Area Forecast Discussion
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FXUS62 KTAE 171900

Area Forecast Discussion
National Weather Service Tallahassee FL
300 PM EDT Fri Aug 17 2018

.NEAR TERM [Through Tonight]...

A subtle upper level trough has slowly moved east this afternoon, on
the western periphery of the upper ridge trying to nose in from the
west. This has once again led to an early start of convection off
the coast of the FL Panhandle. This convection eventually formed a
line and moved SW to NE along the mean upper flow. It is a very
broken line as of 2PM stretched from Dothan AL to Tallahassee. The
outflow boundary is stretching out ahead of it to the east which has
become the focus for additional afternoon thunderstorms. Outside
of that, the normal popcorn like showers have begun to develop.
Like any summer afternoon, a boundary collision could cause a
strong storm but the heaviest threats will be the heavy rainfall
due to PW values analyzed around 2.2 inches. The earlier
mentioned line of storms has an extensive cloud deck with it so
much of the area has likely already reached their high temps, in
the mid to upper 80s. Areas in SW GA may still have a deg or two
more to warm.

Storms should diminish shortly after sunset as usual but the hires
guidance has really backed off on overnight convection. The usual
showers over the open waters look likely but the coastal convection
may hold off till closer to 8-10AM. Lows will be mild, in the lower

.SHORT TERM [Saturday Through Sunday Night]...

A mid-level trough will remain poised just northwest of the area
through Saturday, gradually lifting northeast on Sunday, as the
subtropical ridge begins building in for Monday. A low to mid-
level southwest flow will maintain a plume of Precipitable Water
(PW) values up 2.2 inches from the FL Big Bend and northwest on
Saturday, then dropping to between 1.5 and 2.0 inches across the
entire area on Sunday. A 1000-700 hPa mean flow around 10 knots
will support a seabreeze circulation on both Saturday and Sunday.
Meanwhile, shortwaves transiting the trough will enhance synoptic
scale lift leading to heavier than normal precip, especially
northwest of the FL Big Bend on Saturday. PoPs are 50-60 pct
across this area on Saturday, decreasing to 30-40 pct to the
southeast. As the ridge builds in on Sunday, PoPs decrease about
10-15 pct across the board.

On Saturday, instability is more limited northwest of the FL
Big Bend owing to greater cloud cover (Mixed Layer CAPE at or
below 1.0k J/Kg), yet 500 hPa level winds increase up to 20
knots across this area, which is above average. Cannot rule out
some isolated thunderstorm wind gusts around 50 mph. On Sunday,
Mixed Layer CAPE is modeled around 1.5k J/Kg from the FL Big Bend
into portions of southwest GA, while Downdraft CAPE reaches 1.5k
J/Kg. Isolated, sub-severe thunderstorm wind gusts are possible.

High temperatures on Saturday and Sunday will generally range
85 to 90 degrees, up to 5 degrees below normal at the inland
climate sites, mainly owing to cloud cover and precipitation.

Patchy fog will be possible both nights with the moist ground
and any partial clearing.

.LONG TERM [Monday Through Friday]...

The low to mid-level flow will become more anticyclonic out
of the southwest on Monday into Tuesday as the subtropical ridge
builds in. This will support mainly diurnally driven, sea-breeze
induced convection, with PoPs ranging from 30 to 50 pct. A
longwave trough entering the TN Valley mid-week, with 500 hPa
heights of 2 Standard Deviations below normal, will bring a cold
front into the southeastern US. Both the GEFS and Euro ensemble
means bring the front into our area by Wednesday, but there is
plenty of uncertainty given the time range and dispersion amongst
the individual ensemble members. We have followed the
climatologically favored solution of this boundary undergoing
gradual frontolysis, stalling in the vicinity of the area on
Thursday and Friday. This leads to the potential for heavier than
normal rainfall, as well as more organized convection.
Temperatures are expected to be near or slightly below average
during the long term period.


.AVIATION [Through 18Z Saturday]...

A broken line of showers and storms is moving SW to NE between DHN
and TLH now. ECP should just see vicinity showers or a few light
showers through the evening. DHN will see some storms as this line
approaches and added a TEMPO group for that with TLH now seeing
lighting strikes in their vicinity. ABY and VLD will see a few
storms later this afternoon. For now, kept it VCTS until we have
more confidence in the placement of the storms. Winds will be
light and CIGS will be VFR outside of thunderstorms. Brief IFR
conditions are possible in stronger storms. Dry conditions persist
overnight with storms returning early for ECP once again.



Winds out of the south to southwest at 5 to 10 knots through
Tuesday night. Seas will range from 1 to 2 feet through Saturday,
increasing to 2 to 4 feet from Saturday night through Tuesday



With RH values above critical thresholds, hazardous fire weather
conditions are not expected.



In general, 1-3" of rain is forecast along and west of the
Apalachicola and Chattahoochee rivers over the next couple of days
and around 1" to the east. Isolated higher amounts are possible
especially into early next week. Ensemble guidance for area rivers
suggests the potential for action stage along the most reactive
basins west of the aforementioned delineation. Scattered `nuisance`
urban and rural flooding will also be possible through the next



Spotter activation is not requested. However, spotters are always
encouraged to safely report significant weather conditions when they
occur by calling the office or tweeting us @NWSTallahassee.



Tallahassee   73  90  74  90  74 /  20  50  20  40  10
Panama City   75  88  77  87  76 /  20  60  50  50  20
Dothan        72  87  73  86  73 /  20  70  40  60  30
Albany        73  88  73  87  73 /  20  60  30  60  30
Valdosta      72  90  73  91  73 /  20  50  10  30  10
Cross City    74  91  74  91  75 /  20  20  10  20  20
Apalachicola  77  88  77  88  77 /  20  40  30  40  10




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