740
FXUS64 KBRO 242028
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
328 PM CDT Tue Sep 24 2024

...New SHORT TERM, LONG TERM, MARINE...

...Deteriorating surf, marine, and beach conditions likely
Thursday through Friday...

.SHORT TERM...
(Tonight through Wednesday night)
Issued at 319 PM CDT Tue Sep 24 2024

Through this evening...Very localized convection has fired up in
southern Cameron County...right over Brownsville. Convective-
allowing models (CAMs) are not too bullish on too much additional
development beyond this area...will need to keep an eye a bit
farther west along the river but that`s about it.  Better activity
in the Gulf should stay there due to sea breeze stability over the
coastal/near coastal areas. Model blends actually show this fairly
well and updated the forecast to cover...with a pocket of likely
where it`s raining now and lower (isolated/scattered) elsewhere
inland except for the upper Valley/Rio Grande Plains. Another area
to watch is Kenedy/Brooks which is closer to an area of higher
coverage of showers this afternoon.

For tonight, skies will clear as they have in recent nights and
winds will lay down to near calm.  The one exception is late at
night near the coast, as another round of isolated to scattered
showers over the Gulf should edge toward shore. There are some hints
of deeper moisture (compared with recent nights) so could see a few
more late night clouds inland...but this is not explicitly forecast
right now.  Temperatures will be largely unchanged from recent
overnights...plenty muggy with morning lows in the mid to upper 70s.

For Wednesday...models have backed down a bit with rain chances and
QPF...but as northwest flow aloft becomes established by late in the
day we`ll need to watch the approach of a shear axis on the tail of
the actual trough which dives into the lower Mississippi Valley
toward evening. For now...maintained mainly sea-breeze driven
convection, pulling the rain chances away from the nearshore/coastal
areas by mid to late afternoon which is common.  Temperatures and
heat index should be similar to today except if/when/where it rains.
The aforementioned shear axis moves across the region overnight
and the forecast could be a bit tricky. Global models are
diurnally bearish...but CAMs are a bit more bullish including the
HRRR and HRW which show band(s) along/just ahead of the wind shift
(front) that is expected to pass and bring drier (lower humidity)
conditions beginning Thursday. Exactly where any stronger cells
may develop is unclear but the threat is there. For now a stripe
of 20-30 percent chances moving from north to south through the
night, which should exit the U.S. between 4 and 7 AM, if not
sooner. The onset of the drier air will filter across the northern
ranchlands and perhaps reach the RGV by daybreak on slightly
increased northerly winds...and low temperatures will end up near
seasonal averages (70 to lower 70s) by daybreak.

Lastly, as Tropical Cyclone Helene moves toward the Yucatan Channel
and begins its potential rapid intensification cycle overnight
Wednesday, coastal impacts may be delayed until the start of
Thursday (long term period).  That said, tides remain well above
predicted levels...which are in "King" Tide mode and among the
highest levels of the calendar year.  However, the lack of wave
energy should keep coastal flooding (i.e. water up to the dunes)
out of the picture at the early Wednesday morning high tide...and
probably just out of the picture before daybreak Thursday though
we`ll need to keep tabs for a possible Coastal Flood Statement
then, before we begin to consider Advisories Thurday into Friday
due to the back swell from Helene.

&&

.LONG TERM...
(Thursday through next Tuesday)
Issued at 319 PM CDT Tue Sep 24 2024

Key Messages:

-Tropical Storm/Hurricane Helene to have little impact to Deep South
Texas and the Rio Grande valley.

-A cold front will move through the region late week bringing a bit
of relief from the oppressive heat.

The main story and impact to Deep South Texas and the Rio Grande
Valley for the long term period will be the approach and passing of
a cold front. On Thursday, a midlevel low pressure system located
across the middle Mississippi Valley will begin to ingest what will
be Hurricane Helene and cause the aforementioned low to linger
across the middle/lower Mississippi Valley through the end of the
week. The low finally drifts northeast through the weekend and into
early next week with high pressure returning to Deep South Texas a
dry conditions into early week.

At the surface the aforementioned low will drag a cold front through
the CWA Thursday. Forecast CAPE values remain around 1000 J/kg and
PWATs between 1.50 and 1.75 inches on Thursday morning we could see
some isolated to scattered showers and thunderstorms with the
potential for some brief heavy rainfall in spots as the front moves
through the CWA. Rain chances will be confined to the lower Valley
and along the coast with POPs under 30%. In wake of the FROPA, much
drier air will filter into the region.

