Scientific Forecaster Discussion
fxus62 ktbw 211121
Area forecast discussion
National Weather Service Tampa Bay Ruskin Florida
721 am EDT Tue Aug 21 2018
VFR conditions are expected to prevail outside of any thunderstorms.
Light and variable winds this morning will shift onshore near the
coast early this afternoon with the sea breeze. Scattered showers and
thunderstorms are expected through the afternon and evening, with
activity generally shifting inland through the day. Storms will
dissipate within a few hours of sunset, with dry VFR conditions
Prev discussion... /issued 412 am EDT Tue Aug 21 2018/
08z water vapor and h4 rap analysis continues to show a rather ill-
defined middle and upper level pattern in place over Florida this
morning. 21/00z ktbw sounding profile measured a precipitable water value of just
over 2", with remain in the vicinity of climo for the time of year.
Goes16 multi-layer WV imagery also shows no significant area of
above or below normal moisture over the region. The last of the
convective high level debris cirrus from Monday evening's storms has
dissipated, resulting in mostly clear skies across a good portion of
the peninsula...along and south of the I-4 corridor. At the surface,
the subtropical ridge axis is aligned through the central part of
the peninsula and into the eastern Gulf of Mexico. This ridge
position puts the forecast area in a light/variable wind/convection
pattern for today.
The light synoptic flow has allowed the nocturnal land breeze
circulation to take control of the air flow early this morning, with
most stations reporting light offshore winds. Isolated coverage
of typical early morning storms has formed along this land-
breeze boundary over the coastal waters. From the Tampa Bay
area southward, this flow should be strong enough to keep
this activity offshore through the morning hours. Further
north along the Nature Coast, a position north of the ridge
axis and resulting light SW synoptic flow is keeping the
land breeze less dominant and will likely allow some of
these isolated showers/storms to occasionally make their way
back to the coast during the next several hours.
Short term (today through wednesday)...
today...a light and variable synoptic flow pattern will define the
convective evolution of diurnal storms. Little Signal in the
atmospheric column to enhance or suppress storm development suggests
staying close to the climatological spatial and temporal evolution
for storms during this type of light flow.
Other than a few isolated showers along the Nature Coast this
morning, the rest of the region should see a generally dry first
several daylight hours, as temperatures warm through the 80s. Weak
and feeble sea-breeze development will commence around midday, with
the first scattered storms going up along this boundary in the
vicinity of the coast. As the afternoon progresses, the general
evolution of storms will be to have the favored storm genesis region
progress slowly inland to the north of the Tampa Bay area within a
light SW synoptic flow, and to hold closer to the I-75 corridor from
Tampa Bay southward, with variable to light southeast flow. Beyond the
middle to later afternoon hours, storm evolution will have less and
less to do with the synoptic setup and become highly dependent on
convective outflow. When the flow is light, this outflow finds
little resistance to propagation, but also does not favor storm
development when moving any particular direction. Therefore, this
evening, storm spatial coverage will generally become rather uniform
from the I-4 corridor southward, while still favoring a more inland
position further to the north.
Storms gradually diminish into the later evening hours with the loss
of surface diurnal heating and less instability aloft due to
abundant latent heat release from earlier convection.
Overnight, a weak cold front will progress southward through the
deep south reaching the northeast Gulf Coast and essentially
stalling just to our north by Wednesday morning. Do not get too
excited about a cold front. This is a weak boundary, and will not be
providing any relief from the Summer heat and humidity. Still too
early to expect anything like that. In fact, the presence of the
front to our north will provide a longer fetch of west/SW flow over the
warm waters of the Gulf and likely contribute to even higher
dewpoints through the middle of the week. After midnight into
Wednesday morning, the SW/west synoptic flow will be most defined from
Tarpon Spring northward. The west flow is already climo favored for
morning showers to move ashore, but add the frontal proximity and we
should expect at least scattered, if not numerous showers/storms to
be present along the Nature Coast by the time of sunrise. A few
showers along the coast can be expected further south as well,
however, weaker synoptic flow and further distance from frontal
enhancement should keep coverage considerably less.
The convective pattern for the daylight hours of Wednesday would be
to expect initial scattered storm development near the coast through
midday, but with a pronounced trend toward greater storm coverage
inland with time as the synoptic flow quickly propagates the West
Coast seabreeze inland toward the central part of the peninsula.
Long term (thursday through next monday)...
large scale pattern for the late week and upcoming weekend period
remains fairly consistent from model to model and run to
run...adding higher than average confidence to the forecast. Weak
stationary frontal boundary over the NE Gulf of Mexico and northern
Florida will quickly dissipate during the latter half of the work
week and is replaced by decently strong high pressure building down
the eastern Seaboard. The position of a strong ridge to our north
will provide the region with a well-defined and fairly strong
synoptic east/southeast flow through the upcoming weekend and into the early
portion of next week. This type of flow favors greatest diurnal
thunderstorm coverage each day to occur along the I-75 corridor of
the state, with a generally later start time to the convection
(later afternoon/early evening). Only concern for this part of the
forecast, is if the large scale flow actually becomes strong enough
to prevent a West Coast seabreeze or keeps the feeble boundary just
offshore. If this was to occur, best storm coverage could be
confined to the immediate coast. At this time, will assume some of
the stronger model solutions are overdone, as this type of strong
flow is very unusual for late August. Therefore the forecast will
follow a philosophy of near climo temps and sct-nmrs late
afternoon/evening storms through the end of the forecast period.
Mentioned this last night and will again today. An for your information to boaters...
stronger southeast/east flow can pose a 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/prevent a safe path back to Harbor for anyone navigating away
from the immediate coast. There remains plenty of time to watch the
forecast through the week, but for those looking to get some
pleasure boating done next weekend, it is unlikely that a
significant shift in the forecast philosophy will occur, and it is
important to keep this hazardous potential in mind.
Aviation (21/08z through 22/12z)...
VFR conditions prevail for the morning hours for the west-
central and southwest Florida terminals. Light variable/southeast
winds shift onshore near the coast around midday, with
increasing potential for typical Summer thunderstorm
development into the afternoon hours. Outside of any brief
passing thunderstorm, VFR conditions will prevail. Scattered
storms fade within a couple hours of sunset this evening,
allowing mainly dry/VFR conditions to remain in the forecast
into Wednesday morning.
high pressure continues to reside over the eastern Gulf of Mexico
today, keeping winds and seas (outside of scattered thunderstorms)
low. A weak cold front will approach the region from the north
tonight and then stall over the northeast Gulf Wednesday. This front
will act to shift winds to a more predominant west to southwest
direction, and will likely bring an enhancement to the coverage of
showers and storms, especially to the north of Tarpon Springs. The
front will decay during the later part of the week, allowing a very
weak gradient, and resulting light winds to again develop over the
eastern Gulf of Mexico.
no significant fire weather concerns in the forecast through the
next several days as typical Summer conditions persist over west-
central and southwest Florida. Relative humidity remains well above
critical levels, with a scattering of mainly afternoon and evening
showers and storms each day.
Fog potential...no significant fog or visibility reductions are
anticipated through the next several days.
Preliminary point temps/pops...
tpa 91 78 90 78 / 30 10 30 20
fmy 91 76 90 76 / 30 40 30 20
gif 92 75 91 75 / 60 50 40 10
srq 89 78 89 78 / 20 20 20 20
bkv 90 75 89 75 / 50 20 30 20
spg 91 79 89 78 / 20 20 30 30
Florida...Beach hazards statement through Thursday evening for
coastal Charlotte-coastal Lee-coastal Manatee-coastal