May switch-a-roo

May ends on dry note
SFL Weekly Watersheds Summary: May 21 – 27

WEATHER. A few weeks ago I commented that the month of May is interesting because its 31 days straddle the meteorological transition from dry to wet season conditions. While 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 May averages only 30 percent of the monthly total, with bigger brunt (70 percent) of rain falling in the month’s final half. From a historical standpoint, that makes May sort of like the rain year in miniature — the beginning of May representing the dry season (~30 percent of yearly rainfall), and the end of May representing the wet season (~70 percent of yearly rainfall).

That comparison was more metaphor than clock-work fact. This May the table has been somewhat reversed. Other than Miami’s lower east coast, the last third of the month has been fairly rainless for the rest of the Kissimmee-Okeechobee-Everglades watersheds; and the bulk of rain has fallen in the month’s first half. Don’t forget that the lion’s share of this months rain has fallen on the peninsula’s very southern end where it can’t replenish any other part of the system. Down south Miami-Dade has received 4 inches of May rain in comparison to a paltry 0.7 inches farther upstream in the Upper Kissimmee.

The cause? A high-pressure cell hanging off the Carolina’s has been whipping a strong easterly breeze across the peninsula, not only stoking fire activity, but also cutting off currents of Caribbean tropical air from greasing our rain machine. Meteorologists are predicting an end to that high-pressure cell in the upcoming days, which may bode well for re-emergence of rain showers just in time for June. I noticed that night-time lows in Naples for the past week have dipped back below the critical 70° F mark, presumably as a result of the easterly winds blowing clockwise around the Carolinian High. District-wide, we’ve only totaled 2.4 inches in May. That puts the District about an inch below its 3.5 inch long-term May average.

The really critical month of June hangs in the balance, and means that a short-term end to the drought is in sight. District-wide June has averaged 8.5 inches of rain over the past 10 years, ranking it as the highest rainfall month of the year. July, September, and August have average 7.2, 7.8, and 8.1 inches in comparison. This means that all eye’s will be on June as a make or break rainfall month for putting an end to the multi-season 18-month drought that has slowly tightened its grip on the region. The District received only 6 inches of June rain last year (2006), in comparison to 15 inches of District-wide rain in June of 2005.

The prospect for breaking clear out of the long-term drought mode will be a wait-and-see game, especially with the Lake poised to drop into new low-water territory in the upcoming days.
Keep in mind that this year’s historic drought has been the result of a sustained 18-month deficit of rain that can trace its roots to almost the day after Wilma passed across the peninsula (in October 2005). The drought picked up stream when last year’s rainy season ended early. That gave us a 4-5 week head start right out of the gate by merit of the rainy season fizzling to an early end in late September. At roughly the same time, a burgeoning El Nino raised just enough higher altitude westerly wind sheer to repress tropical storms from forming. That was the good news. And caused a mid-summer reversal to the tropical season outlook. That was a welcome surprise for Florida residents, but proved to be a double edged sword for water planners who had strategically lowered the Lake in Spring of 2006 in preparation for the early-season outlook of yet another hyper-active hurricane season, (not to metion all the storm-strewn leaf litter and branches from active hurricane season’s in 2004 and 2005 that have stoked this year’s dry-season blazes).

The bad news, of course, was that the same El Nino died on the vine by mid winter — and never brought the above-average winter season rains that could have reversed the southern peninsula’s 18-month drying trend. That brings us to our current condition. Forecasters are calling for an especially active hurricane season this year (producing up to 17 tropical storms and hurricanes), and possible formation of a La Nina condition that, if it persists into the winter, could also result in below normal dry-season rain totals. As usual, its all educated speculation until it happens. So stay tuned.

BIG CYPRESS. When will the preserve’s wet prairies reflood with water? Probably sometime by June or early July. Big Cypress National Preserve was a little wetter than the rest of the District for the month of May, at 2.85 inches of rain. (The preserve’s long-term May average is 4 inches.) Preserve-wide stage is currently hovering in the shallow aquifer about a foot below land surface of our deepest wetlands. That put us right at the same level as the long-term average for late May, and also at the same level as late May of last year. That means that water stage will have to rise another foot to rewet our deepest wetland communities (swamp forest and marshes), and rise a full two feet to start re-wetting our herbaceous marl prairies. Last year it wasn’t until the very end of June that the water table rose into our wet prairies. The year before in 2005, prairies re-wetted a few weeks earlier in early June, and rapidly rose into the mesic pines at about the same time.

EVERGLADES. Regulatory stage in WCA3A is about 10 inches below late May of last year, and about a foot below the 5-year average for late May. Slough water depths in southern 3A have dropped to a 1 ft depth. In comparison, slough water depths in northern 3A (north of I75) haven’t been that deep for 6 months (since December 2006), and have been dry since for the past 2 months (since mid March). Down in the Park, late May marks the traditional low-water mark for its sloughs. (The late-Summer high-water mark typically raises waters ~1.5 ft above late-May low). Stage in Shark River Slough at P33 is currently tracking just an inch or two below the 5-year late May average and last year’s late May level. Regulatory water levels in WCA1 and WCA2 are tracking 0.5 and 1 ft, respectively, below their 5-year late May average.

LAKE O. In summary, water levels in the preserve and park at their annual low-water marks, which means that conditions are dry, but from a hydrologic standpoint the drydown has not been severely low or prolonged. That coincides with both areas having recently received more rain in the area south of US41. The brunt of the rain deficit has been in the District’s northern half. Lake Okeechobee is currently tracking 3.7 ft below late May of last year, and 4.2 ft below its 5-year late May average. Such low lake levels have allowed for removal of physical removal of phosphorus-rich much at 6 sites along around 15 miles of the lake’s shoreline, and also led to a 2000 acre fire in the lake’s interior marsh. For a 19-month consecutive period from September 2004 to April 2006, Lake stage stayed above 14 ft msl, followed by 14 consecutive months (and running) of being under 14 ft msl. Its been feast or famine with the Lake since the turn of the new millenium: three consecutive years (2003-2005) with >17 ft msl lake stages and 2 season with historic plunges below 9 ft msl.

CORRECTION FROM TWO WEEKS AGO: I errorously wrote that “(f)orecasters are calling for an especially active hurricane season this year (producing up to 17 tropical storms and hurricanes), and formation of a La Nina condition that could stiffle summer rains.” La Nina condition, if it persists, into the winter could result in below normal dry-season rain totals; and has no relation to summer rain amounts.

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