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National Weather Service Forecast Discussion

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264
FXUS64 KHGX 291909
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
209 PM CDT Tue Sep 29 2026

...New DISCUSSION, MARINE...

.KEY MESSAGES...

-Hot and humid conditions will continue through Wednesday.

-Periods of showers and thunderstorms are expected each day, peaking
from Wednesday through Saturday. Locally heavy rain will be possible.

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

&&

.DISCUSSION...
Issued at 204 PM CDT Tue Sep 29 2026

I think my colleague had a good idea with breaking this forecast up
into short-term, long-term, and longer-term periods, so in order to
keep the details of this forecast more organized and not get lost in
the weeds, we will continue with that set up.

So, let`s start with the short-term forecast period...

We are already seeing the result of persistent onshore flow in the
lower levels this afternoon as showers continue to move into the
Coastal Plains from the Gulf. Looking at satellite derived PWAT
values we can see a wedge of 1.8" where those showers are developing
and moving inland. A closer look at the 850 mb shows decent moisture
convergence in that area; however, mid-upper levels are on the drier
side (that will change in the coming days) and we still remain under
the influence of a ridge (ridge axis is bisecting the area)...all
that to say, should primarily see shallow showers through the rest
of the day. The afternoon sea breeze may be able to generate an
isolated thunderstorm or two, but the favorable dynamics for that
are very limited (decent capping, marginally unstable lifted index).


As we get into the overnight hours tonight into Wednesday morning,
that ridge will continue to shift east, and we will see an increase
in cloud cover over much of the area with the eastern periphery of
our widely discussed trough approaching the area. By this point the
remnants of Hurricane Polo (whose landfall into Mexico is imminent)
are expected to merge into the overall flow of the the mid-upper
level trough. Withe the trough approaching and the ridge exiting,
the resultant tightening pressure gradient may lead to gusty winds
(around 20-25 mph) at times this afternoon through Wednesday.

Wednesday is still anticipated to be more of a transition day
weather wise. The positioning of southeast Texas on the eastern edge
of the trough will provide the area access to mid-upper level
moisture brought in from the Pacific, and we will see this moisture
gradually increase through the day(GFS soundings show a slow
transition to a deeper saturated profile Wednesday into Thursday and
PWAT values approach 2" ). Additionally, models continue to show a
surface trough developing over West Texas, stretching SEward into
Mexico, and with its proximity to the surface high pressure
stretching across the SE CONUS, expect to see the pressure gradient
tighten across coastal areas. Conditions are bordering the criteria
for a Wind Advisory across the Barrier Islands (sustained winds of
25 mph or greater), so will hold off on any advisories for this
forecast package.

Otherwise, we can expect similar conditions to today for
Wednesday...temperatures reaching into the 90s, heat indices getting
into the triple digits for much of the area, and continued showers
streaming in from the Gulf.

Alright...insert intermission and coffee break to process all of
that information here..

Now lets get into the meat of the forecast...Wednesday night through
the weekend

By Wednesday night models are in pretty good agreement with the
position of this mid-upper low being centered over the
Texas/Oklahoma Panhandles. PWAT values across our area will range
from 2.0-2.4" (feel like a broken record here, but this value is
well above the 90th percentile for our area). Models continue to
show the potential for a few embedded shortwaves and associated vort
maxes moving overhead late Wednesday into Thursday, and these could
generate a few showers over the Brazos Valley/Piney Woods. The
environment looks a little more unfavorable for any convection, so
expect this to be primarily showers.

Closer to the coast it looks like we may have decent moisture
convergence in the lower levels, so wouldn`t be surprised to see a
few showers along the coast overnight Wednesday into Thursday
morning (honestly kind of similar to how the radar has looked
through the day today).

During the day Thursday models continue to show some disagreement
with the positioning of this trough. The GFS has the center of the
mid-upper level low lifting into Kansas, with a secondary area of
low pressure developing over eastern New Mexico. ECMWF has a slower
progression (as has been the case for the last few days), with the
center of the low showing a slight eastward progression into the
eastern part of the TX/OK Panhandles. In both cases, we see a slow
moving frontal boundary associated with the parent low, and this is
expected to generate widespread showers from the Brush Country of
South Texas, through the Hill Country, and NE into the DFW area.
Most of the rainfall Thursday is expected to occur just outside of
our CWA...models have been consistently showing that the last few
days, but with the amount of moisture overhead, and embedded
shortwaves and vort maxes continuing to move across the area ahead
of the boundary, our CWA isn`t entirely out of the woods with the
flood potential...additionally, there is some concern with the
models continuing to disagree on where the frontal boundary
stalls...If the front pushes any further eastward and stalls,
interests in the Brazos Valley would be more likely to see the
impacts.

