Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS62 KTBW 240747

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
347 AM EDT Tue Oct 24 2017

08Z water vapor and H4 RAP analysis shows a now highly
amplified upper level pattern over the CONUS...highlighted
by a deepening longwave trough to the east of the
continental divide. Two significant pieces of shortwave
energy are located within this flow that will have a
significant impact on our local weather for the next several
days. The first piece is located within the base of the
trough approaching the TN valley, while the second is
quickly translating southward along the backside of the
trough over the upper midwest. These two features will
"combine forces" during the next 12-24 hours in order to
further amplify this longwave trough all the way down into
the deep south/eastern Gulf of Mexico.

An initial shortwave that passed north of the region Monday has
accelerated up the eastern seaboard...and left behind a cold front
it helped deliver to the Florida peninsula. With deep layer synoptic
flow now nearly parallel to the front in the wake of this energy,
the front has slowed to a crawl southward across the region. This
front will separate the forecast area today...with rain-free
conditions north of the I-4 corridor, and frontal showers and storms
still in the forecast further to the south within a zone of deep
moisture and closer proximity to the frontal focus. However, this
slow down is only temporary. The amplifying trough discussed above
will deliver a secondary cold front to the FL peninsula this evening
and tonight. This secondary front will have plenty of momentum to
clear the entire region and bring the highly anticipated and welcome
fall-like airmass for Wednesday and Thursday to all forecast

Regional radars show the greatest concentration of showers
as of 4AM from Highlands to Lee counties and the adjacent
marine zones. The intensity of this activity has been on the
decrease past several hours, and should see a transition to
more scattered type showers by dawn.


.Short Term (Today through Wednesday)...
Today will be the transition day between the muggy and showery
conditions on Monday, and the dry and fall-like airmass arriving for
Wednesday. The forecast area will be split north and south
today...with northern areas (north of I-4) far enough behind the
initial front to stay dry...with some initial drop in dewpoints. The
front will be located south of the I-4 corridor today...making very
slow progress southward. Therefore...for the Tampa Bay/Lakeland few isolated shower can not be ruled out during
the first half of the morning, but will
turn out generally dry...with increasing amounts of sun over
time. The further south and southeast one travels, the
higher the rain chances today...especially during the
afternoon with the help on diurnal instability. Late in the
afternoon...the upper trough will be approaching quickly
from the northwest...with a secondary cold front reaching
Levy county and the northern marine zones by around 00Z.

Tonight...Strong secondary cold front will quickly translate south
down the peninsula and through the eastern Gulf of Mexico. This
boundary will be dry...with very little moisture to work with. Its
passage will be noted with a slight increase in winds over land, and
a big increase in winds over the eastern Gulf of Mexico...where
advisory level northerly winds will quickly develop overnight. The
initial cold front still down toward Highlands county/Fort Myers
area may support a few more hours with scattered showers through
late evening, but thereafter, the secondary front and digging upper
trough will shift the residual moisture quickly out of our area. The
second half of tonight looks dry for all our forecast zones...with
cooler and much drier air quickly filtering in from the north. Would
say most areas will become mostly clear given the lack of moisture
aloft, however, will not be surprised to see some lower level
stratocu developing off the eastern Gulf of Mexico toward
dawn...given the SSTs...single digit 850mb temps, and strong PVA
associated with the digging trough. We are certainly going to have
the vertical temp differential to support the lower clouds, but the
exact wind direction will determine if these clouds make it onshore
or stay just offshore over the Gulf. Either way, these clouds will
only be an issue for the aviation community. The new airmass will be
noticeable by dawn...especially along and north of the I-4 corridor.
Low temps reach the lower 60s from St Pete to Sarasota and Fort
Myers, but decrease steadily further inland and northward. It is not
out of the question that spots up toward Chiefland drop briefly into
the upper 40s.

