Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 211149
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
649 AM EST Tue Feb 21 2017

...Update to increase PoPs Wednesday with earlier onset of
higher rain potential...

.UPDATE (Wednesday 12Z through Thursday 00Z)... Some
adjustments to the PoP/Wx forecast have been made for
Wednesday to both bring higher rain chances into the FL west
coast earlier, and to increase the overall rain chances at
most locations. Operational and ensemble members from most
reliable guidance are in agreement that it is going to rain
tomorrow. Would not expect different given the synoptic
support and deep layer moisture in place.

By 12Z Wednesday morning...highly cyclonic flow is already
in place overhead...with a large swath of deep layer Qvector
convergence moving in from the Gulf. In addition...efficient
venting aloft with highly diffluent flow between 600-300mb
will only further enhance the already impressive omega
fields in the mid-levels of the column.

No longer appears as through the highest rain chances will
be occurring in the afternoon. We will certainly have high
rain chances remaining for the afternoon...but will be
bringing the likely/categorical PoPs into the coast very
early in the day now. Unlikely that most spots will see much
of a window of dry weather after sunrise (if at all). Those
with outdoor plans Wednesday morning should make their
preparations with rain in mind. Have kept isolated thunder
in the forecast as well. Instability...even aloft is not all
that great. Even showalter indices generally stay
positive...however...given the dynamics in the
vicinity...would expect at least the potential for embedded
storms in the rain bands.

&&

.AVIATION (21/12Z through 23/00Z)... General VFR conditions
will prevail through Tuesday under passing mid/high level
clouds. Do have some BKN Cigs out there this morning...but
generally above 4KFT...so no significant restrictions
anticipated. Aviation concerns do not look to materialize
until late tonight...and especially Wednesday morning.
Expect several bands of rain with embedded storms to move
onshore during this period and impact all terminals of west-
central and southwest Florida. Much of Wednesday after 12Z
has the potential to be prevailing VFR with periods of IFR
both in CIG and VIS.

&&

.Prev Discussion... /issued 300 AM EST Tue Feb 21 2017/

SYNOPSIS...
Aloft - A low over east TX troughed down into southern Mexico while
a ridge above the northwest Caribbean Sea reached north...over the
eastern states...to Quebec. Surface - A low on the Manitoba-Ontario
border trailed a wavy frontal boundary south to TX. High pressure
in eastern Quebec ridged south to Cuba.

SHORT TERM (Today - Wednesday)...
The deep layer ridging over the area slowly slides east...providing
a warm stable and generally dry atmosphere through most of tonight.
Temperatures will run above normal with model PWAT values
in the 0.75 to 1.0 inch range. Patchy early morning low
clouds and fog lift shortly after sunrise with partly
cloudy skies. However increasing high clouds begin working
in from the west later this afternoon.

Meanwhile the TX low and trough move east and southeast...forming a
surface reflection and trailing front. This system will reach the
eastern Gulf waters...between Tampa Bay and the Keys...by
late Wed afternoon. Flow aloft becomes more southerly and
increases with abundant moisture streaming across the
area...with PWAT running 1.6 to 1.8 inches. Anticipate
showers and storms spreading in the coastal waters late
tonight or early Wed morning then pushing onshore later in
the morning and during the afternoon. Coverage starts
scattered but ramps up during Wed to likely to categorical.
The upper low and trough will provide lift for thunderstorms
but the rainfall along with considerable cloud cover will
inhibit heating...will keep storms in the slight chance
range for now. The high temperatures Wed will cool compared
to today but stay near or just above normal.

LONG TERM (Wednesday night-Monday)...
A vertically stacked U/L low and associated surface low will track
southeast over the eastern Gulf of Mexico Wednesday night and the
Florida straits on Thursday.  ECMWF and GFS in good agreement with
concentrated area of showers and possible thunderstorms pushing east
of the forecast area early Wednesday night, with residual scattered
showers persisting over the forecast area through Thursday as the
system pushes very slowly east across the region.  Decreasing
cloudiness across west central and southwest Florida in the wake of
the system Thursday night and Friday. Temperatures will continue to
run several degrees above normal.

A cold front will push across the Florida peninsula over the
weekend, however the front will have little in the way of deep layer
moisture to work with and no U/L support...so no shower activity is
expected as the front moves across the region.  Surface high
pressure will build over the forecast area in the wake of the front
with drier cooler air advecting across west central and southwest
Florida.  Temperatures will return to near or a few degrees below
climatic normals.

MARINE...
High pressure across Fl slides east of the state tonight as
a deep low pressure system moves into the Gulf. This system
tracks into the southeast Gulf and across far south FL Wed
night then exits to the Atlantic Thu. Weak high pressure
builds into the Gulf Fri...followed by a dry front with
stronger high pressure behind it Sat. Winds veer around at
speeds of 15 knots or less. The main concern will be
thunderstorms with higher winds and seas.

FIRE WEATHER...
While high pressure has dominated there has been enough low level
moisture to preclude any low RH concerns. A deep low pressure
system approaching then moving across the area Wed-Thu will
provide high rain chances with increasing humidity into the
weekend.

&&

.Preliminary Point Temps/PoPs...
TPA  80  65  74  64 /   0  20  90  40
FMY  82  64  75  62 /   0  20  90  40
GIF  82  63  76  62 /   0  10  90  50
SRQ  79  64  74  61 /   0  30  90  40
BKV  82  61  76  62 /   0  20  90  40
SPG  78  65  72  64 /   0  20  90  40

&&

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

&&

$$

UPDATE/AVIATION...Mroczka
PREVIOUS DISCUSSION...RUDE/OGLESBY



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