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Week of Jun 5-11, 2006

A look back on this year’s low-water mark.

Surface water systems in south Florida typically hit their low-water mark in mid May. It varies from year to year depending on how much spring rainfall we get, and if and when the summer rains get started. In 2003 for example over 10 inches of rain fell in late April and May leading to an absence of a deep or prolonged dry down. In 2000 a relative lack of May and June rainfall in the Big Cypress extended the drydown well into June. With the past couple of weeks chiming in at about 1.5 inches per week, followed by the sideswiping of TS Alberto*, surface water levels appear to have bottommed out and have begun their rise back up toward summer conditions.

Here’s a quick look back on where this year’s low-water mark fell in each watershed.
Lake Okeechobee stage continues to drop and appears not yet to have reached its low-water mark. As of Sunday Lake stage dropped to 12.37 ft above sea level. That’s a little over a foot below the bottom of the Lake’s littoral zone and almost 3 ft below where Lake stage was this time last year. Inflows from the Kissimmee River are still less than 500 cfs and from what I can see the Kissimmee watershed escaped the bulk of the rain from TS Alberto.

In Loxahatchee (WCA1) surface water levels bottomed out this year at a level where sloughs still remained flooded with water. This contrasts to WCA2 where sloughs went dry as early as late March and April (and remain dry as I type). Water levels in the southern part of WCA3A remained flooded at or above the ridge landscape type during this year’s low-water mark in that area. In comparision, slough surface water levels in the northern part of 3A went dry in the early part of May and still remain dry at Site 63 as I type. Slough surface water levels in northeast Everglades National Park (NE Shark River Slough) also went in the early part of May and remain just below ground surface as I type.

In Big Cypress NP, surface water levels dropped below our lowest lying wetland type (swamp forest) in early April, and as of today most of the preserve’s stations are still reporting water levels at or below the swamp forest wetland type. However, the southern part of the preserve seems to be filling up first with the onset of the summer rains. Most of our stations south of US41 are currently showing water levels at or above the swamp forest wetland type.

In Everglades NP, water levels have risen to the slough level in most areas. Note that currently there are no flows entering the park from its northern boundary — The S12s are all closed, inflows from the US41 culverts between S333 and S334 (ie, NE SRS) are negligible, and inflows to the western arm of the park from the Big Cypress are also still neglible. The ENP watershed summary is still a work in progress which I hope to refine with Kevin Kotun and other ENP staff over the next couple of weeks.


* This week’s update does not include most of the rainfall from TS Alberto which brushed up against southwest Florida starting Sunday through Monday.

Week of May 22-28, 2005

It appears that the wet season has begun. Night-time temperatures have stayed above 70 degrees for the past week. This trend has been accompanied by a greater abundance of cloud cover in the afternoon and re-emergence of cumulonimbus clouds anvilling out at the top of the troposphere (bottom of stratasphere). So it looks like the south Florida summer rain machine is back up and running.

Typically we average somewhere around 2 inches of rain per week (or about 8-9 inches per month) during the wet season. However, the wet season never drops its rain uniformly throughout the summer, and each year the wet season seems to unfold in a slightly different pattern.

Last year we started the rainy season fast out of the gate with 20 inches in June. In the Big Cypress Basin, June is our most abundant rainfall month, averaging 10.6 inches of rain in June over the past 10 years. June consistently gets the most rain of any month because there is still a large amount of instability in the air from springtime fronts swooping down from the north and funnelling in moisture from the south.

Somewhere in July a less active second phase of the wet season tends to take control. July is our least abundant summer rainfall month, averaging 7.4 inches over the past 10 years in the Big Cypress. This dip in rainall is caused by the presence of more homogeneous atmosperic conditions up north, which tends to bring more stability and less rain to the south Florida weather machine. Other factors such as the position of the Bermuda high (which steers the directionality of the easterly trade winds blowing across the peninsula) and upper atmospheric warming are also at play.

As the summer progresses and sea breezes from both coasts collide with increasing severity in the middle of the peninsula, the localized convectional showers of the early wet season season give way to convergence fronts that form immense cells of storm clouds that can drop heavy amounts of rain over large areas, hitting the preserve first then moving on towards Naples. These convergence storm fronts mark the third phase of the wet season. (These are the really impressive, and often very scary storms). Some old-timers consider the onset of the convergence storms to be the start of the rainy season, even though these things don’t start up in earnest until late July or August. The months of August and September have averaged 8.3 and 8.9 inches of rain, respectively, over the past 10-years for the Big Cypress.

The fourth and final hurricane phase of the wet season tends to become a factor from August through October. South Florida seems to get the heaviest drenching from tropical storm activity (waves, tropical storms, and hurricanes) in September and October, but these events are hit and miss and more difficult to predict than the first three phases of the wet season. The 1998 wet season seemed to be over by mid October only to be hit by almost a foot of rain by Hurricane Mitch at the start of November. The same thing happened last year with Wilma.

Let’s all hope for a less eventful fourth phase of the wet season this year.