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Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 221732 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1232 PM CDT Wed May 22 2019

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

&&

.AVIATION...Visible satellite and local obs confirm plentiful 
low cloud cover, though ceilings are lifting/scattering out on
moderate to breezy southeast winds. Mid level ridging is ensconced
over the lower Southeast United Sates, with high pressure over the
North Gulf of Mexico. This will support persistent southeast winds
over the next 24 hours, with continued low level moisture influx.
Guidance advertises a lowering of ceilings to MVFR this evening 
and overnight, with light to moderate southeast winds. Ceilings 
will lift and scatter out Thursday during the day with another 
round of breezy southeast winds.

&&

.PREV DISCUSSION... /issued 737 AM CDT Wed May 22 2019/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...IR and Vsbl satl imagery over the RGV shows the MVFR 
cld decks remain in place this morning. So will maintain MVFR 
ceilings for all 3 RGV airports throughout the midmorning period. 
The increasing S-SE low level winds will then combine with the 
daytime heating to mix out the lower morning ceilings allowing for
VFR conditions to prevail into the afternoon and early evening. 
The persisting low level moisture values will likely result in 
another round of MVFR ceilings after sunset today.

PREV DISCUSSION... /issued 409 AM CDT Wed May 22 2019/ 

.Very high heat indices or "Feels Like" temperatures likely over
the Mid-Upper Rio Grande Valley today...

SHORT TERM (Now through Thursday): A pair of 500-mb closed lows 
will dominate the flow pattern over the northwestern half of the 
lower 48 states throughout the short term period. The northeastern
of these lows will lift out to the northeast later today and 
Thurs eventually reaching the Great Lakes region by midday Thurs. 
Meanwhile the more southwestern of these two 500 mb lows will dig 
more to the south along the U.s. West Coast. This will result in a
building ridge axis over the Gulf Coast and the southeastern U.S.
which will in turn maintain the RGV heat. The 1000-500 mb 
thickness values peak in the 584-586 dam range across the region 
by late this afternoon. This will drive the excessive heat values 
this afternoon with the surface dewpoints remaining high enough to
push the heat index values into Heat Advisory criteria for some 
of the western counties for a couple of hours this afternoon. 

The 1000-500 mb thickness values then lower just a bit on Thurs 
dropping into the 580-582 dam range which will allow for a little 
lower high temps and heat index readings tomorrow afternoon. 

The building 500 mb ridge over the area will maintain a pretty dry 
atms over the RGV. The 00z BRO sounding indicated a very high CAPE 
near 5400 J/KG, but with a very low PWAT of only 1.26 inches. This 
low PWAT is not very conducive for conv. So will be going near zero 
pops throughout Thurs.

LONG TERM (Thursday Night through Tuesday): Hot, often breezy,
and essentially rain-free conditions are likely to continue right
on through Memorial Day weekend, and perhaps beyond. The larger-
scale pattern across the CONUS will continue to feature troughing
across the West and ridging over the Southeast through at least
Monday (Memorial Day). The western edge of this ridge will be the
dominant player in the weather for the Rio Grande Valley and Deep
South Texas, with the stable/subsident mid-troposphere keeping any
appreciable moisture bottled up below 850mb. Consequently, precipitable
water values (per 00Z GFS run) are not forecast to rise above
about 1.5 inches (near the climatological normal for BRO) during 
this time. SE surface winds will maintain elevated dewpoints,
though, contributing to heat index values of 105-109F on Friday 
in the Upper RGV and approximately 101-106F across much of the RGV
for the remainder of the holiday weekend. So seek shade and stay 
hydrated during those outdoor activities!

Some subtle differences begin to show up between the GFS and ECMWF
model output late in the forecast period. GFS finally nudges the
stubborn western trough more into the central portions of the
CONUS, with the ridge over the Southeast flattening. Lee
cyclogenesis ensues Monday/Monday night, perhaps sending the
tail-end of a front, with some modest moisture pooling along it,
toward South Texas by Tue/Wed (Day 7/8). Meanwhile, the ECM holds
the SE CONUS ridging stronger, hence slowing down the progression
of the western trough. Not surprisingly, it is even slower and
less aggressive with any moisture return than the GFS. WPC
suggesting a lean toward the ECM, given that GFS tends to have a
somewhat fast bias out toward D6-D7. Still plenty of time to sort
this out. Have nudged PoP's up just a bit in this timeframe, but
held at silent levels.

Should also note rip current risk looks like it will be elevated
on Friday and Saturday at least. With the heat likely to send
large crowds to the beach over the holiday weekend, swimmers will
need to monitor conditions and take precautions.

MARINE (Now through Thursday):  Buoy020 currently reports a SSE wind 
at 16G19kts with swells near 6.6 feet, while the surface obs near
the Laguna Madre are checking in from the SSE at around 15 kts 
roughly. 

With the large surface ridging sprawling over much of the 
southeastern U.S. and the Gulf of Mex, the 500 mb closed low/surface 
low pressure area building across the Desert SW will likely result 
in a steadily strengthening PGF along the lower TX coastline. This 
will steadily degrade the wind and swell activity across the lower 
TX coastline. Accordingly will post up an SCA for the Laguna Madre 
today and will extend the ongoing SCA for the Gulf waters out to 60 
NM offshore throughout today and tonight. 

Current tidal data for the SPI Brazos Santiago gage indicates that 
the current observed tide levels are running <1 foot above predicted 
levels. The next high tide for SPI is 921 am today. With the 
prevailing wind direction today remaining from the SSE, this will 
not be very conducive for excessive runup on the area beaches. So 
for now will hold off on issuing any Coastal Flooding statements 
this morning.

Thursday Night through Monday: Small Craft Advisories (SCA's)
likely to be ongoing Thursday evening, as the pressure gradient
between troughing over the Plains and a high over the SE U. S.
remains tight. Moreover, the SE fetch should have built Gulf seas
to hazardous levels as well, which should last into Friday night,
at least. The SE U. S. high weakens a bit through the remainder 
of the weekend, eventually leading to a more moderate wind and sea
regime by Sunday. Exercise Caution (SCEC) conditions will still 
be possible, though, especially for the Laguna Madre during the 
daytime and Gulf waters overnight. SE winds may tick back up again
by later on Monday if a new low-pressure area develops in the lee
of the Rockies, as some guidance is suggesting.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Heat Advisory until 6 PM CDT this evening for TXZ248-252-253.

GM...Small Craft Advisory for winds until 7 AM CDT Thursday for 
     GMZ130-132-135.

     Small Craft Advisory until 7 AM CDT Thursday for GMZ150-155-170-
     175.

&&

$$

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