Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 212330
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
730 PM EDT Mon Aug 21 2017

.UPDATE (Overnight through Tuesday)...
23Z water vapor and H4 RAP analysis shows the Florida
peninsula resides between a large expanse of upper ridging
from Carolinas/Georgia to the western Atlantic...and an
upper trough retrograding westward across the
central/western Gulf of Mexico. A swath of quite dry
mid/upper level air has pivoted over the central and
southern peninsula during the past 24 hours from the SE
within the large-scale flow between these two features.
This drier air as significantly held back the coverage of
storms from the I-4 corridor southward this
afternoon/evening. We were actually able to mix high enough
to tap into this drier air...and evaporate quite a bit of
the cumulus field. This process that began a bit before the
effects of the passing eclipse shadow was actually enhanced
by the loss of solar insolation. An even more dramatic
decrease in cumulus coverage was evident in satellite
imagery closer to where totality passed well off to our
north.

Have seen a few widely scattered storms the past few hours
across the Nature coast where the impacts of the drier air
have not been as great today. However, the swath of drier
air will continue to pivot slowly north and northwest the
next 24 hours, and it will be these nature coast zones
(north of I-4) that see the greatest impact/suppression of
storms from the dry air during Tuesday.

So, for the rest of tonight, the few isolated shower that
have been able to survive the hostile thermodynamic
environment aloft will dissipate with sunset and loss of
diurnal heating. The remainder of the overnight will see a
dry forecast, with typical August overnight temperatures.

Tuesday, the dry forecast continues over the landmass
through the morning hours, with just a slow development of a
few-sct cumulus field. As mentioned above, the driest mid-
level air will have move up over the Nature Coast zones to
the north of I-4, and it will be these areas that struggle
to see much in the way of showers/storms during the
afternoon/evening.

A renewed influx of deep moisture will be slowly trying to
move back into the central and southern part of the FL
peninsula, with greatest influence the further south and SE
one travels. Therefore, isolated storms (20% PoPs) are
expected in and around the I-4 corridor the second half of
Tuesday, ramping up to scattered coverage (30-40%) down
toward Sebring/Punta Gorda/Fort Myers).

Temperatures look quite warm for your Tuesday, with many
spots...even close to the coast...reaching the middle 90s.

Have a great evening everyone!

&&

.AVIATION (22/00Z through 23/00Z)...
VFR conditions will prevail for all terminals through much,
if not all, of the the TAF period. Light easterly or
northeast winds overnight will continue into the daylight
hours of Tuesday, with some potential for winds to turn NW
during the afternoon at KTPA/KPIE/KSRQ. Thunderstorm
potential will still be below normal, although returning
moisture will increase the chance for a passing storm at
KPGD/KFMY/KRSW late in the afternoon or evening. Potential
further north is still low enough to exclude form current
TAFs.

&&

.Prev Discussion... /issued 351 PM EDT Mon Aug 21 2017/

SHORT TERM (Tonight-Tuesday)...
One tropical wave lifting off to the NW over northern FL,
another wave in place over the eastern Bahamas, and high
pressure ridging in between with a little dry slot across
the central and southern zones this afternoon. This drier
mid level air is limiting rain chances for this afternoon,
with the greatest chances in place across the Nature Coast.
These features will continue to shift to the W/NW through
Tuesday with the dry slot lifting north and moisture
returning from the south as the tropical wave approaches and
then moves over southern FL through Tuesday. For Tuesday
afternoon, the Nature Coast will have low rain chances,
while the southern half of the area increases to 40-50
percent chances. Generally E/NE flow will be in place
through the period, with a weak sea breeze possible near the
coast Tuesday afternoon. Temperatures will be near normal
for lows, generally in the 70s, and highs will be on the
warm side in the lower to mid 90s.

LONG TERM (Tuesday Night through Next Monday)...
The start of the long range period will feature a strong
upper trough centered over the Great Lakes with upper
level high pressure from the western CONUS into the Gulf of
Mexico. At the surface, high pressure will extend across the
western Atlantic Ocean and the Gulf of Mexico with a weak
tropical low across the far eastern Gulf.

The forecast remains highly uncertain with respect to the
tropical low/easterly wave expected to move across the
southern tip of Florida or Florida Straits. This feature is
currently being affected by an area of drier air and weak
wind shear, which is good because it limits the potential
for this feature to strengthen into a tropical cyclone.
However, conditions may become slightly more favorable by
Wednesday. So if this low makes it into the eastern Gulf of
Mexico, it will have to be monitored closely. Given the
feature is generally being poorly resolved, models vary in
its position over the next 72 hours, with some solutions
showing it over the Gulf and others just of the east coast
of Florida. In the current forecast, we expect the low to
track across south Florida, bringing greatly increased rain
chances to areas along and south of I-4. The most recent
guidance shows this low will remain a relevant feature
through the latter half of the week as it slowly meanders
either over south Florida, or just off the southwest Florida
coast. It goes without saying, this will need to be watched
closely for potential tropical development. At the very
least, this feature will maintain high rain chances each day
as deep tropical moisture mingles with the seabreezes and
daytime heating. Locally heavy rainfall may also become a
concern, especially south of I-4 where the greatest QPF
signal is apparent.

Heading into the weekend, a potent shortwave is expected to
lead to a deepening of the Great Lakes trough, allowing a
cold front to drop southward into the southeast U.S. The
more aggressive European brings the drier post frontal air
all the way into northern Florida with the more reserved GFS
confining it more to Georgia and the Carolinas. In both
cases, an interaction between the remnants of the tropical
disturbance and the frontal boundary will likely promote
heavy rainfall across portions of the southeastern U.S.
including Florida.

Temperatures will remain seasonal, becoming slightly cooler
for the latter half of the week into the weekend as
humidity increases.

MARINE...
Winds will increase with an easterly surge overnight as the
area remains in weak high pressure between two tropical
waves, one over northern Florida, and the other over the
eastern Bahamas. The features will shift west through mid-
week, with high pressure then returning over the waters for
the end of the week. Scattered showers and storms possible
as moisture returns across the area through the rest of the
week. No headlines expected through the period.

FIRE WEATHER...
No concerns.

&&

.Preliminary Point Temps/PoPs...
TPA  78  94  78  92 /  10  20  10  30
FMY  77  95  77  92 /  10  40  40  60
GIF  75  95  76  94 /   0  30  10  40
SRQ  77  95  78  92 /  10  20  30  30
BKV  72  95  75  94 /  10  10  10  20
SPG  80  94  79  92 /  10  20  20  30

&&

.TBW WATCHES/WARNINGS/ADVISORIES...
FL...None.
Gulf waters...None.

&&

$$

UPDATE/AVIATION...Mroczka
PREVIOUS DISCUSSION...HUBBARD



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