322
FXUS64 KBRO 190909 CCA
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
406 AM CDT Thu Sep 19 2024

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

.SHORT TERM...
(Today through Friday)
Issued at 404 AM CDT Thu Sep 19 2024

Key messages:

* Oppressive heat continues today, with heat index values ranging
  from between 105 to 112.
* A Special Weather Statement for elevated heat indices is in effect
  this afternoon.
* There is a low chance (15 to 25%) of showers and isolated storms
  this afternoon. Better chance (20 to 35%) of showers/storms Friday
  afternoon.

Midlevel high pressure over Texas will continue to minimize rain
chances today. 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 will be capable of producing brief heavy rainfall.
However, the overall upper level ridging over our area will favor
mostly dry weather and efficient daytime heating so have largely
kept PoPs at or below 20 percent. Given the lack of large scale
synoptics, showers or isolated storms that develop would be
triggered by the sea breeze boundary.

Forecasting heat index values for this afternoon is quite tricky,
mainly because of the potential for boundary layer mixing. Hi-res
guidance are quite consistent in depicting an uptick in southeast
winds this afternoon, gusting to 15-20 mph. Forecast soundings
indicate an inverted v profile, or an increase in dew point
depression with height. The National Blend of Models (NBM) is likely
too moist with the boundary layer, so blended in the 25th percentile
for dew points especially west and along the US-77/I-69E corridor.
Our CWA`s Special Weather Statement (SPS) criteria is 108-111 while
Heat Advisory criteria is 111-115 for two or more hours over 50
percent of the forecast zone. The thinking is that dew points should
get into the 73-76 range away from our coastal zones. With ambient
air temperatures around 94-98, that air temperature/dew point
combination would yield heat indices in the 107-111 range, maybe
locally 112-113 out towards southern Hidalgo. The wild card would be
the delayed onset of mixing, but this would be a low probability
scenario. Regardless of SPS or Heat Advisory, today will once again
be uncomfortably if not oppressively warm with temperatures only
cooling into the 77-81 range this morning across the RGV and dew
points staying elevated in the 76-79 range. Whether the heat index
value is 110 or 112, please exercise precaution if spending extended
amount of time outdoors today.

With sunset, there are some indications of convection developing
across our western zones overnight tonight but given the overall
upper level ridging in place, confidence in any widespread
convection is low. With the same air mass in place, it would be
another muggy night with lows generally in the 76-82 range. Heading
into Friday, model guidance suggests a slight erosion of the upper
level ridge. This would favor a slight uptick in shower and
thunderstorm chances for Friday afternoon. Therefore, PoPs are
marginally higher than the past couple of days, rising into the 20
to 35 percent range. Current maximum heat index values for Friday
afternoon exhibit a wider spread, ranging from 102 to 110. A SPS may
once again be needed, but the confidence is lower given the more
widespread rainfall chances.

&&

.LONG TERM...
(Friday night through Wednesday)
Issued at 404 AM CDT Thu Sep 19 2024

To begin the long term Friday night, a mid-to-upper level ridge will
continue to hold its grip over Deep South, situated to to the east-
southeast of an upper level low and associated trough, centered over
southern California. Throughout the course of the weekend, the upper
level low will advance northeastward and over the Rocky Mountains,
riding along the ridge`s western and northern side, gradually
flattening the ridge`s curvature and overall strength. As the ridge
still holds over Deep South Texas, chances of showers and
thunderstorms are limited through Sunday across much of the CWA,
aside from a 20% to 30% chance of isolated afternoon showers and
thunderstorms along the seabreeze boundary for the coastal counties
(Cameron/Willacy/Kenedy) and the middle RGV.

