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Area Forecast Discussion (AFD)

Issued by Brownsville/Rio Grande Valley, TX (BRO)

FXUS64 KBRO 171439 AAB

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
939 AM CDT Tue Sep 17 2019

.DISCUSSION...Updated PoP/Wx/Sky grids in alignment with latest
satellite/radar trends and short-term model guidance, in addition
to collaboration with CRP. With fairly robust low-level SW flow 
already showing up on KBRO VWP, will be tough to get much precip
going, so have kept coverage isolated at best over land. Might
still see a brief coastal shower through late morning...and CAM's
also still hinting at a few showers across the northern 
Ranchlands this afternoon, so won't go totally silent.  Temps 
could spike up a few degrees, particularly from Hwy. 281 westward,
this afternoon, but have opted to just monitor obs for now. Text 
products updated and disseminated.


.PREV DISCUSSION... /issued 635 AM CDT Tue Sep 17 2019/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Mostly clear skies with light and variable winds
prevail across deep south Texas this morning. Light west to
southwest winds later this morning will become southerly by late
this afternoon. Expect VFR conditions to continue for the next 24
hours. However, a brief period of MVFR conditions will be 
possible in areas of heavier showers and thunderstorms this 
afternoon. The best rain chances should remain north of the Rio 
Grande Valley aerodromes, so will not mention any TEMPO wording
for convection at this time. 

PREV DISCUSSION... /issued 436 AM CDT Tue Sep 17 2019/ 

SHORT TERM (Today through Wednesday): Satellite imagery this 
morning continues to show an upper level over the northwestern 
Gulf of Mexico. Showers and thunderstorms have increased overnight
with the associated tropical disturbance/broad surface low 
pressure area. This mid to upper level feature is expected to move
over the middle to upper Texas coast tonight into Wednesday. The 
latest National Hurricane Center Tropical Weather Outlook has a 
low chance, 30 percent, of this feature developing during the 

Deep South Texas will remain on the south side of the 500 mb low. 
The west to northwest flow aloft and slightly drier air in the mid 
levels, should allow for limited rain chances through the short 
term. Lowered POPS from the previous forecast but higher than the 
latest MAV and ECMWF guidance due to ample moisture in place. BRO 0Z 
Upper Air data from Monday evening indicates Precipitable Water of 
2.17 inches. Can not rule out a heavier shower or thunderstorm at 
any location today or Wednesday as some of the activity could be 
efficient rain producers. Temperatures today and Wednesday will be 
mainly depend on rainfall, with highs generally near normal to 
slightly above normal, especially across western portions of the CWA 
with better rainfall chances further north and east.

LONG TERM (Wednesday Night through Monday): The 500 mb closed low
and associated broad surface troffing that is expected to bring
widespread rainfall to the middle and upper TX coastline is
expected to continue moving slowly northwards from Wed Night
through Fri ahead of a deep and broad 500 mb trough digging across
the western third of the lower 48 states. As this closed low lifts
northwards, 500 mb ridging will then rebuild over the Deep South
TX region which will help dry out and stabilize the atms resulting
in less conv potential. Enough residual moisture will likely
linger over the region to warrent a mention of some low end conv
chcs mainly near the coastline and over the offshore areas. 

As the 500 mb ridging builds steadily over the area and the deeper
layer tropical moisture values start dropping off a bit, expect
daytime high temps to be a bit warmer in comparison to the temp
trends over the last couple of days. Will mention some highs near
the century mark for Zapata County throughout the longer range
forecast periods with highs across the central and eastern
counties remaining above climo for the latter half of Sept. 

Heat indices will also be on the uptick with the highest readings
reaching the 100-108 range pretty much each day through next

The GFS and ECMWF are in reasonable agreement through Days 7/8 in
the handling of the 500 mb features over the region. Run to run
consistency with both models are also pretty decent. The biggest
differences with the models remain the pops over the longer range
with the GFS coming in a bit wetter versus the ECMWF. Overall
confidence in the longer range forecast wording is above average
at this time. 

MARINE (Now through Wednesday): A broad area of low pressure may
develop just north of the coastal waters later today and slowly 
move inland across Southeast Texas tonight. A slightly elevated 
easterly swell will continue to be directed towards the lower 
Texas coast today then slowly subside tonight and Wednesday. Light
westerly winds will prevail across the coastal waters today 
before becoming southerly tonight as weak low pressure moves 
inland. Isolated to scattered showers and thunderstorms are 
expected through the period. 

Wednesday Night through Saturday Night: After the broad 500 mb low
and associated surface troffing to our north lifts farther
northward into the central Plains states, expect the broad surface
ridging over the SE U.S. to remain in control over the Gulf of
Mex. This will maintain a light to moderate S-SE low level flow
across both the Bay and Gulf waters with moderate seas expected
through Sat Night. The PGF may be strong enough to produce some
potential SCEC conditions from Wed Night through Thurs Night.
However widespread SCA conditions are not expected at this time.  




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