Along the coast, the combination of the frontal passage and the
arrival of long period swell from Helene will result in increased
wave heights and adverse beach conditions Thursday and Friday.
Enhanced rip current risk and minor coastal flooding will be
possible at beaches along the Lower Texas Coast.

As for temperatures, we won`t see much relief from the warm
temperatures (in some respects) with highs remaining in the low to
mid 90s, but with less humidity and lower dewpoints heat indices
will stay below 100 degrees through the forecast period.


&&

.AVIATION...
(18Z TAFS)
Issued at 1248 PM CDT Tue Sep 24 2024

Scattered showers have prevailed across the Lower Texas Gulf
waters, but the typical "jump" along the sea breeze has been slow
to go. Convective-allowing models as well as global models have
been rather bearish for this afternoon...and with limited forcing
underneath the upper level ridge this makes sense. So...given low
confidence have elected to roll a VCSH for all terminals until the
sea breeze passes and stabilizes things. This would be around 2-3
PM in Brownsville and by 4 PM in Harlingen. Added to McAllen as
well and maintained through sunset.

Otherwise...light winds becoming east around 10 knots post-sea
breeze this afternoon before laying to to near calm by mid
evening, with skies clearing except for few-scattered MVFR
cumulus near Brownsville.

As for Wednesday...incoming models are also more bearish on rain
chances even though energy arrives from the north via an upper-
level trough dipping into the lower/mid Mississippi Valley with
associated shear axis running from Louisiana through south Texas
by midday. This, combined with any early morning showers sneaking
onshore, may be enough to trigger isolated activity for
Brownsville/Harlngen after 9 or 10 AM. For now, called it VCSH
with periodic MVFR ceilings through 1 PM., with McAllen scattered
for now.

&&

.MARINE...
Issued at 319 PM CDT Tue Sep 24 2024

Now through Wednesday Night:  Outside of Isolated to scattered
convection, light winds and slight seas will be the rule through
most of the period through Wednesday.  Wednesday evening will be
fine as well, but conditions could begin deteriorating overnight,
first from a small chance (10 to 20 percent) of a band of
showers/embedded thunderstorms to press south through the waters
during the late evening through postThursday through Monday...What will be Hurricane Helene and the
passage of a cold front will likely lead somewhat adverse conditons
to start the period. As a cold front moves through the waters off
the lower Texas coast on Thursday, isolated to scattered showers and
thunderstorms will be possible along with increased winds Thursday
into Friday. At the same time swell from Helene will arrive on
Thursday as well leading to elevated seas, most likely on the
offshore Gulf waters. Small Craft Should Exercise Caution headlines
or Small Craft Advisories may be needed for the Gulf waters Thursday
and Friday as wave heights look to reach 6-7 feet. Seas are expected
to subside over the weekend, with favorable marine conditions
prevailing through the remainder of the period. midnight hours.
Soon after, winds turn northerly and increase toward 15
knots...while at the same time the first note of easterly swell
and building seas may arrive in 20-60 nm leg before daybreak. For
now...seas are forecast to increase to near 4 feet at that time,
likely a combination of wind wave and early arrival of easterly
swell.

Thursday through Monday...What will be Hurricane Helene and the
passage of a cold front will likely lead somewhat adverse conditons
to start the period. As a cold front moves through the waters off
the lower Texas coast on Thursday, isolated to scattered showers and
thunderstorms will be possible along with increased winds Thursday
into Friday. At the same time swell from Helene will arrive on
Thursday as well leading to elevated seas, most likely on the
offshore Gulf waters. Small Craft Should Exercise Caution headlines
or Small Craft Advisories may be needed for the Gulf waters Thursday
and Friday as wave heights look to reach 6-7 feet. Seas are expected
to subside over the weekend, with favorable marine conditions
prevailing through the remainder of the period.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             78  93  75  89 /  20  40  30  30
HARLINGEN               74  94  71  90 /  10  40  30  20
MCALLEN                 78  96  75  94 /  10  30  30  10
RIO GRANDE CITY         75  94  73  92 /  10  30  30  10
SOUTH PADRE ISLAND      81  87  79  86 /  20  40  40  30
BAYVIEW/PORT ISABEL     77  91  74  87 /  20  40  40  20

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

SHORT TERM...52-Goldsmith
LONG TERM....68-McGinnis