Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 230724
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
324 AM EDT Mon Oct 23 2017

.Synopsis...
07Z water vapor and H4 RAP analysis shows a strong and deep
shortwave trough migration eastward over the Deep South and Northern
Gulf Coast early this morning. The potency of this shortwave is
clear in the amount of drying/subsidence behind the trough axis on
WV imagery. This shortwave axis is already showing signs of becoming
negatively tilted (as NWP guidance has been showing for several
days)...and this evolution during the next 12-18 hours will result
in the greatest synoptic influence related to lift passing generally
to our north today. Even still...enough lift on the fringe of this
energy...combined with abundant moisture and some diurnal heating
should still keep high rain chances in the forecast. The amount of
rainfall and overall durations however should be less than in areas
up toward the I-10 corridor and further north.

As mentioned...atmospheric moisture will be high today. The KTBW 00Z
sounding showed good moisture return already, and this should
continue through the morning hours. WV imagery shows a large plume
of mid/upper level moisture streaming over the FL peninsula...and
this will continue as the shortwave axis gets closer in the hours to
come.

At the surface...A southeasterly wind flow dominates our region
between high pressure east of New England and an approaching cold
front associated with the upper level energy described above. As the
cold front gets closer today and becomes the more dominant synoptic
feature, these winds will continue to veer to a more southerly
direction.

Regional radars at 3AM showed generally quiet conditions
over the Florida peninsula...with a large area of
rain/embedded storms lurking offshore over the Gulf of
Mexico. This area of showers looks ominous for our area
before dawn, however, all indications are that it will be
very slow to move eastward the next several hours.
Thereafter, the overspreading of synoptic support for lift
from the west will evolve this area toward the coast in a
quicker manner.

&&

.Short Term (Monday through Tuesday)...
Today...Shortwave energy moving along the northern Gulf coast and an
associated cold front will increasingly provide large scale lift to
the region...resulting in increasing rain chances. The main region
of height falls/synoptic lift does look to pass to our north as the
shortwave continues to evolve to a more negatively tilted
configuration and pivot northeastward through the FL
panhandle/AL/GA. However, much of our region will still fall into a
region of at least weak synoptic support ahead of the trough
axis...despite meager height falls this far south.

Given the warm Gulf waters, even the weak synoptic lift...combined
with broad diffluence aloft associated with passing jet energy
should be enough in the presence of the low level frontal focus
support areas of showers/storms moving onshore after mid-morning
(Perhaps a bit earlier up over parts of the Nature Coast). The best
overall ingredients for rainfall today will be to the north of the I-
4 corridor, and hence the highest rain chances, rainfall amounts,
and expected rainfall duration will also be north of I-4.

The broad nature of the lift suggests several clusters of showers
are possible...as opposed to a single defined frontal band of
convection. This obviously makes the forecast and outdoor planning
for rainfall timing more difficult.

South of I-4...especially over inland areas...the later arrival of
any organized rainfall is likely to allow for a decent diurnal
recovery...and this is also likely to aid shower development during
the afternoon/evening associated with the arrival of synoptic
influences...and justify higher rain chances.

Tonight...Shortwave energy is quickly exiting the region this
evening...taking most of the synoptic support for lift with it. The
result is going to be a much slower moving cold front across the FL
peninsula. Latest model ensembles suggest the front barely makes it
down to the I-4 corridor by sunrise Tuesday. With this in mind, will
slowly end rain chances over our far northern areas overnight with a
wind shift to the northwest, while further south toward Tampa Bay
and point south keep a chance of sct showers and southerly flow.
With the loss of synoptic forcing, diurnal minimum in heating, and a
slower moving front to taper low level focus, the overall shower
coverage and intensity is going to decrease significantly. Basically
just looking for isolated to scattered light showers for all zones
before midnight and mainly along and south of I-4 after midnight.

Looking far to the north tonight, we will find another series of
strong shortwaves beginning to dig a deep trough that will
eventually cover all the eastern states by Wednesday. It will be
this energy as it quickly drops southward during Tuesday that will
eventually give the initial cold front and associated shower
activity the kick south out of the area...and deliver the secondary
cold front that will bring the significant change in airmass
advertised for Wednesday and Thursday.

