Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 201133

Area Forecast Discussion
National Weather Service Brownsville TX
Issued by National Weather Service Lake Charles LA
633 AM CDT Tue Oct 20 2020

.DISCUSSION...Updated for latest /12Z/ aviation discussion below.


For the 20/12Z TAFs...Little change from previous TAF thinking.
Generally benign conditions expected as sfc high pres ridges
southwestward along the TX coast. This will result in southeast
winds 5-10 KT during the day, with winds shifting more easterly
and diminishing after sunset. Outside of brief IFR cigs at MFE
early this morning, expect VFR conditions to prevail with FEW-SCT
CU with bases around 2500 FT lifting to 4000-5000 FT by aftn. Any
clouds will decrease this evening. Some low clouds are expected to
develop toward daybreak Wednesday, and inserted SCT decks between
1000-1500 FT although some intermittent MVFR cigs may be possible
by the end of the TAF period.



.PREV DISCUSSION... /issued 417 AM CDT Tue Oct 20 2020/

SHORT TERM (Today through Wednesday): The main story continues
to be late summer- like heat with a bit more modest humidity as
region remains under weak steering pattern between broader
subtropical 500 mb ridges along the Mexican Pacific coast and the
southeast U.S. coast. Despite an upper-level weakness between the
ridges...moisture is limited to a fairly thin layer near the
surface that rises with daytime heating/mixing to near 850 mb.
Perhaps a touch more low level moisture by midday Wednesday with
subtle but somewhat notable increases in precipitable water values
(from ~1.25 today to ~1.5 Wednesday)...though these values are
below average today and near average for late October on

For the sensible weather, the lack of deeper moisture and the time
of year argue for no mentionable rain in the forecast today...with
any morning low clouds in the mid/upper Valley and ranchlands
burning off to a mix of sunshine and a few clouds with temperatures
once again between 90 and 95...5 to 8 degrees above average. A
mainly clear and mild night once again with low clouds forming up in
the same areas.  For Wednesday...with lighter easterly flow and the
hint of higher moisture, went ahead and added isolated sea-breeze
showers along the US 77/IH-69C corridor mainly between 10 AM and 2
PM before mixing of dry/very dry air aloft kills off the
opportunity. Guidance trimming temperatures a hair but not seeing
where this comes from so will lean toward the warmer ECMWF which
matches closely with persistence.

Great weather or a trip to the pool or the beach...which brings us
to potential tidal impacts on the barrier islands.  As mentioned
this time Monday...a lull in waves/swell/push did occur at the most
recent high tide...and departures from predicted of ~+0.3 kept water
above dry land on the beach front to around 1 foot...which thinned
the beaches but did not totally flood them. Forecast daytime low
tides with perhaps a slight uptick in west-moving swell should keep
the beaches in good stead today but the combination of tonight`s
highest seasonal tide (0.72 feet above Mean Higher High Water at
Brazos Santiago, just inshore of the Isla Blanca Jetty, with a bit
more swell "push" should get water levels close to the dunes at
narrow beaches and leave little usable beach elsewhere beginning
around or just after sunset (high tide is at 1007 PM).

At minimum a Coastal Flood Statement is a lock, but an Advisory is
possible for water reaching the dunes or moving farther into the
gaps between them. Will have the oncoming shift issues one or these
hazards by early to mid afternoon as tides begin to rise. Wednesday
night looks like the best opportunity for these conditions with
predicted high tides similar to tonight`s but with a more pronounced
perpendicular push of higher waves due to gradient between broad low
pressure near the Yucatan and persistent high pressure over the mid
Atlantic region.

Needless to say rip current threat will be an issue as surf will
look deceptively inviting but moderate intensity rips are likely by
this afternoon and even more on Wednesday.  Could see high intensity
as early as Wednesday afternoon but a little too soon to pinpoint so
going with moderate for now.

LONG TERM (Wednesday Night through Monday): The general weakness
in the 500 mb ridging sprawling across the southern portion of the
country will persist into Wed Night/Thurs morning. This broad
500 mb troffing over the RGV/Deep South TX area will start lifting
out to the NE on Thur as ridging builds eastwards over the region
from northern Mex. The 500 mb ridging will remain in place over
southern TX through this upcoming weekend. Then a large 500 mb
troough will dig into the Rockies and the central portion of the
lower 48 states this weekend into early next week. This southward
digging 500 mb trough will a couple of cold fronts into south TX.
The first being a fairly weak front that will struggle to move
through the RGV on Sat. Then the 500 mb trough axis will push a
much stronger cold front through the Border Region late Mon
setting up the stage for a pretty significant cool down of temps
next Tues.

Overall moisture values remain pretty limited across the region
through the upcoming weekend with the 500 mb ridging building over
the area. Significant moisture levels do not return to the RGV
until the stronger cold front approaches the area early next week.
Will likely see enough moisture return ahead of this front to
justify some isold/sct conv starting Mon Night into Tues.

The GFS and the ECMWF models are in decent agreement in the
overall synoptic pattern through Days 7/8. However, the ECMWF
continues to show a warmer bias on temps for much of the upcoming
forecast period versus the slightly cooler GFS. Tend to prefer the
warmer ECMWF solution for temps through next Mon and then prefer a
model blend for temps after the main fropa next week.

Overall forecast confidence in the longer range period is above
average this morning due to the GFS and ECMWF models coming into
better agreement.

MARINE (Now through Wednesday):Gradient between the mid Atlantic
ridge and the broad low pressure over the Yucatan Peninsula has
brought speeds up to or just above 15 knots overnight...but expect
the diurnal drop to arrive by daybreak and a steady 10-12 knot
easterly flow dominate today over the Gulf. Winds should pick up
again later this evening before dipping again on Wednesday to
similar values...backing a bit more easterly in the process.

Seas at NOAA Buoy 42020 were running just under 4 feet this morning
and a bit above 4 feet at TABS Buoy 45 (40 nm east of South Padre)
and NOAA Buoy 42002 (200 miles east of South Padre). Swell was in
check so far but beginning to rise at buoy 42002. NOAA WaveWatch III
is a bit slower to rise seas tonight but NWPS is a bit more bullish
on overnight swells into Wednesday (peaking beyond the short term).
Would not surprise to see need for Small Craft Exercise Caution on
Wednesday for seas (6 feet) if trends are stronger than the
WaveWatch for for now capped at 5 feet.

Laguna Madre conditions look ideal through the period with just a
freshening of easterly winds each afternoon at 10 to 15
knots...continuing through midnight before diminishing.

Wednesday Night through Saturday Night: The persistent east to
west surface wind flow pattern over the Gulf of Mex will likely
maintain pretty elevated Gulf swells at least through Thurs. This
could potential produce some marginal SCA conditions for the lower
TX Gulf waters Wed Night and Thurs. The PGF then relaxes somewhat
Fri and Sat as a weak cold front approaches from the north
allowing for the winds and seas to weaken. The more sheltered
Laguna Madre waters should be more relaxed throughout the period
with no SCEC/SCA conditions expected through Sat Night.



BAYVIEW/PORT ISABEL  89  74  88  72 /  10  10  20   0
BROWNSVILLE          91  73  90  73 /  10  10  20   0
HARLINGEN            92  71  91  71 /  10   0  20   0
MCALLEN              94  74  93  69 /  10   0  10   0
RIO GRANDE CITY      94  72  93  70 /  10   0  10   0
SOUTH PADRE ISLAND   85  77  84  78 /  10  10  20   0




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