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National Weather Service Forecast Discussion

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921
FXUS64 KHGX 302255
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
555 PM CDT Wed Sep 30 2026

...New AVIATION...

.KEY MESSAGES...

- Periods of showers and thunderstorms are expected each day,
  peaking Thursday through early Sunday. Locally heavy rain will
  be possible and could lead to instances of flash flooding.

- The heavy rainfall threat shifts from the Brazos Valley on
  Thursday/Friday to the coast on Saturday/Sunday.

- Daytime temperatures are expected to drop to more seasonal
  levels in response to rain and increased cloud cover.

&&

.DISCUSSION...
Issued at 206 PM CDT Wed Sep 30 2026

Continuing on with the next segment of the Southeast TX forecast
series...btw, have been printing out and annotating the previous
AFD and this morning`s was 6 pages long, so A LOT of information
to process. If you enjoy a light and nerdy read, grab a beverage,
kick back and relax, and let`s dive in...

Going to continue breaking it down into the near-term and long-term
to break up the information load a bit. We are *just* starting to
get hi-res guidance for the timeframe we are most concerned with,
and I will touch on that in a bit.

So where are we at right now? Satellite derived PWAT values are
getting juicy...we have a pocket of 1.9-2.2" PWAT vales south of I-
10 and west of I-45, and if we take a look at SPC`s mesoanalysis
page we can see that there is a significant moisture transport in
the lower levels. This can be attributed to our position on the
western side of high pressure in the lower levels. Since we have
that pretty hefty low pressure system approaching from our west,
it`s creating a tighter pressure gradient, which leads to stronger
winds pulling in more moisture from the Gulf. In addition to that,
we have some moisture convergence in that area, so once again seeing
showers and a couple of thunderstorms moving north from the
Matagorda Bay area. We will see that trend continue through the day,
with more development along the sea breeze possible this afternoon.
This tighter pressure gradient has also led to breezy winds
across the area with several locations gusting to 20-25 mph. I
have not seen this trend across the Barrier Islands just yet, so
holding off on a Wind Advisory for now. This may be re-evaluated
this evening if winds trend higher than what is currently
forecast. At the synoptic level, remnants of Polo have merged with
the mid-upper level trough, which is currently centered over New
Mexico.

Going into tonight models continue to show a nocturnal LLJ
developing with a southwest to northeast orientation across Central
Texas to E Oklahoma. The main region of lift lies just to our west,
and with a slow-moving cold front pushing southeastward through the
northern/central portion of the state, we continue to expect that
this is where most of the rainfall will be focused tonight into
Thursday.

Getting into Thursday, PWAT values increase to 2.0-2.5", with the
highest PWAT values residing in the Brazos Valley and Piney Woods
areas. While the main front will still be west of our area, moisture
convergence out ahead of the front, along with embedded shortwaves,
and a nearby coastal trough, could provide an opportunity for
scattered showers and thunderstorms throughout the day Thursday.
Highs for Thursday will be mainly in the 90s, with slightly cooler
temperatures wherever rainfall occurs.

Alright...so the other thing we will be closely monitoring tomorrow
is how this low pressure system evolves. I mentioned yesterday (as
did the evening forecaster) that the GFS had a cutoff low developing
on the tail end of the mid-upper level trough...well, looks like
several models have hopped on this trend with the 12Z run
today...albeit, those solutions are far less aggressive than the GFS.

The reason our attention is drawn to that is that we have already
been seeing models consistently showing the high PWAT values,
embedded shortwaves, as well as some frontogenic forcing
(particularly across the Brazos Valley) Thursday night into Friday.
If it trends more towards the GFS solution, we could see the heavy
rain axis drift a little more eastward into the Brazos Valley. This
would increase the flash flood threat for portions of the Brazos
Valley, and with deep tropical moisture overhead, we could easily
see rainfall rates peak above the 3-4"/hr range with heavier
downpours. Again, this potential depends heavily on the position of
the front, whether or not it stalls, and if this cutoff low develops
(and does it drift southward).

That`s on the synoptic level, and I think it is worth noting that
several other models have hopped on the cutoff low trend.

Mesocale gets a bit messier...Hi-Res guidance is just starting to
dip into the timeframe of concern (and not sure it shows the full
picture quite yet). The 12Z run of the HRRR seems to have
initialized well today, so that`s what I will be primarily using
for the mesoscale level.

As mentioned in the above paragraph, during the day Thursday we will
see a line of storms become focused along and ahead of the frontal
boundary that is draped across Texas from Big Bend up to DFW. We
will see this line slowly approach southeast Texas during the day
with coverage of showers and thunderstorms across our area
potentially becoming more scattered (again, have a nearby coastal
trough that is influencing the dynamics closer to the coast in
addition to moisture convergence ahead of the trough). That first
line of storms looks to arrive in our western counties sometime
Thursday afternoon, and looks to weaken as we get closer to sunset.

