Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 201917
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
217 PM EST Fri Jan 20 2017

.SHORT TERM (Tonight-Saturday)...
Strong U/L southern stream flow will continue from California across
the southern tier of the U.S.  A S/W disturbance will ride over a
baroclinic zone across the northern Gulf of Mexico tonight with
numerous showers and thunderstorms developing over the Lower
Mississippi Valley and north central Gulf of Mexico. This system
will push across the southeast U.S. on Saturday with an increasing
chance of showers and thunderstorms over the extreme northern
forecast area Saturday afternoon.  Surface high pressure along with
a strong mid level subsidence inversion will hold across the
remainder of west central and southwest Florida which will inhibit
shower development.  This should create a rather sharp POP boundary
over the northern nature coast...with most of the forecast area
under partly cloudy skies and continued warm conditions.

.LONG TERM (Saturday Night - Friday)...
Upper level ridging will be moving east of Florida into the Atlantic
at the start of the period as a deep upper level trough digs into
the Lower Mississippi Valley and Gulf Coast. Ahead of this trough,
an upper level disturbance will move across the northern peninsula
during Saturday night, bringing with it chances for showers and
storms some of which could be strong or severe in nature especially
across northern portions of the Nature Coast where pops in the 50 to
70 percent range will be advertised. Now during Sunday and Sunday
night, models continue to be in fairly good agreement with regards
to a more widespread severe storm event for the forecast area,
especially during Sunday afternoon and evening.

An intense cut-off upper level low moving eastward through the
southeastern states will aid in the development of a fast moving pre-
frontal squall line which will move southeast through the forecast
area during Sunday afternoon and night. Ample moisture (Pw`s
increasing into the 1.5 to 1.6 inch range) combined with afternoon
temperatures climbing into the upper 70s ahead of this line of
convection should result in ample instability across the forecast
area.

This instability combined with warm air advection, increasing large
scale ascent ahead of the cut-off low, increasing wind shear
(helicity values increasing into the 200-300 m2s2 range), and a 50
to 55 knot low level jet which will be moving into the region from
the Gulf during Sunday afternoon will all favor strong to severe
storms along the squall line as it progresses south through the
forecast area with damaging winds, large hail, and possibly a few
tornadoes the main hazards especially with any discrete or
supercells that develop out ahead of the main line.

At the current time a blend of model guidance shows the squall line
moving into the Nature Coast early Sunday morning, then through the
the remainder of the region during Sunday afternoon and evening. As
always this timing could change slightly during the next day or so,
so all residents and visitors of West Central and Southwest Florida
should continue to listen to latest forecasts as we move into the
upcoming weekend.

During Sunday night the pre-frontal convection along the squall line
will shift south of the forecast area with the main cold front
moving south through the region. With the trough axis of the cut-off
upper level low still to our west lingering showers will continue to
be possible over the area overnight into early Monday morning before
all of the rain ends from north to south by mid day on Monday. In
the wake of the front a very strong westerly wind flow will advect
in somewhat cooler and drier air into the region. The strong onshore
flow will support very hazardous marine conditions over the adjacent
Gulf waters where solid Small Craft conditions are likely with
frequent wind gusts to gale force along with building seas and a
gale watch or warning may be required in later forecasts. The strong
onshore flow will also bring a threat of some minor coastal flooding
especially at times of high tides Sunday night into Monday, as well
as high surf and a high risk of rip currents along area beaches.

During Tuesday and Wednesday pleasant dry weather with diminishing
winds will return to the forecast area as high pressure surface and
aloft builds in over the region. Another cold front will approach
from the west on Thursday with the next chance of some rain. At the
moment the ECMWF is faster with this front moving it into the Nature
Coast during Thursday afternoon while the GFS is slower and depicts
the front across the eastern Panhandle. Given these model
differences and seeing that this is day seven of the forecast will
take a blend of the models for now and depict slight chances for
some showers across the Nature Coast during Thursday afternoon,
otherwise mainly dry conditions will continue. Above normal
temperatures at the start of the period will fall back to near
normal levels during Monday and Tuesday in the wake of the front
before rebounding back to above normal levels from mid week on.

&&

.AVIATION...
VFR conditions the remainder of this afternoon and evening.  Areas
of fog will develop late tonight and may impact all terminals with
areas of MVFR vsbys, and lcl IFR cigs/vsbys.

&&

.MARINE...
Increasing southerly winds will develop over the waters Saturday
night ahead of a strong cold front.  An area of showers and strong
to severe thunderstorms is expected to push across the waters Sunday
associated with the cold front.  The gradient will tighten in the
wake of the front as an area of low pressure deepens near the mid
Atlantic coast.  Frequent gusts to gale force will be possible
Sunday night and into Monday across the near and offshore waters
with hazardous seas developing. High pressure will build over the
waters Tuesday with winds and seas subsiding.

&&

.FIRE WEATHER...
No fire weather hazards are expected through the weekend as
relative humidity values will remain above 35 percent each
day.

&&

.Preliminary Point Temps/PoPs...
TPA  67  79  68  76 /   0   0  40  80
FMY  64  81  68  81 /   0   0  20  60
GIF  64  83  65  81 /   0   0  20  80
SRQ  65  76  69  78 /   0   0  30  80
BKV  65  81  67  79 /   0  10  50  70
SPG  66  77  68  78 /   0   0  30  80

&&

.TBW WATCHES/WARNINGS/ADVISORIES...
FL...Beach Hazards Statement through Monday evening for Coastal
     Manatee-Coastal Sarasota.

Gulf waters...None.
&&

$$

SHORT TERM/AVIATION/MARINE/FIRE WEATHER...13/Oglesby
MID TERM/LONG TERM/DECISION SUPPORT...57/McMichael


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