Tuesday...Cold front continues to move very slowly during the day,
dropping south of the Tampa Bay/I-4 corridor by the afternoon. Will
keep sct showers in the forecast for this region during the morning,
but allow things to dry out in the afternoon. Further south...shower
and storm activity is likely to increase once again during the
afternoon in response to diurnal heating and the presence of the
frontal focus. However, as mentioned above, the "atmospheric kicker"
digging trough aloft will be quickly approaching, and begin to speed
the progress of the front toward the very end of the day, and
certainly into Tuesday night.

A secondary cold front along the leading edge of the height gradient
associated with the trough is shown to arrive in the Nature coast
Tuesday evening, and exit our southern zones Wednesday morning.
While some airmass drying (dewpoint drops) will have occurred the
first front, it will be this secondary surge that brings the really
comfortable air for the middle of the week.

&&

.Mid/Long Term...
The first cold front of the fall season will pushing through the
peninsula at the start of the mid-term forecast period. Cloud cover
and rain will diminish, while winds become northerly allowing for
cooler, drier air to move into the area. The day will start
noticeable cooler Wed morn with lows in the 50-65 range, coolest
across the nature coast where the FROPA will occur much earlier.
Throughout the day Wednesday, CAA will continue across the area and
afternoon high temps will be some 7-10 degrees below normal with
highs in the low to mid 70s expected across the area. Wednesday
night/Thursday morning looks to be the coolest night with lows
ranging from the mid 40s to mid 50s. This will be the coolest air
we`ve seen across the forecast area since early April! Thursday will
remain cooler with highs again in the low to mid 70s. Temps will
begin to moderate some Friday and Saturday as surface high will
shift out into the Atlantic Ocean with a more NE to E flow setting
up bringing moderating temperatures and increasing moisture. At the
very end of the forecast period, GFS/ECMWF models diverge with
respect to tropical moisture/disturbance moving north out of the
western Caribbean so for now will show some increasing rain chances
for Sunday but this is low confidence at the present time. However,
both models do continue to show some upper-level troughing again
setting up over the central and eastern U.S. so looks like another
shot of cooler weather will be possible to end October but this is
still more than a week away so will have to see how things
play out.

&&

.Aviation (23/06Z through 24/06Z) VFR conditions prevail across west-
central and southwest FL early this morning, and should generally
continue for the next several hours. The chances for showers and
isolated storms will begin to increase after dawn from KSRQ
northward...but likely hold off until the afternoon for
KPGD/KFMY/KRSW. A few periods of MVFR cigs/vis will be possible with
any of the passing areas of rain/storms today, but generally
expecting conditions to remain VFR.

&&

.Marine...
The approach of a cold front will shift wind from the southeast to
the south through today...with periods of cautionary level winds.
Numerous showers and storms can be expected over the coastal waters
ahead of this front. The cold front will slowly sag through the
coastal waters tonight and Tuesday...with winds shifting to the
northwest in the wake. These northwest wind will reach cautionary to
advisory levels Tuesday Night through Wednesday, and then slowly
diminish into the later portion of the week as high pressure builds
over the region.

&&

.Fire Weather...
A cold front will approach today keeping rain chances in the
forecast and high levels of relative humidity. This front will
slowly sag southward through the region tonight and Tuesday. The
slow movement of the front will prevent any significant intrusion of
drier air through Tuesday. A secondary cold front arriving Tuesday
night will usher in cooler and much drier air for Wednesday and
Thursday. A few area...especially north of the I-4 corridor are
forecast to see relative humidity approach 35 percent or drop
slightly below both Wednesday and Thursday afternoon. Low level
moisture will begin to return to the region Friday and into the
upcoming weekend.

Fog Potential...No significant areas of fog or reduced
visibility are expected the next couple of days.

&&

.Preliminary Point Temps/PoPs...
TPA  83  73  81  61 /  80  50  30  10
FMY  85  73  84  64 /  60  50  60  40
GIF  84  71  81  58 /  60  50  30  20
SRQ  83  73  81  62 /  80  50  40  10
BKV  83  69  80  55 /  80  50  20  10
SPG  83  73  81  63 /  80  50  30  10

&&

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

&&

$$

SYNOPSIS/SHORT TERM/AVIATION/MARINE/FIRE WX...Mroczka
MID TERM/LONG TERM/DECISION SUPPORT...McKaughan



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