Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tampa Bay Ruskin Florida
901 am EDT Tuesday Jul 26 2016
upper low over Cuba this morning wrapping drier air aloft over the
Florida Peninsula as seen in WV imagery. 12z ktbw sounding showing the
drier air above 20k ft while lower-mid levels remain moist and
unstable. Sensible surface weather will keep warm summertime temps
and humid dewpoints in place for hot heat index values ongoing.
Pops expected to run slightly below seasonal levels with 20-40%
areawide this afternoon along seabreeze and outflow boundaries in
weak S-southeast steering flow. Latest grids and forecasts on track.
expect VFR conditions to continue at terminals except brief
cig/vsby restrictions in and near isolated to scattered afternoon
and evening thunderstorms and rain to keep vc in tafs.
Prev discussion... /issued 437 am EDT Tuesday Jul 26 2016/
08z water vapor and h4 rap analysis shows the Florida Peninsula
resides between a large ridge of high pressure centered over the
Carolinas...and a pair of upper level lows across the Gulf of
Mexico and northwest Caribbean Sea. The upper level low over the Gulf has
moved far enough west to not directly be impacting our weather
anymore...however some fringe influence from the upper low
passing over Cuba/northwest Caribbean is helped support more robust
convection along the west Florida land breeze that has been able
to migrate onshore near the coast early this morning. Normally
much of this activity would stay offshore...however...the synoptic
low level flow was light enough to allow cold pool propagation
eastward onto the coast during the last 2-4 hours. This cold pool
development has essential destroyed the land breeze and seeing a
rapid decrease in the original convection from Sarasota northward
to the Tampa Bay area. Further south...the combination of a
residual land breeze and more upper support from the passing low
to our south is helping fuel additional development of storm over
the coastal waters. Expect these storms to migrate northward
through dawn...possibly also impacting the immediate coastal
areas. Have kept shower mention in the forecast at the coast
through the next several hours.
At the surface...a broad subtropical ridge axis runs across the
central/northern parts of the peninsula. This ridge then runs up
against troughing across the north-central Gulf of Mexico
underneath the parent upper low. The result in a tighter gradient
than usual over our offshore waters early this morning...however
as the day progresses...the troughing is forecast to
subside...which should also decrease the gradient and slacken the
Short term (today through wednesday)...
the synoptic pattern described above will not change much during
today...although the upper low to our south will continue its slow
migration westward into the southeast Gulf of Mexico. Global
guidance shows a swath of drier air aloft trying to move into our
region from the southeast...but at the moment it does not look to
arrive in time to significantly decrease the convective potential
during the afternoon and evening.
At least for this morning...eventually the land will heat up
enough to destroy the land breeze altogether...and even at the
coast we should see several hours with very low rain chances.
During the afternoon...the typical development of showers and
storms can be expected...although at least some arrival of drier
air aloft may hold the coverage down. The question will be...does
the weak support from the passing upper low counter the negative
thermodynamics from the dry air. The subtle nature of deep pulse
convection makes this a difficult question to answer...so as usual
for a Summer day...be prepared for a brief storm...but we may find
it less active than usual. Several of the more reliable hires
convection allowing models are hinting at a less active diurnal
convection event. Otherwise...seasonable temperatures by the
afternoon with lower 90s inland and upper 80s to around 90 at the
Tonight...any evening storms should dissipate by midnight...and
then will see the development of more sct storms over the coastal
waters along the land breeze. Have brought a slight chance for
rain into the forecast along the immediate coast after 2
am...similar to this morning. The pattern does not change
much...so at least some potential must be accounted for. The
influence of the upper low should be decreasing by early tomorrow
morning...so coastal convection should be more isolated.
the drier air aloft looks to be more influential on our diurnal
convection during Wednesday afternoon and evening. Although storms
will be around...confidence in coverage below climo is higher.
Have gone with a 20-30% rain chance for most areas. The exception
is along the Suncoast...mainly south of Sarasota where have gone
with a 40% chance. This 40% chance is still well below the climo
60-70% rain coverage during southeast flow. Went with the slightly
higher pop for these areas due to the more favorable convergence
setup...and the closer proximity to some residual upper support
from the upper low to our southwest over the Gulf. Beyond
Wednesday...all the upper lows are gone...and upper level high
pressure takes complete control over the peninsula. More on the
weather for the later part of the week in the mid/long term
Have a great Tuesday everyone!
Mid/long term (wednesday night-monday)...
stagnant pattern to remain in place through the period. A strong
u/L Ridge will persist over the western Atlantic and Florida with
the main u/l flow remaining across the northern tier of the U.S.
And Canada. Although the u/L Ridge will hold over west central and
southwest Florida through the remainder of the week and the
weekend, an u/l disturbance will dig over the Great Lakes and Ohio
River valley late in the week with heights lowering a bit over the
forecast area. Potential for a TUTT cell to push across the
Florida Straits and western Caribbean this weekend with the
forecast area on the northern side of the TUTT. Some drier cooler
air aloft may push over the forecast area associated with this
feature which may enhance the threat for damaging wind gusts
associated with afternoon thunderstorms Saturday and Sunday.
Surface high pressure will remain anchored over the central Atlantic
with the ridge axis extending across the central Florida Peninsula
through the period. Generally east to southeast boundary layer flow
will continue which will promote scattered afternoon and evening
showers and thunderstorms across the forecast area each day, with
areal coverage increasing along the West Coast sea breeze boundary
each late afternoon/evening. Temperatures will continue to run a
few degrees above climatic normals each day.
high pressure will remain fixed over the eastern Gulf
of Mexico through the remainder of the week. Lighter winds will
keep seas low as well. Mariners can expect the development of
scattered showers and storms near the coast during the early
morning hours each day...with brief locally higher winds and seas
in the vicinity of any storms.
high pressure remains over the Florida Peninsula through the
remainder of the week. Periods of drier air aloft will likely
result in lower coverage of storms during the afternoon and
evening than is typical for late July...however scattered storms
will still be in the forecast. Relative humidity values will be
typical of Summer and generally remain well above critical levels.
Therefore...no significant fire weather concerns are anticipated.
Preliminary point temps/pops...
tpa 92 77 91 78 / 30 20 30 30
fmy 93 76 91 77 / 40 20 40 20
gif 93 75 94 76 / 40 30 30 20
srq 90 77 91 77 / 20 20 30 30
bkv 92 73 93 74 / 30 30 30 30
spg 91 79 92 80 / 30 20 20 30