Wednesday...The advertised fall-like airmass will have arrived.
Still may be seeing some sct-bkn stratocu along the immediate coast,
but otherwise plenty of sunshine will be in the forecast. Will be
almost unknown from the surface, but appears a strong
shortwave/vorticity lobe will be swinging across our skies during
the second half of the day. Luckily there is simply no moisture for
this feature to work. High temperatures are going to struggle into
the middle 70s...and even lower 70s up toward Citrus/Levy Counties.
Dewpoints are forecast to reach very comfortable readings in the
40s. The dry airmass fully in place will allow for the coldest night
of the week Wednesday Night. Hard to believe but widespread 40s and
50s appears likely. We have not seen an airmass like this in many
months. Enjoy it if you can.


.Mid/Long Term...
The forecast period begins with cool, dry fall air in place over the
peninsula as high pressure settles in behind a cold front. Skies
will be clear overnight Wed into Thursday morning allowing temps to
drop into the 40s and 50s across west-central and SW Florida. This
will be some of the coolest air we`ve seen in over 6 months.
Wednesday night/Thursday morning looks to be the coolest night with
temps Thursday afternoon topping out in the mid 70s. Temps begin to
moderate Friday and into Saturday as high pressure will
shift to the east with a moderating NE to E flow setting up over the

As we get into the weekend, models continue to show the potential
for an area of low pressure to develop in the western Caribbean in
association with a Central American Gyre currently over the region.
These features are notoriously difficult for global models to
forecast but nevertheless the latest tropical weather outlook from
NHC gives this area a 50% chance of tropical development over the
next 5 days. Model trends over the last few days has been for a
significant moisture increase over the area for the weekend
associated with this feature. Given the trends, have increased PoPs
quite a bit from the previous forecast for Saturday and Sunday.
While this feature moves north, deep upper-level troughing and
another cold front will swoop down across the central CONUS and move
towards the FL peninsula. This will quickly lift the moisture
associated with the aforementioned low, whether tropical in nature
or not, across the region before moving it out into the western
Atlantic by Sun afternoon. Again, these features are tough for
global models to forecast so this is a very low confidence forecast
for Saturday and Sunday. Later Sunday, the front will push through
the area and bring another round of below normal temps to the


.AVIATION (24/08Z through 25/06Z)... Cold front is very
slowly moving southward down the Florida peninsula early
this morning. Widespread showers/storms from around
KFMY/KRSW should become more scattered approaching dawn, but
will remain in the forecast south of the front. KTPA and
nearby terminals may see a brief shower through 12-14Z, but
thereafter, VFR conditions and improving sky conditions
should progress through the day. Terminals around KFMY could
see a few showers at any time during the day Tuesday,
however rain chances decrease with time after late
afternoon. All terminals begin a long duration of prevailing
VFR conditions behind a secondary cold front Tuesday
Evening / Night.


A cold front will continue to sag south through the eastern
Gulf of Mexico today...shifting winds northerly in its
wake. These winds will occasionaly approach 15 knots. A
secondary and stronger cold front will quickly move through
the forecast waters tonight. Winds increase rapidly to
advisory levels behind this front and continue strong into
the day Wednesday. Winds will then subside into Thursday as
high pressure builds over the region.


A cold front will continue to slowly sag south across the
region today...keeping chances for showers generally south
of the I-4 corridor. A secondary cold front will arrive
tonight and clear out the entire region. A cooler and much
drier airmass will arrive for Wednesday...lingering through
Thursday. Relative humidity values both Wednesday and
Thursday afternoons are expected to approach or fall briefly
below 35 percent. However, due to low ERC values, and
generally light winds, no red flag conditions are
anticipated. Although fair weather will continue...low level
moisture and relative humidity values will begin to
increase once again on Friday.

Fog Potential...No significant areas of fog or lower
visibility are expected over the next several days.


.Preliminary Point Temps/PoPs...
TPA  81  61  74  54 /  20   0   0   0
FMY  84  64  76  54 /  60  20   0   0
GIF  82  58  73  51 /  30   0   0   0
SRQ  79  63  74  54 /  20  10   0   0
BKV  80  55  73  44 /  10   0   0   0
SPG  81  63  74  59 /  20   0   0   0


Gulf waters...None.



MID TERM/LONG TERM/DECISION SUPPORT...McKaughan is the U.S. government's official web portal to all federal, state and local government web resources and services.