textproduct: Houston/Galveston

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

(18Z TAF Issuance) Issued at 206 PM CDT Wed Sep 30 2026

Scattered SHRA will impact the western portions of the area this afternoon. Placed PROB30s in the KCLL/KUTS TAFs for 20-24Z. Expect this activity to remain mostly west of the Houston terminals today. Southeast winds will gust 15-20 knots this aftn. MVFR ceilings will develop late tonight for the far inland areas. Only placed BKN ceilings at KCLL for now. An MCS is expected to develop over the Hill Country/Edwards Plateau tonight and progress eastward across Central TX late tonight. SHRA/TSRA associated with this system will approach the western areas Thursday morning. Have VCTS starting 15Z Thursday for KCLL with VCSH other terminals by late morning. Placed PROB30s for thunder at the Houston terminals starting 16Z.

35/MM

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.