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Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 220555 AAA
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1255 AM CDT Wed May 22 2019

.DISCUSSION...Updated for latest aviation discussion below.

&&

.AVIATION...IR satl imagery over the RGV shows the MVFR cld decks
are starting to expand over the area. So will maintain MVFR
ceilings for all 3 RGV airports throughout the overnight and early
morning period. The increasing S-SE low level winds will combine
with the daytime heating to mix out the low morning ceilings
allowing for VFR conditions to prevail. The persisting low level 
moisture will likely result in another round of MVFR ceilings
after sunset Wed. 

&&

.PREV DISCUSSION... /issued 629 PM CDT Tue May 21 2019/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...VFR is prevailing at this time, but MVFR is expected
to develop within the next few hours. Although surface winds will
diminish around sunset, they are expected to increase overnight 
tonight. Windy conditions are anticipated tomorrow as cloud decks 
transition from MVFR to VFR. 

PREV DISCUSSION... /issued 312 PM CDT Tue May 21 2019/ 

.Very High to Extreme Heat Indices or "Feels-Like" Temperatures
Expected Wednesday...

SHORT TERM (Now through Wednesday night): Little change in the 
recent hot and windy weather as the synoptic pattern remains 
persistent. Mid-level trough over the Western US and mid-level ridge 
extending over the Gulf remains in tact for the next 36 hours and 
beyond. Guidance and raw model data show little to no change in 
temperatures, dew points, thicknesses or heights as well as other 
parameters to warrant any significant changes to the ongoing 
forecast.  If anything, will raise high temperatures a few degrees 
for Wednesday with a slight 10-20 decameter increase in heights and 
850mb temperatures progged to be 23-35C or about one degree warmer 
than today as the ridge axis drifts westward. Pressure gradient 
Wednesday is not expected to be as strong as Monday or today (in the 
east) but remains respectable for another breezy to windy day. 

A heat advisory for "feels-like" temperatures of greater then 110
degrees may be warranted for most of the Rio Grande Valley and as
well as portions of Zapata county at least 2 to 4 hours Wednesday
afternoon. The frontal boundary that is moving into Zapata and 
Jim Hogg county should retreat north or dissipate overnight 
allowing for dew points to recover well into the 70s later tonight
with little mixing out tomorrow. Dew points remain elevated over 
the Gulf, 79 degrees at buoy 42020, and with the persistent 
southeast fetch do not see any reason for dew points to lower. 
Guidance and raw data agree in this reasoning. 

LONG TERM (Thursday through Tuesday): As the steering flow
becomes more amplified towards the end of the week, low pressure 
will deepen across the Western U.S., resulting in a continuously 
unsettled pattern the next several days. This will allow ridging 
to build and retreat westward across the Central Gulf of Mexico, 
and bring hot and dry conditions to Deep South Texas. Models keep 
surface dew points relatively high through the long term with 
solid southeasterly low-level flow and little diurnal mixing. This
combination with hot afternoon temperatures in the mid to upper
90s would result in heat indices periodically exceeding 105F. 
Though hot temperatures are common across Deep South Texas, we are
running several degrees above normal in some areas, so we want to
stress the importance of heat safety. Model time-heights show 
moisture limited to below 850mb with strong dry air and subsidence
aloft, therefore, rain chances will generally be 5% or below each
day.

MARINE (Now through Wednesday night): Pressure gradient has been 
weakening over the Laguna and Gulf waters much of the late morning 
and early afternoon allowing for the onshore flow to diminish, seas 
however continue to be agitated and will likely remain elevated much 
of the night. Have cancelled the SCA for the Laguna Madre but have 
maintained the SCA for the Gulf waters mainly for the elevated seas. 
The hazardous marine conditions will continue to slowly improve as 
the gradient is likely to maintain current strength which may allow 
for the SCA to be cancelled before 7 am tomorrow morning. This is 
likely to be temporary as pressure begin to lower out west Wednesday 
afternoon and evening as the next upstream shortwave trough 
approaches west Texas inducing another Southern Plains low pressure 
area. SCA's are likely for the Laguna Madre Wednesday afternoon and 
evening and once again for the Gulf Wednesday night, if not already 
cancelled. 

Thursday through Tuesday: A series of surface lows developing in
the Plains and a surface high sitting across the Northern Gulf
will result in a tight pressure gradient through much of the long
term period. Small Craft Advisories will be possible through
at least Saturday. Models indicate winds should improve
thereafter, but Small Craft Exercise Caution conditions will be
likely.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 7 AM CDT this morning for GMZ150-155-
     170-175.

&&

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

Short Term/Aviation...60
Long Term...53