Area Forecast Discussion
Issued by NWS Jacksonville, FL

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FXUS62 KJAX 260824
AFDJAX

Area Forecast Discussion
National Weather Service Jacksonville FL
424 AM EDT Wed Jul 26 2017

...LOCALIZED FLOODING POSSIBLE OVER PORTIONS OF SOUTHEAST GEORGIA
LATE THIS AFTERNOON AND EVENING...
...STRONG THUNDERSTORMS POSSIBLE ALONG I-10 CORRIDOR LATE THIS
AFTERNOON AND EVENING...

.CURRENTLY...
Early morning surface analysis depicts the axis of Atlantic
ridging situated across Lake Okeechobee and the southeastern Gulf
of Mexico. Meanwhile, a backdoor cold front was moving slowly
southward through the Carolinas, with high pressure (1024
millibars) centered over upstate New York. Aloft...a mid-level low
was becoming cutoff over south central Georgia, while deep-layer
ridges exist over the Florida peninsula and the southern Plains
and Ozarks. Low stratus ceilings were overspreading inland
southeast Georgia, which is on the east side of the developing
cutoff mid-level low pressure center. Debris cloudiness from late
evening convection over coastal southeast Georgia prevails
elsewhere, with temperatures generally ranging from 75 to 80
degrees and dewpoints in the mid to upper 70s as of 08Z.

.NEAR TERM /Today and Tonight/...
Mid-level low pressure will cutoff near the Altamaha/Ocmulgee
River basin this morning and should pivot southward through this
evening. The axis of Atlantic ridging will remain in place over
the Florida peninsula today, keeping southwesterly flow in place
and advecting a ribbon of dry air from the Nature coast
northeastward across north central Florida. Deep moisture will
pool in the vicinity of the cutoff low, with PWATS of 2.2-2.4
inches overspreading most of southeast Georgia and far western
portions of the Suwannee Valley this morning, with a tight
moisture gradient prevailing south of Interstate 10 by this
afternoon, as PWATS of 1.2-1.3 inches are forecast over Marion and
Alachua Counties.

Short-term high resolution models suggest that convection will
initiate this morning to the east of the mid level low center
across inland southeast Georgia, with activity increasing in
coverage and migrating southeastward towards coastal southeast
Georgia towards noon and across the Interstate 10 corridor during
the mid-afternoon hours. Storm motion should be very slow over
southeast Georgia, with some concerns for localized flooding by
late afternoon and evening as additional rounds of convection
potentially develop. Model soundings depict mid-level dry air
remaining in place along the I-10 corridor, which will enhance the
threat for wet microbursts as activity from southeast Georgia
migrates southeastward this afternoon. Downdraft CAPE values at
Jacksonville are near 1000 j/kg in the NAM model sounding and over
1100 j/kg in the GFS sounding. The Weather Prediction Center has
placed all of southeast Georgia and the I-10 corridor within a
marginal risk for excessive rainfall today, where widespread
totals of 1-2 inches are forecast by sunset, with localized totals
up to 5 inches possible where cells train and/or multiple rounds
of convection develop through this evening. An early start to
convection should keep highs over most of southeast Georgia in the
mid to upper 80s, while highs over north central Florida will
soar into the mid 90s. The dry air aloft will mix down to the
surface this afternoon across north central Florida, resulting in
dewpoints dropping to near 70, keeping max heat index values near
or just below 105 degrees.

Thunderstorm activity may linger into the early evening hours over
coastal southeast Georgia and along I-10, with dry conditions
expected to prevail over north central Florida through the predawn
hours. The mid-level low should continue to migrate slowly
southward towards the Big Bend coast, with south-southwesterly
flow possibly bringing a predawn round of convection into the
Suwannee Valley and southeast Georgia before sunrise. Low stratus
ceilings will likely overspread much of our area overnight, with
lows ranging from the lower 70s inland to the upper 70s near the
coast.