We continue to expect the heaviest rainfall for our area to occur
Thursday night into Friday, and this is also where the forecast gets
a little tricky...particularly with the disagreement among
models...so going to try to break it down a bit.

By this point in the forecast period, our overhead environment is
primed for heavy rainfall. We have DEEP tropical moisture, NBM
probabilities for PWAT values greater than 2.0" range from 60% to
100% from east to west respectively...Ensembles are showing the same
picture with NAEFS and ECMWF ensembles both showing PWAT values at
max levels during the Thursday night into Friday timeframe.

My concern lies with the 12Z run of the GFS (not to put all of the
eggs in one basket...but in this environment, certainly not
something to discount either). The 500mb level shows a second area
of low pressure forming as the parent trough lifts NE. This new low
becomes centered over west Texas with several embedded impulses
moving overhead in a closer proximity to the Brazos Valley, and a
stationary front draped across Central Texas. This scenario could
result in showers with heavy rain rates(3-4"/hr) across the Brazos
Valley as strong moisture convergence occurs out ahead of the
stationary front. This would be the area to keep an eye on over the
next day/day and a half (hi-res guidance window ends just before
this period).

WPC currently has all of southeast Texas in a marginal to slight
risk for excessive rainfall Thursday into Friday morning. I do want
to point out that the Brazos Valley is just outside of the axis of
heaviest rainfall (and WPC`s moderate risk area), so any deviation
to the east could result in an upgrade to the excessive rainfall
outlook...it could also trigger a Flood Watch for that area (holding
off on that for now).

WPC maintains a Marginal Risk for the entire area through the
majority of the weekend, so will need to continue to monitor the
flood potential through this time. In any case, it is important
to continue to monitor the forecast as we approach this event.

With the previous paragraph, it`s relevant to say that rain is
expected to continue through the weekend as elevated moisture
persists. Current QPF for the area through the weekend sits around
2-4" with the higher totals further north and west (coastal areas
could see some 3" totals with the influx of Gulf moisture).
Again, with the uncertainty in the forecast, don`t get too fixated
on the current QPF...it is still subject to change and could
increase as we get closer to the event.

A brief nod to temperatures...they`ll be more seasonal later in the
week and into the weekend (thanks rain and clouds).

Bailey

&&

.AVIATION...
(18Z TAF Issuance)
Issued at 1236 PM CDT Tue Sep 29 2026

Isol showers west of I-45 continue to move inland and may increase
in coverage as the sea breeze. The SE breeze will continue to
increase over the afternoon with gusts up to 20kts across all
terminals. Tonight, winds prevail out of the SE between 5-10kts.
CIGs begin to fill in during the morning hours as tropical moisture
arrives creating the potential for MVFR CIGs and becoming the main
flight category impact. With the arrival of moisture, PROB30 for
showers are becoming more confident. At this time TS has been left
out due to lower probabilities and confidence on terminal impacts.
Coverage and timing will continue to be monitored going forward.

CMW

&&

.MARINE...
Issued at 204 PM CDT Tue Sep 29 2026

Light to occasionally moderate onshore flow and 1-3 ft seas will
persist through this afternoon before winds persist closer to the
20-25 kt range late Tuesday into Wednesday. Small Craft
Advisories will likely be needed during this time period due to
the elevated winds and subsequently increasing seas (into the 5-8
ft range in the Gulf). Water levels during the latter half of the
week will peak around or just above 3.0 ft MLLW, not enough for
coastal flooding hazards but high enough 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.

Batiste/Bailey

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)  74  95  76  93 /   0  40  30  90
Houston (IAH)  78  92  79  92 /  20  40  30  60
Galveston (GLS)  83  89  83  89 /  20  40  30  50

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Should Exercise Caution through Wednesday evening
     for GMZ330-335-350-355-370-375.

&&

$$

DISCUSSION...Bailey
AVIATION...CMW
MARINE...Bailey/Batiste

NWS HGX Office Area Forecast Discussion