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National Weather Service Forecast Discussion

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451
FXUS64 KHGX 212308
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
608 PM CDT Thu May 21 2026

...New MESOSCALE UPDATE, AVIATION...

.KEY MESSAGES...

- A Flood Watch remains in effect for counties along the coast
  through Monday morning.

- Multiple rounds of thunderstorms are expected through the
  holiday weekend and into next week. Some may be strong at times.
  Locally heavy rainfall is possible leading to the potential of
  flash flooding.

- Mariners should be prepared for the multiple rounds of storms.
  Winds and seas will be higher in and near storms.

&&

.MESOSCALE UPDATE...
Issued at 607 PM CDT Thu May 21 2026

Shortwave from earlier today has mostly exited the region off to
the east. Attention is turning to the shortwave and associated
shra/tstms tracking SEwd toward the I-35 corridor in the Hill
Country. This activity looks like it might pivot into parts of the
Brazos Valley this evening where there is still some lingering
instability that can be tapped into complements of some breaks of
late afternoon sunshine. The overall instability should be
declining as the hours wear on from debris clouds and sunset, but
have nudged POPs up a touch across NW parts of the CWA for the
next 6 hours or so and will continue to monitor trends. 47

&&

.DISCUSSION...
Issued at 143 PM CDT Thu May 21 2026

As we did yesterday, I`ll be keeping this discussion mainly focused
on what`s expected and the potential impacts as a result of that. If
I use any fancy sciency words, I`ll break em down and explain what
it means and why its relevant. We are going into a holiday weekend,
but that doesn`t mean that it`s time to completely disconnect from
the weather forecast. The heavy rainfall threat Saturday into Sunday
is of particular concern for the area. Continue to stay weather
aware by keeping up with the latest forecasts and have multiple ways
to receive alerts (especially if you outdoor plans or have plans to
be on the road for the holiday weekend).

This morning`s (Thursday morning) round of convection remained
mostly offshore, but did drop 0.5-2" of rainfall along the coast.
This time yesterday, we were anticipating some of the heavier
rainfall to occur along the coast...and for what it`s worth we did
see some radar estimated 3-6" amounts...in the Gulf waters. Now why
am I admitting this? Because it goes to show just how difficult it is
in this environment to be precise on the exact timing and placement
of the heaviest convection. PW values remain near or above the 75th
percentile (~1.60") this afternoon [moisture availability remains
elevated] and there is some lingering shortwave energy [a source of
lift], so some isolated to scattered showers/storms will be possible
this afternoon. This is most likely to occur north of I-10 closer to
a lingering frontal boundary. Fortunately, this spotty round of rain
will be falling in areas that are not as vulnerable as the coastal
areas.

Keep in mind that soils along the coast remain fairly saturated
after receiving 4-6+" of rainfall for areas along the coast (mainly
Matagorda and Brazoria Counties). As a result, a Flood Watch remains
in effect along the coast through Monday morning. There is decent
potential that we`ll expand the watch further northward and eastward
by Friday morning based on rainfall/model trends. If you`d like to
see more in-depth details on the Flood Watch, see the Hydrology
section below.

Speaking of rainfall, let`s talk about when the next rounds of rain
(after this afternoon) is expected. Some of the latest high
resolution model guidance depicts a line of gradually decaying
storms pushing into the area from the west later this evening. The
models that reflect this have another round early Friday morning
mainly along the coast/offshore. We saw this morning that model
guidance did not capture the correct placement of convection. So,
could this convection occur further inland, yes! Could it occur even
further offshore, also yes! Either way, the threat for heavy
rainfall along the coast persists which explains the current extent
of the Flood Watch. There is some generally decent consensus on a
dry-ish period going into Friday night, then comes our main
timeframe of concern over the weekend.

An upper level low that is currently situated off of the coast of
California is gradually pushing its way eastward towards Texas. It
looks to arrive generally in the Saturday to Sunday window. This
will pair with PW values peaking near or above 2" (90th percentile:
~1.80") and the potential for frontogenetic banding. Think of
frontogenetic banding as an area of locally enhanced convection...so
heavier rainfall. In a moisture-rich environment such as this,
rainfall rates could easily exceed 4" per hour in these types of
bands and this potential extends into at least Sunday morning. There
remains uncertainty on exactly where this will set up, but the
potential is certainly there...so there will likely be an expansion
of the Flood Watch area going into the weekend.

