Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS62 KTBW 221158

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
758 AM EDT Sat Jul 22 2017

Showers and thunderstorms this morning near the coast will
expand inland through the afternoon. Have a tempo at KSRQ
where there is currently a thunderstorm in place, and VCTS
elsewhere for nearby storms. Activity should subside after
01z, with light and variable winds overnight. 


.Prev Discussion... /issued 313 AM EDT Sat Jul 22 2017/ 

07Z water vapor and H4 RAP analysis show the Florida peninsula under 
a region of weak flow aloft. In the mid-levels our region resides 
between an area of upper ridging over the Atlantic...and a weak 
upper low retrograding westward across north-central Gulf 
of Mexico. While the low to our west has vertical extent 
into the upper troposphere, the upper ridge to our east does
not. Above the H5 level, this ridge quickly breaks down and
transitions to a TUTT cell migrating westward over the 
Bahamas. All-in-all the upper levels are rather complex over
our vicinity of the world, yet no feature is really going 
to have a significant impact on our weather today. 

The main belt of northern stream westerlies are displaced well to 
our north (where they should be this time of year), and any synoptic 
features of lift associated with it, will also stay well away from 
our region. 

At the surface, the subtropical ridge axis remains aligned over the 
south-central Florida peninsula, providing our region with a light 
1000-700mb SW flow pattern, that will persist for the extent of the 
weekend. This deeper low level SW flow pattern is climatologically 
favorable for late night/early morning convective activity forming 
along the land-breeze over the coastal waters...and then
tending to migrate onshore. We have seen this the past 
couple of mornings, and today will be no different. Activity
will not be widespread, but those out early this morning 
near the coast can expect to dodge a brief shower or two, 
perhaps a few rumbles of thunder. This flow pattern, because
winds are weak but also tend to push convective cells to 
the coast, is also favorable for the development of isolated
brief and generally weak waterspouts near the coast within 
a few hours either side of sunrise, as cells migrate 
nearshore and encounter old/residual boundaries to enhance 
updraft vorticity stretching. Keep in mind, it does not take
strong convection to produce these morning waterspout 
events. Even a weak updraft encountering a favorable 
thermodynamic and vorticity profile can spin up a brief visible

Short Term (Today through Sunday)...
Today...Light southwest flow dictates the main weather features 
through the day. Southwest flow is highly favorable for the 
development of shallow coastal showers/storms during the pre-dawn 
hours, which then migrate onshore during the morning hours. 
Generally speaking these showers do not survive far inland within a 
few hours of sunrise, as they migrate away from their source of 
focus and instability. Therefore, highest rainfall chances through 
15Z will remain near the coast. Thereafter, diurnal 
insolation and resulting instability further inland supports
increasing convective potential along outflow/sea-breeze 
boundaries migrating inland with the synoptic flow. Will 
increase rain chances over inland areas during the afternoon
to coincide with climatology under light SW flow. Generally
speaking, by late in the afternoon, rain chances should have 
significantly fallen off close to the coast...and this will
be in the forecast. However, the light flow can 
occasionally fail to prevent outflow from inland storms to 
migrate back westward, providing focus for another round of 
storms back toward the coast in the evening. This scenario 
is very difficult to accurately predict far in advance...and
is not part of the current forecast. Scattered storms will 
become more isolated with time after sunset and generally 
end over the land zones by midnight. Temperatures will end 
up within a couple degrees of 90 for most locations, close 
to where high temps should be in late July.

Sunday...After midnight the continued southwest flow will favor
re-development of scattered shallow convective activity 
over the coastal waters. There are some indications that the
low level pressure gradient during the pre-dawn hours will 
favor slightly better low level convergence/focus off the 
Nature Coast compared to previous mornings, and a resulting 
higher coverage of showers developing and migrating toward 
shore from Pinellas County northward through the morning 
hours. Will have highest rain chances Sunday morning over 
the northern coastal waters and adjacent land zones, and 
then similar to today, decrease activity near the coast 
during the second half of the afternoon, while increase rain
chances inland from the I-75 corridor. 

LONG TERM (Sunday Night - Friday)...
Very little change in the overall pattern is forecast for the first 
half of next week, with the sub-tropical surface ridge continuing to 
extend from the Atlantic into the southern half of the Florida 
Peninsula. This will keep weak winds in place, although the low 
level flow will generally be a bit more westerly/southwesterly north 
of Interstate 4 along the northern side of the ridge. With abundant 
moisture available, the pattern will continue to feature scattered 
early morning thunderstorms developing over the Gulf and shifting 
east into the Florida Peninsula through the morning and early 
afternoon. Storm coverage will become more numerous each afternoon 
under strong surface heating, before storms dissipate and shift into 
the interior through the early evening hours. 

During the second half of next week, a mid level trough will build 
into the eastern seaboard, shifting the surface ridge to the 
southeast. This will bring a slight increase in the westerly low 
level flow, but the increase will not be enough to drastically alter 
the pattern of morning convection over the Gulf shifting east into 
the interior throughout the day. Scattered to numerous storms will 
still be expected each afternoon, with the highest coverage 
generally expected over the interior. Temperatures will remain 
seasonable, with highs around 90, and morning lows in the mid to 
upper 70s.

Aviation (22/07Z through 23/06Z)...
General VFR conditions can be expected through the morning 
hours. However, southwest flow will promote a scattering of 
showers/storms migrating onshore from the Gulf. Any terminal
may see a brief cig/vis restriction with the passage of one
of these showers. The majority of the showers/storms should
migrate inland from the terminals during the later 
afternoon/evening hours, with the exception of KLAL. Winds 
outside of any storms generally remain less than 10 knots 
through the TAF period. 

Surface ridge axis remain in a position over the southern half of 
the Florida peninsula into the eastern Gulf of Mexico through the 
next several days, providing the region with a generally light 
southwest to south wind. Winds and seas will generally be quite low, 
however mariners should expect scattered thunderstorms, especially 
during the overnight and morning hours. 

No significant fire weather concerns expected as a moist summer 
pattern remain in place through the next several days. Winds will 
generally be light from the south or southwest, with relative 
humidity well above critical values. Scattered storms can be 
expected near the coast each morning...with activity spreading 
inland with time through the day. 

Fog Potential...No significant or widespread fog or visibility 
restrictions are expected through the next several days. 


.Preliminary Point Temps/PoPs...
TPA  89  78  88  79 /  50  30  50  20 
FMY  90  76  90  77 /  40  20  40  20 
GIF  91  75  90  75 /  50  30  50  20 
SRQ  87  78  87  77 /  50  30  50  20 
BKV  89  74  88  74 /  50  30  50  20 
SPG  89  78  88  79 /  50  30  50  20 


Gulf waters...None.

National Weather Service Glossary of Abbreviations