Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 201126
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
726 AM EDT Mon Aug 20 2018

.AVIATION...
VFR conditions are expected outside of any thunderstorm activity.
Light winds this morning will shift onshore near the coast this
afternoon with the sea breeze. Another round of showers and
thunderstorms can be expected this afternoon and evening, with the
greatest chances along and east of I-75. Showers and storms will then
dissipate within a few hours of sunset, with dry VFR conditions
expected tonight.

&&

.Prev Discussion... /issued 521 AM EDT Mon Aug 20 2018/

SYNOPSIS...
09Z water vapor and H4 RAP analysis showing an ill-defined
mid/upper level pattern in place over deep south/FL early
this morning. We really see no significant synoptic
features, dry/wet pockets in place to introduce additional
variables to a typical southeast flow Florida summer
forecast.

At the surface...The subtropical ridge axis from the
Atlantic is aligned across the central Florida
peninsula/eastern Gulf...roughly in the vicinity of the I-4
corridor. This positions provides the clockwise southeast
flow in the lower atmosphere across the central/southern
peninsula...turning more southerly toward our northern
Nature Coast zones. However, the ridge axis nearby is
keeping the gradient and resulting wind regime rather light.
The more mesoscale nocturnal local land breeze circulation
is currently dominating the flow along the coast and into
the eastern Gulf at this early hour. A scattering of
showers/storms can be seen on radar well offshore along this
boundary, and based on the strength of the flow, these
storms will remain offshore through a good portion of the
morning. The only exception being up toward Citrus
county/Cedar Key, where the more southerly flow against the
land breeze is likely to allow the migration of isolated
convection to the coast in the next several hours. Most of
the higher resolution convection allowing guidance is
supportive of this type of short term convection evolution.
Otherwise, its a benign, warm and muggy late summer morning.
Temperatures and dewpoints are generally in the 70s and
should be holding near steady through dawn.

SHORT TERM (Today through Tuesday)...
Today... Surrounding sounding profiles from last evening
and current NWP guidance indicates a fairly static PW
profile across the peninsula of roughly 2" (or slightly
above). This value is within the realm of climatology for
this time of year and should not significantly suppress or
enhance the typical diurnal distribution of storm coverage
one would expect within the current low level southeast
flow regime. As mentioned in the synopsis...most of the
convective activity this morning will remain over the
coastal waters, allowing a mainly dry morning with
temperatures rising through the 80s.

Once the sea-breeze gets organized during the early
afternoon hours, a few isolated storms can be expected near
the coast. However, this pattern generally sees the greater
convective coverage for this side of the state during the
later afternoon and evening hours, and this evolution will
be used in the timing of today`s forecast. Storms will
evolve inland from the I-95 corridor along the east coast
seabreeze into our south- central zones during the
afternoon...providing outflow to interact with the west-
coast seabreeze late in the day and force the majority of
the storms. The greatest coverage of storms will be along
and just east of the I-75 corridor later today, and highest
PoPs will be in these locations. However, with sea-breeze being
rather light, will keep rain mention in the forecast
further west to the coast, as any stronger convective outflow
from the inland storms can certainly overpower the sea-
breeze and make its way inside the I-75 corridor to force
additional storms into the evening hours. Unfortunately for
the meteorologist attempting to pin down the best
probability/timing for each location, there is nothing easy
or straightforward about forecasting the mesoscale (or
microscale) nature of Florida summer convection...
especially as one gets later in the day toward a 3rd or 4th
generation of pulse storm development. Generalized
spatial/temporal patterns that are known for differing
synoptic wind directions provide a good first guess toward
forecasting the day`s convective evolution. However,
unpredictable convective outflow under lighter flow has
ruined many a well reasoned forecast in the past, and
certainly will continue to do so in the future.

Tuesday...Our synoptic low level winds pattern begins to
transition from SE to SW during Tuesday as troughing evolves
over the eastern part of the country and drives a weak cold
front down into the deep south. This progression will force
the subtropical ridge to drop south and result in this more
southwesterly flow into our region. Yes, the term "cold
front" was just used in this AFD, but don`t get too excited.
This front is not packing much of a punch and will stall and
decay before reaching the peninsula. The front`s influence
to provide a longer fetch off Gulf waters in the upper 80s
will prob only make things more humid. Unfortunately, we
still have a ways to go folks until that first refreshing
cold front enters our forecast.

For Tuesday...Although most of the morning will be dry, a
few isolated showers should be expected within the
southerly/SW near the coast and over the coastal waters
during the morning hours. The flow will then rather quickly
push the sea-breeze inland during the afternoon hours.
Therefore, scattered storms will be expected along and
inside the I-75 corridor during the early afternoon, but
should quickly translate inland during the later
afternoon/evening hours. The more defined flow will also
help to prevent convective outflow from coming back westward
during the evening, especially from the Tampa Bay area
northward through the Nature Coast.

LONG TERM...
The middle and end of the workweek will feature a pattern of
continued low level SW flow. This pattern will provide the
best chance for scattered showers/storms near the coast
during the morning/early afternoon hours, followed by the
best storm coverage moving steadily inland.

There is almost unanimous agreement in the longer term NWP
guidance in a shift back to a well-defined southeast flow
pattern for the upcoming weekend. The southeast flow will
keep the higher rain chances on the west side of the state,
and with a generally later start time (late
afternoon/evening). For boaters, stronger southeast flow
does pose of danger of storms developing in the later
afternoon hours and rapidly moving offshore into the coastal
waters. We frequently see instances during strong southeast
flow were storms develop and move offshore rapidly enough
to block the path back to safe harbor. Plenty of time to
watch the forecast through the week, but for those looking
to get some pleasure boating done next weekend, keep this
potential in mind.

AVIATION (20/06Z through 21/06Z)...
High level cirrus dissipating early this morning with VFR
conditions prevailing region-wide. A few isolated storms
over the coastal waters should stay offshore through 12-15Z.
A typical distribution of mainly afternoon/evening storms
expected for Monday, with greatest concentration of storms
along and inland from the I-75 corridor. Winds light E/SE in
the morning shift onshore near the coast in the afternoon.
Showers/storms dissipate within a few hours of sunset this
evening. Remove mention of VCTS after 01Z.

MARINE...
Weak high pressure will remain over the region through
the middle of the week keeping winds and seas low. Winds and seas
may briefly become higher in the vicinity of scattered
thunderstorms.

FIRE WEATHER...
Abundant low level moisture keeps relative humidity values well above
critical levels through the week. A typical summer distribution of
mainly afternoon and evening thunderstorms can be expected each day.

Fog Potential...Fog is not expected the next several days.

&&

.Preliminary Point Temps/PoPs...
TPA  91  77  91  78 /  50  40  30  20
FMY  89  75  91  76 /  60  20  40  30
GIF  91  75  92  75 /  60  50  40  30
SRQ  89  77  89  78 /  40  30  20  10
BKV  91  74  91  75 /  50  50  40  10
SPG  91  78  90  79 /  40  30  20  10

&&

.TBW WATCHES/WARNINGS/ADVISORIES...
FL...Beach Hazards Statement through this evening for Coastal
     Charlotte-Coastal Lee-Coastal Sarasota.

Gulf waters...None.
&&

$$

AVIATION...05/Carlisle


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