The upper level low does look to move out of the area towards the
end of the weekend, but PW values do remain elevated well into next
week as another upper level trough approaches. So, rainfall
potential will essentially continue for the foreseeable future. Keep
in mind that each round of rain will help to saturate the soils for
future rounds of rain. Saturated soils leads to quicker runoff,
which leads to a quicker transition to flash flooding. As a result,
there is daily potential for excessive rainfall for most or all of
Southeast Texas through Monday/Memorial Day. The excessive rainfall
risk alternates between a marginal risk (level 1 of 4) and a slight
risk (level 2 of 4) each day through Monday, but this is subject to
change once we gain confidence on timing/location of the heavy
rainfall potential. Cannot entirely rule out an upgrade in the
excessive rainfall risk on Saturday. As far as rainfall totals
go, we`re still anticipating additional amounts of 4-6" with
isolated higher amounts through Monday...just keep in mind the
confidence of the higher amounts remains low at this time. The
bulk of the rainfall is anticipated to occur within the Saturday
to Sunday timeframe. Be sure to stay up to date on the forecast
for the latest details.

In case you were curious about the temperature forecast, it`s going
to be highly dependent on the timing of these rounds of rain. For
now, we have high temperatures mainly in the low to mid 80s through
next week, but there may be times where the high temperatures remain
in the 70s depending on the timing of rainfall. We may see high
temperatures reach into the upper 80s next week depending on the
next round of rain. Low temperatures will mostly be in the upper 60s
to low 70s.

Batiste

&&

.AVIATION...
(00Z TAF Issuance)
Issued at 607 PM CDT Thu May 21 2026

A few lingering late day cells leftover from the shotwave that
passed through earlier today should be waning early this evening
for most of the area. However, we`re watching another shortwave
and associated storms in the Hill Country that could pivot into
the CLL/UTS area this evening. Overall intensity trend should be
declining with time...but we`ll keep an eye on things. Otherwise,
mainly VFR and dry for the rest of the region during the overnight
period. With light winds, wet ground, and thinner cloud cover I
wouldn`t be suprised to see some patchy late night fog development
across parts of the area. Next fairly weak shortwave and remnant
light precip coming out of Mexico should make it into our southern
terminal (IAH southward) in the 14-18z timeframe Fri. Unsure on
overall coverage so have some PROB30s in the TAFs for now. Rest of
Friday appears primarilly uneventful outside of some iso-sct tstm
pop-ups associated with afternoon heating. 47


&&

.MARINE...
Issued at 143 PM CDT Thu May 21 2026

Multiple rounds of showers and thunderstorms are anticipated through
the holiday weekend, so mariners should take note for the potential
for intermittent periods of elevated winds and seas in and around
any of the stronger thunderstorms that push through the waters. The
next round of showers and thunderstorms is expected either late this
evening or Friday morning. Timing of storms beyond that is a bit
uncertain, but additional rounds are expected into at least Monday.
Outside of the storms, expect light to occasionally moderate onshore
flow with 3-5 ft seas.

Batiste

&&

.HYDROLOGY...
Issued at 143 PM CDT Thu May 21 2026

Multiple rounds of moderate to heavy rainfall are expected through
at least Monday. Areas along the coast, especially Matagorda and
Brazoria Counties, received 5-8" over the past 48ish hours. The next
round of rain is expected to move in either late this evening or
Friday morning. Some of this rainfall may occur along the coast,
which brings the potential for flash flooding. As a result, a Flood
Watch continues for coastal counties through Monday morning. The
Flood Watch will likely be extended northward and eastward within
the next 24 hours to cover the threat area for the Saturday into
Sunday timeframe of concern.

With PW values near or above the 90th percentile (~1.79") through
the weekend, rainfall rates are expected to peak in the 3-4+" per
hour range in the heaviest downpours. Anywhere we see these intense
rainfall rates, localized flash flooding is possible. That being
said, we are not anticipating widespread river flooding just yet;
however, this will help prime the soils as we continue with a wet
pattern. As we go into the weekend, multiple rounds of moderate to
heavy rainfall on top of primed soils will generate runoff and cause
rises on area rivers and watersheds. The greatest threat for heavy
rainfall, flash and riverine flooding comes over the holiday weekend
as intense rainfall rates will bring accumulated values of 4-6+"
across most of Southeast Texas through Monday.

Widespread action to minor stage flooding is expected with moderate
to isolated major stage flooding possible depending on where the
most rainfall accumulates. Remember that you can monitor updated
river forecasts via the NWS NWPS webpage (https://water.noaa.gov/).

Batiste/Landry-Guyton

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)  69  85  71  83 /  60  40  10  90
Houston (IAH)  70  86  73  84 /  30  40  10  90
Galveston (GLS)  75  85  78  85 /  30  40  10  70

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Flood Watch through Monday morning for TXZ226-227-235>238-
     335>338-436>438.

GM...None.
&&

$$

DISCUSSION...Batiste
AVIATION...47
MARINE...Batiste

NWS HGX Office Area Forecast Discussion