Celestial summer done

Summer ends, but rainy season still not over
Week of September 17-23

Onset of Fall Temperatures

As we enter the first days of celestial Fall (which officially started on September 21st), it begs the question of whether Fall exists in Florida, and if so, when does it start. (Florida temperature comparison)

Up north, in Baltimore for example, daytime highs have dropped into the 70s (although a spell of Indian Summer recently ratcheted temperatures back into the 80s) and night-time lows have been averaging in the 50s (and even dropped into the 40s). (Baltimore Air temperature)

How does this compare to Naples. (Naples air temperature) The shortening of daylight hours has contributed to a cooling effect, especially in the mornings and evenings. But daytime highs are still rising into the low 90s, and night-time lows are still hovering at a balmy 75 degrees.

Winsberg (Florida Weather) defines the onset of Fall in Florida as the first week in which night-time temperatures drop below 60° F consecutive days. That hasn’t happened yet in Naples. Nor has it happened in Tallahassee. Tallahasee’s coldest night to date has been 64° F, but have more typically been hanging around in the 70° F range. (Tallahasee air temperature)

Using Winsberg’s criteria, last year Fall arrived in Naples during the final week of October. In Tallahassee, closer to the continental air masses, Fall arrived at the start of October last year. If you remember from last year, even after the onset of Fall, it proved to be a mild fall and winter overall. Despit quite cold Thanksgiving in Naples, seemingly beckoning a cold winter to come, December and January that followed were mild in comparison, with it only becoming cold (by Naples standards) in February.
Onset of Winter Dry Season

The onset of the meteorological Dry Season typically occurs in the first part of October. That’s when the regular occurrence of summer rain shows comes to an end, and is usually signalled by a drop in the night-time low temperature below the 70° F mark.

October is still considered a shoulder-season Wet Season month (Big Cypress NP averages 4 inches of October rain in comparison to 2 inches per month in the Dry Season months and 9 inches during the core summer months) because tropical storms can still bring significant preciptation. In 2005, the Dry Season clock got a late start due to Wilma’s late October passing. In comparison, 2006’s Dry Season clock started ticking in early October due to the conspicuous late-season absence of tropical storm systems.

This Year’s Wet Season

I tabulate Wet Season rain as running from the start of May to the end of October. June, July, August, and September are the core rainy season months — averaging around 8 inches per month District-wide — and May and October bookend the rainy season with a 3-4 inch monthly total. (District-wide rainfall history)

The Wet Season started very slow, but is ending on a strong note in Naples and Ft Myers. September has already recorded more precipitation than either June, July, or August did — and we still have a week to go before September ends. The southwest Coast (roughly the area from Naples to Ft Myers) has only received 31 inches of rain since the start of May. Last year around 42 inches fell through the same period. (Southwest Coast rainfall history)

Lake Okeechobee has only received around 20 inches of rain since the start of May. That’s the same amount of rain it received last year, in comparison to a 30 inch 5-yr Wet Season average over the past 5 years. (Lake O rainfall history) That deficit of Wet Season rain matches the District-wide rainfall aggregate. This 30 inches (to date) of Wet Season rain is similar to last year’s subpar 32 Wet Season inches. In the previous few years from 2001 to 2005 Wet Season rainfall totals were chiming in at around 40 inches.

This Wet Season has not been uniformly dry. Miami-Dade was hammered with 13 inches of June rainfall, but only received a paltry 3 inches in August. July rainfalls in the Kissimmee and East Caloosahatchee both totalled out near 10 inches, but were followed by under 5 inches in August. On the flip side of the coin, the monthly rainfall totals in the Southwest Coast had yet to tip above 7 inches in June, July, or August — but the regular arrival of afternoon showers so far this September has filled its rain buckets with nearly 8 inches of rain with a week yet to go before the month. (Basin by basin rainfall overview for south Florida)

Watershed Report

How has this affected our watersheds. The southern half of Big Cypress National Preserve is one of the few spots where normal Wet Season conditions have prevailed. Currently water levels in the southern half of the preserve are touching up into the mesic pineland habitat. When the wetting front reaches that high it means that water depths in the cypress domes and strands are close to 2 ft deep.

Water levels have been tracking about a half foot lower in the northern half of the preserve due to less rainfall. What’s so surprising about the wetter conditions to the south is that its been achieved without benefit from flows from the north. U.S. Geological Survey’s flow monitoring shows a combined discharge of only 500 cfs under the Trail’s bridges, in comparison to a 4,000 cfs discharge last September, and an 5-yr average September discharge of 3,000 cfs.

Those discharges are the only freshwater inflows feeding into Everglades National Park. The primary structural inflow point into the Park, the S12s, remain closed and flowless, as they have been all summer to date. This has been the driest wet season at Shark Valley Tram tower since the consecutive dry summers of 1989 and 1990. Wetland water depths around the tram are only around a half foot deep (in comparison to 1.5 ft deep in Sep 2006 and 2 ft deep in Sep 2005) and if you climb to the top of the tower and look east towards the center of Shark River Slough, water depth is over a foot lower than the 5-yr September average, placing the Park at a 17-year September low water record.

Upstream in Water Conservation Area 3A is also at a 17-yr September low-water mark. Regulatory stage in 3A finally rose a few inches after almost 7 straight weeks of flat-lining at around 9.1 ft mean sea level. That still places this year stage around 2 ft below the September average and last year’s late September level. Slough water depths in southern 3A are currently around 1.75 ft deep in comparison to being over 3 ft deep for the past 6 consecutive Septembers, and even during the drought-affected September of 2000, late September water depths in southern 3A rose above 2.5 ft deep. Water depths in northern 3A (north of I75 near the S11s) are only 0.5 ft deep in comparison.

Slough water depths in Loxahatchee are just around a foot deep. That’s about normal for September, but about a half foot shallower than September of last year.

At under 9.8 ft msl, Lake Okeechobee is at its record September low-water mark. You have to go all the way back in time to 1982 to find a Summer when Lake stage stayed below 10 ft msl into August. Lake stage eventually rose above 11 ft msl by October of that year.

In Conclusion

As with all our inland water bodies and wetlands, the severity of the upcoming Dry Season drydown to come is still too early to call. We still have Wet Season left on the clock, and October rains can be bountiful — as they were in 1995, 1999, and 2005. What I can say is that Lake O stage is 3 ft lower than late September of last year, and 3A is 2 ft lower than late September of last year. That makes the end of September and any rain wet can muster in October extra important.
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