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Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 231702 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1202 PM CDT Thu May 23 2019

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

&&

.AVIATION...CU cloud streets are evident on visible sat imagery 
while TAF site ceilings have lifted above 3 kft on moderate to 
breezy south southeast winds. The morning sounding revealed a 
modest 1.29 inches pwat concentrated in the boundary layer. Not 
much change expected in the next 24 hours as mid level ridging 
will remain centered over the Northeast Gulf Coast, with surface 
high pressure in control over much of the Gulf. Ceilings will
lower to high MVFR this evening through Friday morning with light
to moderate south southeast winds and partly to mostly cloudy
skies. 


&&

.PREV DISCUSSION... /issued 647 AM CDT Thu May 23 2019/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...IR satl imagery continues to show a ragged area of
MVFR/VFR ceilings around the RGV. The strengthening S-SE low level
flow will likely help mix out the lower ceilings to allow VFR
conditions to prevail throughout much of the daylight hours.
Enough low level moisture will reside in the atms to allow for
another round of some MVFR ceilings tonight through sunrise
Fri. 

PREV DISCUSSION... /issued 411 AM CDT Thu May 23 2019/ 

Hot hazy breezy weather to continue until further notice...

SHORT TERM (Today through Friday): 
SHORT TERM (Now through Friday): The 500 mb ridging will remain 
entrenched over much of the southeastern States and the Gulf of 
Mexico through today and Fri. This will maintain a pretty dry and 
subsident atms over the RGV throughout the short term. The 00Z BRO 
sounding continues to advertise a pretty high CAPE >5000 J/KG. On 
the negative side for conv is yesterday evenings PWAT which was a 
pretty dry 1.5 inches. Apparently, the short term guidance from both 
the NAM and the HRRR models indicates that some type of conv was 
supposed to be occurring near the lower TX coastline over the last 
several hours. However the KBRO radar imagery has been clear as bell 
all night. So it is safe to assume that both of these models have 
not initialized well on the current environment over the RGV. So 
will be maintaining near zero pops throughout Fri which is more in 
line with the drier GFS and ECMWF guidance.    

Despite the presence of the 500 mb ridging over the region, the 1000-
500 mb thickness values apparently will drop somewhat throughout 
Thurs and Fri which will in turn allow the max temps to "cool" 
slightly both days. These temps will still remain above climo for 
late May. The NAM/GFS and ECMWF short term temp trends remain in 
pretty good agreement through Fri so will maintain a general blend 
of these numbers. 

Since the overall max temps will be lowering a bit through Fri, the 
heat index values will remain elevated but will not reach into Heat 
Advisory territory. 

LONG TERM (Friday Night through Wednesday): Strong subtropical 
ridge parked over the northern Gulf states will continue to extend
through the east half of Texas Friday night through Sunday before
being slightly flattened Monday and Tuesday, then nudged a tad 
east (over the southeast U.S. and Florida) by Wednesday as pieces 
of upper level energy ride from the southern/central Plains 
through the mid-Mississippi Valley. Regardless of what's happening
aloft...only subtle changes in sensible weather will be noted 
across the Rio Grande Valley and ranchlands through the period. 
For the weekend, pockets of very dry air should leave mainly clear
skies by day and probably most of the night as well, though the 
usual periods of low clouds can be expected amidst the moon and 
stars that show through them. The drier air will mix down and help
lower humidity a smidge each afternoon for the weekend, and 
probably Monday as well - keeping apparent temperatures (heat 
index) in the 100 to 104 range area wide despite high temperatures
ranging from 94 to 99. In addition, the drier air may help remove
some of the haze and slightly improve air quality...though low 
end moderate conditions are still a good bet from continued low 
level flow originating over agricultural burns in central America 
and eastern Mexico.

The slight breakdown/flattening upper ridge Tuesday and Wednesday
will increase thicknesses and low level temperatures, as well as
return a few degrees on dewpoints. This will both increase high
temperatures toward the upper 90s in most areas...century mark
along the Rio Grande in Starr and Zapata...and produce heat index
values that will likely reach 105-108 once again. 

Models do show some deeper layer moisture in the whole by
Wednesday, but the details still favor no rain chances as solid
layer (mainly 800 to 400 mb) continues to feature humidity at or
below 25 percent. Thus...no mentionable rain in this forecast.

Otherwise...the persistent "Valley Wind Machine" goes on and on.
With slightly lower winds at night than in recent days along with
the expectation of more of a partly cloudy/mostly clear situation
overnight, expect lows to drop into the mid to upper 70s (vs.
upper 70s to lower 80s of recent days) over the weekend into
Monday. That may change for Tuesday and Wednesday morning with
lows at or above 80 possible...but capped at 79 in urban areas and
toward the coast for now. Some increase in low level moisture will
return a few more clouds but may also bring the haze and moderate
to potentially unhealthy for sensitive groups air quality by the
end of the period.
 

MARINE: (Today through Friday)...Surface obs near the Laguna 
Madre and reports from Buoy020 all show the strong SE surface 
winds persisting near the lower TX coastline. The latest ob from 
Buoy020 showed a gust up to 31 kts. The model surface pressure 
fields near the lower TX coastline are pretty tightly packed so do
not expect any improvement to the Bay and Gulf conditions 
throughout at least tonight. Will extend out the SCAs for both the
Laguna Madre and the Gulf waters with this update. 

(Friday Night through Monday Night)...The slightly lessened 
gradient with the westward extending ridge, as well as a bit of a 
marine layer especially nearshore due to upwelling, should keep 
winds in the 15 to 20 knot range over the Gulf waters by night and
Laguna Madre by day. At the same time, Gulf seas will begin with 
residual 7 foot levels so Small Craft Advisories are likely to 
persist into Saturday for the Gulf. Low level wind fields pick up 
Monday night especially over the Gulf, so Advisories are likely 
once again and seas will follow at the end of the period or 
beyond. Overall...not the greatest boating or fishing forecast 
despite the lack of rain in the forecast.

CLIMATE...The oppressive May heat has folks wondering where the
month ranks on the all-time (since records have been kept) list.
While afternoon highs have poked a couple degrees above average,
the sultry nights have been the difference maker, with many
minimum temperatures 7 or more degrees above average during this
stretch. For the total average temperature, the following
statistics are available:  

Through May 22 (preliminary): 

Brownsville (since 1878)...
2nd all time (83.1 degrees). Record:  84.0 degrees in 2003. 

McAllen (since 1941)...
8th all time (83.6 degrees). Record: 86.3 degrees in 2003.

Harlingen (since 1912)...
5th all time (82.7 degrees). Record: 85.0 degrees in 2003.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory for winds until 7 PM CDT this evening for 
     GMZ130-132-135.

     Small Craft Advisory until 6 AM CDT Friday for GMZ150-155-170-
     175.

&&

$$

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