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National Weather Service Forecast Discussion

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204
FXUS64 KHGX 210411
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1111 PM CDT Mon Apr 20 2026

...New DISCUSSION, MARINE...

.KEY MESSAGES...

- Widespread rainfall expected on Tuesday and Wednesday with
  potential for locally heavy rainfall that could lead to
  instances of street flooding.

- Daily chances for showers/storms going into the weekend.

- Gradual warming trend with temperatures reaching the upper 80s
  by the end of the work week.

&&

.DISCUSSION...
Issued at 1111 PM CDT Mon Apr 20 2026

The radar depicted widespread moderate rain, but only light rain
reached the surface. What`s the deal?! Was it magic?! Nope...even
better...SCIENCE! PW values gradually increased throughout the day
up to near the 75th percentile (~1.38") due to the development of a
coastal trough/low down near the South TX coast. Moisture gradually
increased from west to east and there was even a few rumbles of
thunder west of the Brazos River as elevated convection drifted in
from our west earlier in the afternoon. Rainfall rates around the
San Antonio metro were near 2"/hr leading to instances of flooding,
but why did we only see light rain? Drier air above the surface is
the answer! Forecast soundings depicted a pocket of drier air around
3km above the surface for most areas in Southeast TX. As the
moisture gradually increased, the air above the surface became more
and more saturated leading to some raindrops reaching the ground.
This is why most of you saw light rain at most today.

Now how can we tell that the majority of the rain evaporated? The
answer is...our radar! Our radar (the lovely KHGX WSR-88D) is
located to the southeast of Houston in northern Galveston County.
The radar beam height increases the further you get from the
radar...so you may have noticed high reflectivity returns out west,
but those returns became lighter as they got closer to the radar.
The radar was sampling moderate rain aloft, but through observations
at the surface (special thanks to those that used mPING today) we
were able to confirm that evaporation aloft was occuring. At times,
there were enough raindrops reaching the ground to wet
surfaces/roadways, but certainly not enough to cause any rainouts.
You might be thinking "why did he write two paragraphs about
something from the past?!" I do it for the love of the science! :)
Since Tuesday is Big Word Day, I can officially say that science is
supercalifragilisticexpialidocious! I can`t use more than one big
word though because of my hippopotomonstrosesquippedaliophobia.
Before y`all ask, yes spell check did just give up on that one.

Going into Tuesday though, the drier air aloft goes bye-bye as the
atmospheric column becomes fully saturated with an embedded
shortwave trough pushing into the region. PW values will be near or
above the 90th percentile (~1.58"), so there is potential for
locally heavy rainfall. Latest CAM guidance depicts the rain moving
in early Tuesday morning, so plan accordingly for your morning
commute. Instability on Monday was slim to none, but there will be a
bit more in place on Tuesday. Not enough for any worries about
strong or severe storms, but enough for some rumbles of thunder.
This instability should also be enough for efficient rainfall
processes. If heavy rainfall falls over an area for an extended
period of time, then this could lead to localized instances of
flooding. This is especially the case for urban areas or areas with
poor drainage/low-lying areas. As a result, WPC has kept most of
Southeast TX in a marginal risk (level 1 of 4) of excessive rainfall
on Tuesday. Keep in mind that this is taking into account the
somewhat more saturated soils from Saturday`s heavy rainfall, which
could lead to quicker transitions to runoff. Current QPF totals show
a general widespread 1-2", but locally higher amounts will be
possible. HREF LPMM shows the potential for isolated spots of 3+".

We get a small break from the heavier downpours going into Tuesday
night, but another round of rain is expected on Wednesday with
another shortwave passing through. Wednesday looks to start out with
sporadic showers in the morning. Daytime heating aids in the
development of scattered thunderstorms in the afternoon hours. For
Wednesday, the marginal risk of excessive rainfall is currently
mainly for areas east of the Brazos River and south of Conroe.
Convective coverage is not anticipated be as expansive as what we
see on Tuesday, but it still warrants keeping an umbrella with you.
As a matter of fact, go ahead and keep that with you for...*checks
forecast*...ever. Even though moisture availability decreases going
into the end of the work week, there will still be enough in place
to pair with additional embedded shortwaves for daily chances of
showers and storms. Going into the weekend, we`ll be monitoring an
upper level low and a frontal boundary pushing into TX. The front
itself is not expected to push through our area, but it will help
increase chances for showers/storms over the weekend. I know it
seems like it`s rained every weekend this month...and that`s
because it has. It`s for the greater good though! ~79% of
Southeast TX remains in at least a severe drought (not counting
this past weekend`s rainfall). This rain certainly won`t be a
drought buster, but every little bit helps!

As far as temperatures go, widespread light rainfall led to high
temperatures in the mid 60s to low 70s on Monday...so you shouldn`t
be surprised that the same is expected for Tuesday. Ridging aloft
will be in place throughout the work week though leading to a
gradual warming trend after Tuesday. High temperatures are expected
to be back in the upper 80s by Friday and potentially continuing
into the weekend. Just keep in mind that rain chances are present
each day, so some tweaks to the temperature forecast could still
occur as the week goes on. Low temperatures will be in the 50s/60s
for another couple of nights, then only bottoming out in the 60s/70s
midweek and beyond.

Batiste

&&

.AVIATION...
(00Z TAF Issuance)
Issued at 639 PM CDT Mon Apr 20 2026

An upper level disturbance is pushing west to east across
southern TX this evening, producing light shower activity roughly
from I-45 westward. Predominantly VFR conditions this evening and
overnight will go below VFR Tuesday morning as a secondary
disturbance moves into the region from the west. All terminals are
expected to experience scattered shower activity beginning in the
morning and persisting through much of the day on Tuesday. Winds
will gradually increase out of the east area-wide to 10-15 kts
with gusts up to 20-25 kts.

McNeel

&&

.MARINE...
Issued at 1111 PM CDT Mon Apr 20 2026

Small craft will need to exercise caution through Tuesday afternoon
in the Gulf waters. Moderate southeasterly winds persist throughout
the day on Tuesday, but will subside going into Tuesday night. The
elevated southeasterly winds will lead to an increased risk of rip
currents early this week. An extended fetch of moderate
southeasterly winds around midweek will likely lead to another round
of increased seas, especially in the offshore Gulf waters. This may
lead to another period of caution flags on Wednesday into Thursday
for the Gulf waters. Increased chances for storms return Tuesday
into Wednesday as an upper level disturbance pushes through. Water
levels are expected to remain near 3.0 feet above Mean Lower Low
Water during high tide cycles through at least the end of the work
week.

Batiste

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)  58  66  61  80 /  80  90  40  60
Houston (IAH)  62  69  64  80 /  50  80  50  80
Galveston (GLS)  70  76  72  79 /  50  70  50  80

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...High Rip Current Risk through late Tuesday night for TXZ436>439.

GM...Small Craft Should Exercise Caution until 1 AM CDT Tuesday for
     GMZ330-335.

     Small Craft Should Exercise Caution through Tuesday evening for
     GMZ350-355-370-375.

&&

$$

DISCUSSION...Batiste
AVIATION...Young
MARINE...Batiste

NWS HGX Office Area Forecast Discussion