414
FXUS64 KBRO 180833
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
333 AM CDT Wed Sep 18 2024

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

.SHORT TERM...
(Today through Thursday)
Issued at 328 AM CDT Wed Sep 18 2024

Key messages:

* Oppressive heat builds today and likely peaks Thursday, with heat
  index values ranging from between 105 to 112 away from the coast.
* A Special Weather Statement for elevated heat indices is in effect
  this afternoon. Better chance for Heat Advisories is on Thursday.
* Outside of local sea breeze driven showers or isolated storms, no
  significant widespread rainfall is expected through tomorrow.

Midlevel high pressure over Texas will continue to slowly dry out
the mid levels and minimize rain chances today and tomorrow. Any
showers and thunderstorms that do develop will be isolated in nature
and most likely develop along the sea breeze boundary. With that
said, PWATs are still around 2 inches, which is equivalent to the 75-
90th percentile of SPC sounding climatology for the Brownsville, TX
upper air site. Any showers or thunderstorms that do develop over
the next couple of days will be capable of producing brief heavy
rainfall that could lead to ponding or localized minor flooding
along low lying and poor drainage areas, especially with the ground
being saturated across portions of Hidalgo and Cameron counties from
recent rainfall.

Heat index values are expected to top out in the 108 to 112 range
this afternoon. While some locations, especially along and west of
the I-69C/US-281 corridor, could briefly approach Heat Advisory
criteria (i.e. heat indices of 111+ for 2 or more hours), better
confidence in Heat Advisory issuance will be on Thursday when the
mid level ridge builds in stronger across our region. Therefore, we
have opted to hoist a Special Weather Statement for elevated heat
indices today roughly along and east of a Rio Grande City-Encino
line. There is a low probability of an upgrade to Heat Advisory for
parts of the area this afternoon depending on actual conditions but
once again, confidence in more widespread 111+ heat index values is
higher for Thursday afternoon. Our forecast thinking is in line with
the WPC experimental heat risk tool, which indicates widespread
moderate (level 2 of 4) heat risk across the RGV this afternoon but
a mix of moderate and major (level 3 of 4) heat risk tomorrow
afternoon. Overnight lows are only expected to cool into the mid 70s
to low 80s tonight.

As mentioned above, Thursday is likely to feature the most
widespread oppressive heat across the RGV. Actual air temperatures
will likely make a run for mid to upper 90s across portions of
Hidalgo and inland Cameron county. With dew points in the mid to
upper 70s, this will yield maximum heat indices in the 109 to 115
range, which would trigger the issuance of a Heat Advisory. There
does remain some uncertainty in how high or prolonged the 111-115
heat index values get for Thursday, as some hi-res CAM guidance
indicate the potential for isolated to widely scattered showers with
embedded thunderstorms developing across the RGV. Additionally,
drier air from top of the boundary layer may well mix down to the
surface, keeping heat index values more manageable in the 105 to 110
range. Either way, please exercise precaution if spending extended
amount of time outdoors today into tomorrow.

&&

.LONG TERM...
(Thursday night through Tuesday)
Issued at 328 AM CDT Wed Sep 18 2024

The long term begins Thursday night with a mid-to-upper level ridge
centered over central/western Texas, which will continue to result
in near-normal to slightly above normal temperatures across Deep
South Texas on Friday and Saturday. Additionally, with dewpoints
reaching into the upper 70s, combined heat indices across most of
the County Warning Area (CWA) are expected to range from around 105
F to 111 F on Friday with the highest indices across the mid and
upper RGV though still nearing 110 across inland portions of
Cameron, Willacy and Kenedy counties. A Special Weather Statement
(SPS) will likely be needed on Friday for these areas. Saturday is
expected to be a bit cooler as an upper level trough over California
pushes eastward and begins to break down the ridge over Texas. As a
result, the heat indices are expected to not range as high on
Saturday though an SPS may still be needed across the mid and upper
RGV as heat indices may reach up to 108 F.

Models are in fair agreement that precipitable water (PWAT) values
are forecast to be at least 1.5 inches to near 2.0 inches on Friday,
which is near-to-above normal for this time of the year.
Additionally, with the exception of Zapata and Jim Hogg counties,
there is about a 20% to near 30% daily chance for afternoon
showers and thunderstorms along the seabreeze boundary as it
pushes westward with the best chances of rain along and east of
the I-69 Corridor. Drier air may filter southward into the CWA
and temporarily reduce POPs on Sunday. As the new week begins,
the combination of lowering mid-level heights and the approach of
an inverted trough over the Gulf of Mexico may increase cloud
coverage and bring an uptick in convection along the afternoon
seabreeze boundary, especially along and east of the I-69
Corridor, where POP`s could reach as high as 35% across the lower
RGV.

Temperature-wise, afternoon highs are expected to generally reach
into the lower 90s across inland Deep South Texas with mid 80s
across the barrier islands. Overnight lows are anticipated to fall
into the mid to upper 70s away from the coast Friday and Saturday
morning and then ranging from the low to mid 70s through the rest of
the period. Meanwhile, along the lower Texas coastline, lows will
remain in the upper 70s to lower 80s.

&&

.AVIATION...
(06Z TAFS)
Issued at 1239 AM CDT Wed Sep 18 2024

VFR conditions with light southeast winds 5-10 kt are expected
through the TAF period. The only exception is the potential
development of patchy fog at KHRL and KMFE right around sunrise
between 11-13z. Otherwise, isolated pop-up showers or even a brief
thunderstorm could occur this afternoon due to sea breeze
interaction. However, confidence is too low to indicate in the
TAFs.

&&

.MARINE...
Issued at 328 AM CDT Wed Sep 18 2024

Now through Thursday...Generally favorable marine conditions will
prevail with light winds and seas around 1 to 2 feet. Chances of
isolated showers and thunderstorms will continue at times through
the forecast period that could lead to locally enhanced winds and
seas.

Thursday night through Tuesday...Light to moderate winds and light
seas are expected Thursday night through Tuesday with seas generally
ranging 1 to 2 feet through Saturday becoming 2 to 3 feet through
Tuesday. There is a slight chance of showers and thunderstorms
daily, which may result in heavy rainfall and elevated winds.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             92  79  93  78 /  20   0  30  10
HARLINGEN               93  76  94  75 /  20   0  30   0
MCALLEN                 97  79  98  79 /  10   0  20   0
RIO GRANDE CITY         95  78  96  77 /  10  10  20  10
SOUTH PADRE ISLAND      88  82  88  81 /  10  10  20  10
BAYVIEW/PORT ISABEL     90  78  91  77 /  10   0  20  10

&&

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

$$

SHORT TERM...63-KC
LONG TERM....65-Irish
AVIATION...63-KC