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FXUS62 KTBW 170710

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
310 AM EDT Tue Sep 17 2019

07Z water vapor and H4 RAP analysis showing a highly amplified 
northern stream upper level pattern over North America. Split flow
arriving over British Columbia dives south into a sharp trough 
along the Pacific Northwest coat before a rapid cyclonic turn 
northward into a broad and amplified upper ridge extending over 
the middle section of the CONUS, northward into central Canada. 
The flow then finally dives back to the southeast, carving out a 
broad trough over New England and Nova Scotia. It is this slowly 
amplifying trough that is expected to capture Hurricane Humberto, 
now spinning of the southeast conus coast, and eject the system 
northeastward into the north Atlantic.

Closer to home, the Florida peninsula resides under a deep layer 
NW/W flow between Hurricane Humberto and an area of upper ridging in 
place across the NW Caribbean / SE Gulf of Mexico. The 00Z KTBW 
sounding profile still showed an appreciable PWAT value over west-
central Florida around 1.84" (right about normal for mid-September), 
however, a much drier column will be arriving from the north/NW 
within the mean flow through the next 12 hours. The 00Z KTLH 
sounding from NWS Tallahassee sampled this drier column not far to 
our northwest with a PWAT value of only 1.18". 

At the surface, the pressure gradient is becoming rather light as 
Hurricane Humberto continues to drift east/NE away from the region, 
and a weak ridge of continental high pressure attempts to build down 
over the region and eastern Gulf. The radar is very quiet early this 
morning and with the drier air starting to arrive as we speak, think 
this quiet nature will continue through the duration of the day. 


.SHORT TERM (Today through Wednesday)...
Today/Tonight...Benign weather day on tap for the region. A much 
drier column airmass will be working southward through the Florida 
peninsula, with most reliable ensemble members showing PWATS 
dropping below 1.5" for all areas by early this afternoon (some 
areas considerably below 1.5"). This drier air will keep the 
forecast rain-free for all areas today and tonight. Not much in the 
way of clouds will allow for maximum insolation through the 
afternoon hours, and temperatures are going to respond, reaching the 
middle 90s for most. Lower 90s can be expected close to the coast 
where onshore flow develops after 12/1PM. Seasonable 
temperatures...mainly in the lower to middle 70s are in the forecast 
for lows tonight. 

Wednesday...Global guidance showing an interesting flow pattern 
developing aloft during Wednesday. Stronger flow diving down the 
eastern seaboard around the backside of Hurricane Humberto is shown 
to propel a pseudo-backdoor frontal system across the region from NE 
to SW. This frontal system should be exiting the FL west coast and 
across the eastern Gulf of Mexico by later Wednesday night. Most 
ensemble members are showing just enough moisture pooling (PWATS 
back up toward 1.7-1.8") and focus for ascent along this boundary, 
especially when it interacts with the west coast sea-breeze, to 
force a round of scattered late day showers. These shower chances 
will be greatest near the coast within this enhanced convergence 
zone. Shower chances then push westward into the Gulf of Mexico with 
the boundary Wednesday night. A significant intrusion of drier 
column air is then forecast to arrive in the wake of this boundary 
for Thursday. More on this drier air in the long term discussion 

.LONG TERM (Thursday through next Monday)...
A significant push of dry air arrives from the northeast on Thursday 
in the wake of a frontal boundary. This airmass will again make rain 
potential very low across the region. There may be just enough 
moisture in the very lowest levels to support a few speed 
convergence showers developing along the FL east coast and moving 
inland, however, it is unlikely any of these make it far enough to 
impact Sumter, Polk, Highlands counties...and therefore have PoPs 
10% or less for all areas. What may be most significant for us with 
this airmass is that with good diurnal mixing during the day, we 
will mix down some rather comfortable air for mid-september. 
Dewpoints Thursday afternoon are likely to drop into the 60s for 
many of us. These dewpoints would feel rather pleasant compared to 
the humid summer air we have experienced for several months. 

Friday into the weekend we watch a disorganized tropical wave slowly 
migrate westward toward and through the Florida straits. This system 
will increase the moisture levels the most across south Florida, and 
rain chances will be highest well south of I-4. However, the trend 
has been further north with the moisture return the past few model 
runs, so have added at least low end rain chances for areas up into 
Tampa Bay/ Polk county for the weekend (especially in the 
afternoon). This general pattern persists into Monday.  


.AVIATION (17/06Z through 18/06Z)...
VFR conditions expected to prevail through the forecast period. Dry 
air in place will keep rain chances very low. Light variable to 
north flow through the morning hours becomes northwest 10-15 knots 
in the afternoon/early evening. 


High pressure holds over the eastern Gulf today and tonight with 
generally light winds and seas. Light winds and seas are expected 
for most of Wednesday, however, an approaching front from the 
northeast is likely to bring a round of late day showers and storms. 
These showers end Wednesday night, but easterly winds quickly 
increase to cautionary levels Thursday into Friday. A period of 
advisory level winds and seas is possible Thursday night. Mariners 
planning to boat on the eastern Gulf later this week should monitor 
the latest forecast for updates on these anticipated rough 


Drier conditions this week in terms of rain chances and dewpoints 
compared to what we have seen the past several months. A few late 
day showers, especially near the coast on Wednesday, but otherwise, 
very low rain chances the next several days. Relative humidity 
during the afternoon hours will be lower than we have seen lately, 
but still remain above critical levels, and no significant fire 
weather concerns are anticipated the next several days. 

No significant smoke or fog concerns through the middle of the week.


.Preliminary Point Temps/PoPs...
TPA  93  75  93  73 /   0   0  40  10 
FMY  93  74  92  73 /   0   0  40  10 
GIF  95  73  93  72 /   0   0  30   0 
SRQ  93  75  93  73 /   0   0  40  20 
BKV  94  71  93  71 /   0   0  40  10 
SPG  93  76  93  74 /   0   0  40  20 


Gulf waters...None.

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