Heavy rain anticipated late week into the weekend

The Weather Prediction Center's rainfall forecast for the Big Bend and South Georgia from...
The Weather Prediction Center's rainfall forecast for the Big Bend and South Georgia from Tuesday evening through Sunday evening.(WCTV First Alert Weather via WPC/NOAA)
Published: Sep. 6, 2022 at 6:08 PM EDT|Updated: Sep. 7, 2022 at 12:50 PM EDT
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TALLAHASSEE, Fla. (WCTV) - A complex, but large-scale weather pattern will set up over the Big Bend and South Georgia late week to bring the potential of 3 to 7 inches of rainfall over five days.

Heavy rain during the end of the week into the weekend could bring a threat of flash flooding, according to the National Weather Service Forecast Office in Tallahassee.

The Setup

Slightly drier air was in place in the mid to upper levels of the atmosphere in the viewing area Tuesday, which has left the Big Bend and South Georgia with lower rain chances compared to what was observed in recent days. The higher moisture content resided just to the west of the area over the Florida Panhandle and locations north. This moisture was associated with a trough of low pressure aloft that stretched from the Great Lakes southwest to eastern Texas.

The trough aloft is forecast to become elongated and dip southeastward to the central Gulf Coast by Friday and become a closed low. There are three main ingredients that will set help set the stage for this higher chance of rain and potential high-end rainfall totals.

At least three ingredients - moisture and lift at the mid to upper levels of the atmosphere -...
At least three ingredients - moisture and lift at the mid to upper levels of the atmosphere - will contribute to the chance of 3 to 7 inches of rain over parts of the Big Bend and South Georgia into the weekend.(Charles Roop / WCTV First Alert Weather)

The first is lift from the mid to upper levels of the atmosphere. A trough in the mid levels will help create spin which, without getting too complicated, induces lift ahead of it. In the upper levels, the outward spreading of the winds will lead to good ventilation and support lift. A weak front and surface low are also forecast to develop over the region.

The lifting mechanisms are there, but what about the moisture? There is plenty of that. Straight out of the Caribbean Sea, this stacked moisture is anticipated to be above normal across the Southeast U.S. starting Thursday and into early next week.

On top of that setup, the pattern is expected to remain sort-of locked in late week and into the weekend.

How much rain

This meteorologist is siding mainly with the Weather Prediction Center’s outlook more so than the ensemble average guidance (which seemed to be under-doing it).

The Weather Prediction Center's rainfall forecast for the Big Bend and South Georgia from...
The Weather Prediction Center's rainfall forecast for the Big Bend and South Georgia from Wednesday through Monday morning.(WCTV First Alert Weather via WPC/NOAA)

It does coincide with the European guidance model, but the American GFS throttled back with the rainfall totals as of the Wednesday morning runs. Based on the setup and the duration of the pattern, totals of 1 to 3 inches in South Georgia through Sunday evening and 3 to 7 inches for most of the Big Bend is more realistic. Locations closer to the Gulf Coast will likely see the highest amount of rainfall.

The Impacts

The Weather Prediction Center is expecting a 5% (marginal) chance of isolated flash flooding Thursday across the entire area, but the odds increase to 15% (slight) on Friday throughout the area and the Southeast U.S.

The Weather Prediction Center placed the Big Bend and South Georgia under a 15% risk of...
The Weather Prediction Center placed the Big Bend and South Georgia under a 15% risk of possible scattered flash flooding for Friday.(WCTV First Alert Weather via WPC/NOAA)

There were a few rivers worth watching as the rain moves in starting Thursday. The Aucilla River at Lamont was in “action stage” at around 52 feet as of Tuesday afternoon while the St. Marks river at Newport (just north of St. Marks) was just under “action stage” at just under 7 feet, according to river data from the National Weather Service. The Econfina River near Perry was almost at “action stage” as of Tuesday afternoon.

Flooding of roadways, low-lying areas, and places that flood easily will be a possibility as the rainy pattern sits in one location for an extended period of time. Those in location near rivers that are slightly elevated need to monitor the weather and river levels over the next several days.

When will the rain end?

Longer-range guidance - including ensemble averages - hints at the potential of drier air moving into parts or most of the area next Tuesday or Wednesday. Given that boundaries have a harder time moving south of the area in early September as well as the events in question are a week away, there is little confidence in the timing of the pattern breaking apart and a return to some normal pattern.

Summary

Rain odds will increase starting Thursday as a large-scale weather system is forecast to set up over the Southeast U.S. The viewing area could see rainfall amounts between 2 to 7 inches through Sunday evening with the highest rain amounts likely along the coast. Locally higher rainfall amounts are also possible. The rainy pattern will likely last into early next week.

The forecast is subject to modifications as newer data arrive; therefore, keep checking back this week and during the weekend. Stay aware of any weather advisories and warnings that could be issued by the National Weather Service.

This story was updated around noon Wednesday with new information on the forecast rainfall amounts, the risk of flash flooding, and differences with the model guidance.