.SHORT TERM /Thursday Through Friday Night/...
Thursday and Thursday Night: Upper trough/weak upper low will be
near the FL/GA border and stretch into the western Atlantic, with
weak surface to low level area of low pressure near/over the
region. Models have slight differences in the short term, which
lead to significant precipitation discrepancies. The GFS and ECMWF
have placement differences of an 850mb low, with the ECMWF across
southeast Georgia and the GFS near Tallahassee. Due to this, the
GFS continues to pull in drier air across northeast Florida
through the day from the Gulf, and keeps the high rain chances
only across southeast Georgia. The ECMWF brings higher moisture
into northeast Florida due to the placement of the weak low, with
higher precipitation chances across the eastern half of southeast
Georgia and much of northeast Florida. The ECMWF begins to bring
in drier air near the Altamaha River late in the day on the back
side of the trough. For the forecast, leaned towards the ECMWF,
with 50-70% PoPs areawide. Expect an early start to convection
across the Suwanee Valley due to the west southwest onshore flow,
with showers/storms moving inland and towards the coast in the
afternoon. An east coast sea breeze is forecast to develop in the
mid afternoon, and storms are forecast along the sea breeze merger
as well. Temperatures at 500mb are forecast to be warm to -5 to
-6.5C, so only expect only isolated strong storms. Storms should
end early in the evening, with quiet conditions until the early
morning. Scattered showers and a few storms are likely to
redevelop over the northeastern Gulf and then move inland across
the Suwanee Valley and possibly much of northeast Florida before
daybreak. Highs will be in the upper 80s to lower 90s, with lows
in the 70s.

Friday and Friday Night: Weak trough over northeast Florida will
begin to weaken. A strong shortwave trough over the Great Lakes
Friday morning will dive southeast across the Ohio Valley Friday
afternoon, and begin to carve out a longwave trough across the
eastern sea board and into the southeast. An early start to
convection is expected once again on Friday, with showers and a
few storms ongoing by daybreak across northeast Florida. Slightly
drier air is forecast to move into southeast Georgia on the
backside of the trough, and lower rain chances are forecast there.
Storms should end by early evening due to the early start, with
mainly dry conditions overnight. Highs are forecast to be mainly
in the lower 90s, with lows in the 70s.

.LONG TERM /Saturday Through Wednesday/...

Wet pattern is forecast through the extended period. An upper
trough will dig southward into the region on Saturday, with a
frontal boundary moving into southern Georgia in the afternoon.
This will lead to numerous showers and storms across southeast
Georgia, and scattered storms elsewhere. Nocturnal convection is
then forecast for Saturday night as the boundary moves across the
region, with scattered to numerous showers and storms overnight.
Models are in good agreement with the boundary then stalling near
the FL/GA border or northeast Florida through the middle of next
week. So, slightly lower rain chances are forecast northwest of
Waycross, with numerous to widespread showers/storms elsewhere.
Heavy rainfall is possible in the region the boundary stalls.
Highs will be in the mid 80s to lower 90s, with lows in the 70s.

&&

.AVIATION...
Ceilings around 1000 feet are possible at SSI through around 13Z.
Otherwise, VFR conditions are expected through the morning hours
at the regional terminals. Widespread showers and thunderstorms
will develop by the late morning hours on Wednesday across
southeast Georgia, with this activity expected to reach SSI
towards 16Z. Periods of IFR visibilities are possible through 00Z
at SSI as waves of heavy showers and thunderstorms potentially
impact the terminal. This activity should spread southward into
the Duval County terminals towards 20Z, with impacts possibly
extending to SGJ after 21Z. Highest confidence in significant
impacts is at SSI, where a tempo group was included from 16Z-20Z.
Gusty winds in excess of 30 knots will also be possible with
heavier thunderstorm activity this afternoon at SSI and the Duval
County terminals. VFR conditions are expected to prevail at GNV.

&&

.MARINE...
The axis of Atlantic ridging will remain positioned south of our
waters during the next several days. Weak low pressure will
develop over inland southeast Georgia today and will move little
through Thursday, before moving slowly eastward and weakening on
Friday. This feature will result in light southwest winds this
morning, becoming more southerly later today and southeasterly on
Thursday. Winds both days will become onshore in the near shore
waters as the Atlantic sea breeze develops. Winds will then shift
to a southwesterly direction by Thursday night and Friday as the
weak low pressure center weakens. Our local pressure gradient will
tighten by Friday evening as a cold front moves southward and
enters the southeastern U.S. Wind speeds should reach caution
levels during the evening wind surge on Friday for both the near
shore and offshore waters. This front will approach the southeast
Georgia waters by late on Saturday, with strong thunderstorms
possibly overspreading the waters during the afternoon hours. This
front will likely stall over the northeast Florida waters by late
this weekend, with stormy conditions expected to persist into at
least early next week.

Rip Currents: Low risk due to low wave heights, with a small long
period easterly swell expected to continue today.

&&

.PRELIMINARY POINT TEMPS/POPS...
AMG  86  73  91  74 /  70  40  60  30
SSI  85  78  87  77 /  60  60  60  50
JAX  91  74  91  75 /  70  50  50  40
SGJ  92  79  89  75 /  30  30  40  20
GNV  93  73  91  74 /  20  20  40  20
OCF  94  74  91  74 /  10  20  30  30

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...None.
GA...None.
AM...None.
&&

$$

Nelson/Kennedy



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