-- Discussion containing changed information from previous version are highlighted. --
961
FXUS64 KBRO 171716 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1216 PM CDT Tue Sep 17 2024

...New AVIATION...

.SHORT TERM...
(Today through Wednesday)
Issued at 309 AM CDT Tue Sep 17 2024

Mid-level ridge will be the main influence for the weather over Deep
South Texas and the Rio Grande Valley. This will bring more drier
air to the mid and upper levels of the atmosphere. While this will
make the environment less favorable for the development of showers
and thunderstorms for the short term period. Isolated showers and
thunderstorms are still possible with the sea breeze during the
afternoon hours. In addition, with the PWAT values are still
expected to be over 2 inches through the period as well, which
signals the potential for any showers and thunderstorms that do
develop to have some heavy downpours with them. PWAT values are
expected to decrease further into the short term forecast period as
more dry air moves into the region.

As for the temperatures, the highs are expected to steadily increase
for today and tomorrow. For the most part the high temperatures are
really expected to be in the low 90s. However by tomorrow, the high
temperatures will be in the mid to upper 90s for the area. With the
surface flow still out of the southeast bringing in plenty of low-
level moisture, the heat indices are expected to climb today as
well. For today the heat indices are expected to be in the range of
104 to 107, which is just shy of the need for a Special Weather
Statement for the elevated heat indices today. Tomorrow is a
different story, as the heat indices are expected to be even higher
tomorrow, with most of the area experiencing heat indices of 110 to
113. With those current expectations a combination of Special
Weather Statement for elevated heat indices and Heat Advisory could
be needed. On the other hand, the low temperatures for tonight are
expected to be in the 70s for the entire region.

&&

.LONG TERM...
(Wednesday night through Monday)
Issued at 309 AM CDT Tue Sep 17 2024

Key messages:

* Oppressive heat builds and peaks on Wednesday and especially
  Thursday, where Heat Advisories may be needed.
* Slightly more comfortable conditions arrive this weekend.
* Outside of local sea breeze driven showers or isolated storms, no
  significant widespread rainfall is expected.

Overall, the extended period looks to feature slightly above normal
temperatures for deep south Texas, but with the arrival of somewhat
more comfortable outdoor conditions by this weekend. A series of
unseasonably potent shortwave troughs will dig into southern
California, allowing the swath of below normal 500mb geopotential
height field to expand eastward. However, the area of below normal
500mb height field across the southeast U.S. will also expand
westward into the Appalachian Mountains. The result is for a
neutrally tilted omega block to become positively tilted along an
axis from Great Bend, TX into the Upper Peninsula of Michigan. In
other words, deep South Texas and the RGV will likely see continued
late summer warmth through the start of astronomical fall, which
begins on the morning of Sunday, September 22. For reference,
typical daytime highs are in the 91-94 range, while overnight lows
are around 73-76. The ridging regime with weakly forced mesoscale
and synoptic environment means that the main rainfall and convective
chances will likely come from local sea breeze boundaries. This
means that on most days, PoPs will generally remain below slight
chance (less than 15 percent) west of I-69C/US-281 corridor, with a
slight to low chance PoP (15 to 25 percent) along and east of it.
Temperatures wise, with dew points remaining elevated in the mid to
upper 70s, ensemble guidance shows elevated probabilities of
apparent temperatures AOA 105 degrees on Wednesday and Thursday. The
experimental WPC heat risk tool currently shows pockets of moderate
(level 3 of 4) heat risk on Wednesday and more widespread moderate
heat risk on Thursday. This is in line with our current thinking
that there is increasing likelihood we might need SPS on Wednesday
and Heat Advisories on Thursday for portions of the RGV,
particularly Hidalgo as well as inland portions of Cameron, Willacy
and Kenedy counties when heat indices top out in the 111 to 115
range.

Heading into the weekend, rain chances increase into the 25 to 35
percent range especially west of the I-69E corridor as an upper
level trough approaches Texas from the northwest. There remains
questions on how far south the trough gets into south Texas, never
mind deep south Texas and the RGV. If that is the case, the daily
shower and thunderstorm chances for the RGV will still be primarily
driven by mesoscale features such as our typical sea breeze
boundaries. We do have higher confidence in somewhat more
comfortable conditions for outdoor activities, as heat index values
top out in the upper 90s to mid 100s. PWAT values are forecast to
fall into the 1.4 to 1.7 inch range, which is between 25th
percentile to median of SPC sounding climatology for the
Brownsville, TX upper air site. In other words, the air would feel
less oppressive and more tolerable outside, just in time for the
arrival of autumnal equinox. This is in line with the WPC minor heat
risk category (level 1 of 4) on both days of the weekend. Overnight
lows would also be closer to climatological values for this time of
the year, bottoming out in the 72-75 range this weekend.

&&

.AVIATION...
-- Changed Discussion --
(18Z TAFS)
Issued at 1212 PM CDT Tue Sep 17 2024

VFR conditions will prevail through the forecast period. Some
showers and thunderstorms have already developed around KBRO and
isolated activity is possible through the rest of the afternoon
across all terminals that could briefly lower visibility to MVFR.
If a heavy shower or thunderstorms passes directly over a
terminal brief IFR conditions can not be ruled out but confidence
is low at this time on direct impact.
-- End Changed Discussion --

&&

.MARINE...
Issued at 309 AM CDT Tue Sep 17 2024

Today through Wednesday...Mostly favorable conditions are expected
for the Laguna Madre and Gulf waters. Light to moderate
southeasterly seas and low seas are expected to persist through
Wednesday. Isolated showers and thunderstorms are possible and could
result in heavy rainfall and elevated winds and seas.

Wednesday night through Monday....Outside of any convection,
favorable marine conditions (light to moderate winds and light
seas) are expected to prevail Wednesday night through next
Monday.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             91  78  91  79 /  40  10  30  10
HARLINGEN               92  76  93  76 /  40   0  30   0
MCALLEN                 96  78  97  79 /  40  10  20   0
RIO GRANDE CITY         93  76  95  77 /  30  10  10   0
SOUTH PADRE ISLAND      88  81  88  82 /  30  10  20  10
BAYVIEW/PORT ISABEL     89  77  90  78 /  30  10  20  10

&&

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

$$

SHORT TERM...64
LONG TERM....99
AVIATION...68