By Monday morning, the ridge will be significantly flattened and
stretching across the Gulf coast states. Meanwhile, a strong upper
level ridge will push into the Pacific Northwest states and dig the
trough, mentioned above, southeastward, causing the ridge over Texas
to weaken into the middle of next week. Meanwhile, at the surface,
an inverted trough over the Gulf of Mexico may fuel more moisture
into Deep South Texas, bringing an uptick in afternoon showers and
thunderstorms through Wednesday, rising POP`s to at least 20% across
all of the CWA for Monday and as high as near 40% across the mid to
lower RGV and coastal counties. By Wednesday, further breakdown of
the ridge aloft could result in at least a 30% chance of rain across
all of Deep South Texas as well as an increased risk of convection,
up to 45%, with heavy downpours across the coastal counties and the
mid RGV.

Temperatures are expected to be near normal with inland Deep South
Texas daily maximum temperatures reaching into the low to mid 90s
and minimum temperatures falling into the mid to upper 70s, at least
through the weekend. Special Weather Statements (SPS) could still be
needed for the mid and upper RGV, especially on Saturday, as heat
indices may briefly hit 111 F in these areas for an hour or two. As
cloud coverage and chances of showers and thunderstorms build
through Wednesday, daily minimum temperatures may range from the
lower to mid 70s and highs could peak into the lower 90s with mid
90s still possible across the mid to upper RGV Monday through
Wednesday. As for the barrier islands, daily high temperatures in
the mid to upper 80s and lows in the upper 70s to lower 80s are
expected.

It should also be noted that the National Hurricane Center (NHC) has
given an area of the northwestern Caribbean Sea and southeastern
Gulf of Mexico has a medium (40%) chance of tropical development in
the next 7 days. They indicate that a broad area of low pressure may
form in the western or northwestern Caribbean Sea over the weekend,
or early next week, and could undergo gradual development into a
tropical depression as it tracks to the north or northwest over the
northwestern Caribbean and into the southeastern Gulf of mexico into
the middle of next week. We will continue to monitor and relay NHC
updates as changes in intensity and, or track could alter the
upcoming forecasts.

&&

.AVIATION...
(06Z TAFS)
Issued at 1241 AM CDT Thu Sep 19 2024

Expect VFR conditions to prevail at all terminals for the TAF
period with southeast winds at 5-10 kt. Between 10-12z, there
could be brief SCT-BKN MVFR ceilings and/or a period of localized
fog. Still thinking that there could be some isolated to widely
scattered showers with possible embedded thunderstorm between
18-22z time frame, but confidence in the temporal and spatial
details remains too low to indicate a prevailing or tempo group in
this set of TAFs. If thunderstorms do develop and pass over or
near the terminals, there could be brief localized higher gusts or
brief MVFR or even IFR conditions.

&&

.MARINE...
Issued at 404 AM CDT Thu Sep 19 2024

Now through Friday...Generally favorable marine conditions will
prevail with seas 1-2 feet and southeast or east southeast winds at
6-12 mph, with 15-20 mph gusts at times this afternoon into early
evening. There is a 15-25 percent chance of showers and
thunderstorms this afternoon, rising to 20-35 percent chance Friday
afternoon. Waves and winds may be briefly higher during
thunderstorms.

Friday night through Wednesday...Light to moderate winds and light
seas of 1 to 3 feet are expected through Tuesday and possibly
building to 4 feet on Wednesday. Chances of daily showers and
thunderstorms remain slight through the weekend but could still
result in brief periods of heavy rainfall, gusty winds and elevated
seas. Chances of showers and thunderstorms increase slightly on
Monday through Wednesday.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             92  79  90  79 /  20  10  30  10
HARLINGEN               92  77  91  75 /  30  10  30   0
MCALLEN                 97  80  96  79 /  20  10  30   0
RIO GRANDE CITY         97  79  96  77 /  10  10  20  10
SOUTH PADRE ISLAND      87  82  87  81 /  20  10  20  20
BAYVIEW/PORT ISABEL     89  78  89  78 /  20  10  20  10

&&

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

$$

SHORT TERM...99-Chai
LONG TERM....65-Irish
AVIATION...99-Chai