — both years at right around 18 ft above mean sea level. Typically Florida’s inland water bodies typically crest in early Fall (this time of year).

The Lake seems to have peaked this year at right around 10 ft msl. That is low by historical standards for any time of year, let alone early Fall. That puts this year’s current Lake stage at 8 ft below October of 2004 — the year of Francis and Jeanne — and 4 ft lower than October 2001.
Don’t forget that Lake stage was proactively lowered in Winter and Spring 2006 in anticipation of an active hurricane season that failed to materialize — in part because of the El Nino which emerged half way through Summer 2006. That was a welcome relief to hurricane weary Florida residents, but was also a contributing cause that accummulated into this year’s record low. Now ENSO’s swing back to the La Nina phase may portend more dry weather for the Lake through the next half year, but don’t forget in the short term that the same La Nina will keep Tropics brewing up storms that may steer our way later than usual this year, even into November. (Lake O watershed summary)

As usual, its a wait and see fuzzy picture of the future with a keen 20/20 vision in retrospect once it happens.

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