A second line of storms looks to develop behind the first line and
is forecast to move through the area overnight Thursday into Friday.
This is where we start to see more of the potential for heavier
downpours, and is going to be the time when we would monitor for the
potential for flash flooding to occur. Again, the highest risk
during that timeframe would be for the Brazos Valley area.

This frontal boundary is projected to move through sometime Friday
and may linger around the area...This is where we are continuing to
monitor for the potential for an increased risk of flooding along
our coastal counties this weekend. By this point we will still have
high PWAT values, embedded shortwaves, the nearby aforementioned
front, and a coastal trough/low in the NW Gulf. Most of the rainfall
looks to occur just offshore, but as mentioned by the previous
forecaster, a ~10-20 mile margin of error could make a difference.

WPC`s Day 2 ERO now has a Moderate Risk clipping portions of the
Brazos Valley, with a Marginal to Slight Risk for the rest of the
area. A Marginal Risk for excessive rainfall remains in place for
most of the area through Saturday, with a Marginal Risk for the
coastal areas through Sunday.

So how much rain are we looking at? Generally speaking, most of the
area will fall into the 2-4" range through Friday, and the Brazos
Valley could see isolated higher amounts.. Coastal areas could see
more this weekend (2-3").

The frustrating thing with this forecast is that our area is
dry...we haven`t had a lot of rainfall lately (except for the
southern counties that have seen some showers the last couple of
days. Taking that into account, our soils could handle a good amount
of rainfall...the concern comes with how fast that rain will fall
and for how long will we have those high rain rates...and that is
hard to pinpoint, even with Hi-Res models...so trying to not "cry
wolf" but also want to point out that there are some concerns that
we are monitoring.

Without getting too far into the weeds (I`ll let my cold front
fanatic colleague dive into the details with that), there is the
potential to see cooler weather next week. An upper level trough is
projected to dip into the northern CONUS, dragging a cold front and
a surface high in its wake.

We could see the tail-end of this front push into our area during
the early to middle part of next week. We will continue to monitor
the trends, but not going to put too much focus on it that far out.

Bailey

&&

.AVIATION...
(00Z TAF Issuance)
Issued at 541 PM CDT Wed Sep 30 2026

VFR conditions to start this TAF period, with SE winds around
10-15 kts and gusting to around 20 kts. Visible satellite shows
some SCT-BKN clouds at around FL040. Winds decrease to the 8-10
kt range overnight, while remaining out of the SE. Early tomorrow
morning, an MVFR level cloud deck will likely impact KCLL and
KUTS, and could reach KCXO as well. Expect MVFR CIGS at FL020-030
beginning at around 08Z at KCLL, then moving eastward through the
early morning hours. A boundary pushes east into our region,
increasing chances for showers and thunderstorms tomorrow
afternoon. Storms will be numerous in nature. Showers may start as
early as 14Z, with the peak of the activity being in the
afternoon hours. Because of the excess moisture available to these
storms, any showers/storms that do develop may be capable of
producing heavy downpours, limiting VIS at terminals. Intermittent
MVFR CIGs may develop with these storms, with low-end VFR CIGs
prevailing. SE winds increase back to the 10-15 kt range, gusting
to 20 kts through the afternoon hours. Shower and thunderstorm
activity looks to start tapering off towards the end of this TAF
period, but a VCSH/VCTS is upheld through 00Z tomorrow.

MLG

&&

.MARINE...
Issued at 206 PM CDT Wed Sep 30 2026

Moderate southeasterly winds and gradually building seas (peaking in
the 5-8 ft range late Wednesday/early Thursday) will persist into
Thursday. A Small Craft Advisory is in effect for the bays and
Gulf waters through at least Thursday morning. Water levels during
the latter half of the week will peak around or just above 3.0 ft
MLLW, so not enough for coastal flooding hazards but high enough
for mariners to take note. Daily chances of showers/storms
continue especially during the overnight to early morning hours
over the Gulf. Expect rain chances to increase after midweek as an
upper level disturbance pushes into TX along with a slow-moving
frontal boundary. Rain chances look to peak over the weekend for
coastal areas and the Gulf as a frontal boundary and a coastal
weather system interact.

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)  75  93  72  84 /  20  80  80  70
Houston (IAH)  79  93  77  89 /  20  70  60  90
Galveston (GLS)  83  89  82  88 /  10  40  40  60

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...High Rip Current Risk through Thursday morning for TXZ436>439.

GM...Small Craft Advisory until 7 AM CDT Thursday for GMZ330-335-350-
     355-370-375.

&&

$$

DISCUSSION...Bailey
AVIATION...MLG
MARINE...Bailey

NWS HGX Office Area Forecast Discussion