NWS Houston/Galveston, TX wrote its last area forecast discussion before the first event at Sep 11, 4:07 AM CST, 53 minutes earlier.
Area forecast discussion, Sep 11, 4:07 AM CST, 53 minutes before
Area Forecast Discussion
National Weather Service Houston/Galveston TX
507 AM CDT Tue Sep 11 2018
.DISCUSSION...
Rain with showers and briefly heavy rain in embedded storms are
occurring on the Gulf Coast, and the disturbed weather will
continue today, and likely deep into the week. An area of showers
and storms near the Yucatan are being monitored for potential
tropical development as it moves towards the Texas coast towards
the weekend. Regardless of whether or not development occurs, one
major impact will be another injection of tropical moisture and
the continuation of our rainy period.
.NEAR TERM [Through Today]...
A large shield of rain covers much of the upper Texas Gulf Coast
early this morning, with embedded thunderstorms and cores of
moderate to heavy rainfall, mainly right along the shores of the
Gulf. Where rainfall is heaviest, the rain is piling up quickly,
with rain rates around 2 inches per hour. At an extreme, a Harris
County gauge near Clear Creek reported half an inch of rain in 5
minutes.
In the last hour or so, IR satellite shows the most significant
new development of convection has moved eastward and is now
occurring over the Gulf waters. This will help to reduce the
intensity of rain over coastal areas, and should lead to at least
a brief respite from any flooding potential.
The main question for today will again be where the rain falls. In
the short term, the most significant convection will likely be
over the waters, while lighter rain falls over coastal areas.
Additionally, we'll look for showers and perhaps a storm to
continue farther north as convection fires off the frontal surface
aloft.
We then look to Deep South Texas, the next shortwave through to
impact our area will be coming from. Given the water vapor
imagery, this shortwave looks to roll more or less up the
coastline, and a shift of only a handful of miles could make a
relatively large difference in impacts on coastal areas. Will play
it safe, and hedge a little towards the greater support of the
right entrance region of the upper jet streak slowly workings its
way northward away from us through the day, and move the highest
PoPs more inland around mid-day for the afternoon. But, it's
important to note that a path that is more offshore would keep the
heaviest rain more offshore, leaving us a little drier. Because of
the uncertainty, will keep the existing flash flood watch in place
for the inherited area (two rows of counties nearest the Gulf) and
time (until 6pm). However, this will have to be evaluated through
the day to account for what actually happens. All in all, this
forecast ends up hewing relatively close to the SPC HREF ensemble
mean, which shows 1-3 more inches of rain along the coast, and
isolated maxes around 5 inches. Along with that, have also
undercut guidance on temps, as clouds, rain, and evaporation of
rain from wet ground will surely take its toll.
.SHORT TERM [Tonight Through Thursday]...
Tonight's forecast is, at least from a bird's eye view, more or
less persistence. The new shortwave trough will travel a path very
near the one currently working through the area (perhaps a touch
more eastward as the longwave trough edges along). This subtle
slide eastward may be enough to keep the heaviest rains over the
waters, with lighter rain onshore...and if this is the case, we'll
likely be able to allow the flash flood watch to expire. But, if
the heaviest rains again set up over land, the watch may need to
be extended deeper into the night.
For Wednesday into Thursday, unsettled weather continues. The NAM,
GFS, and Euro all develop a weak upper low around the Texas Gulf
Coast, which will keep chances for showers and storms up deep into
the week. It will also keep the subtropical ridge from building
back across Texas, and provide the weakness that will draw up
Invest 95L towards Texas in the long term. In addition to higher
rain chances, will also keep high temperatures below normal, but
with a gradual warming trend back towards seasonal averages by
Friday, particularly in the north; that area should be relatively
drier.
.LONG TERM [Thursday Night Through Tuesday]...
Finally...eyes are beginning to turn back to the tropics, and
towards Invest 95L. This is an area of convection located just off
Mexico's Yucatan Peninsula, and has a medium chance of development
into a tropical cyclone before making landfall per the NHC. The
aforementioned upper low will keep the subtropical ridge weak over
our area, and will likely help draw the Invest towards the Texas
coast. The Euro continues to be the most enamored deterministic
model when it comes to this feature, which still likely only ends
up as a tropical storm before making landfall around Baffin Bay.
Regardless of whether or not development actually happens, the
expectation should be for storms to be sheared to the right side
of the lowest pressures/center, and this will result in a slug of
tropical moisture keeping moisture levels over Southeast Texas
high, and keeping rain chances high for us into the weekend.
After a long, wet wait, we will finally see things start to dry
out for the late weekend into early next week. I don't expect
things to go completely dry, as a moisture axis with about 2
inches of precipitable water still may be present for some of us
per the GFS. Still, coverage may be more representative of a
typical late summer day, and temperatures are likely to bounce
back up towards the lower 90s.
&&
.AVIATION [06Z TAF Issuance]...
Not much change to the previous package. Storms developing along
the coast from LBX southwestward as expected. Expecting storms to
fill in along the coast and over the Gulf then spread inland on
the slightly enhanced low level flow. PW of 2.2-2.4" will stream
inland this morning and expect scattered to numerous showers and
thunderstorms across the coastal areas expanding inland during the
morning hours into the afternoon hours. Eventually some drier air
should nudge storms east and southeast back out into the Gulf
probably this afternoon. Across the north still expect periods of
MVFR/IFR beneath the frontal inversion these may briefly reach
down into CXO/IAH area after 08z. 45
&&
.MARINE...
Periods of scattered to numerous showers and thunderstorms
(some with locally heavy rainfall) will persist through
the end of the week. Higher winds and seas as well as
reduced visibilities can be expected in the stronger activity.
A tropical wave may approach the Texas coast late in the week
and bring increasing winds and seas to the area. The National
Hurricane Center is monitoring this system for possible
tropical development, and mariners are urged to keep up with
the latest forecasts. 42
&&
.HYDROLOGY...
Flood advisories for urban areas and small streams have been
issued, as well as one flash flood warning in Galveston County.
Despite this, river gages show no serious flooding concerns. There
is an indication of some rise, and faster flows should be expected
as a result of this rain, but for now there do not appear to be
any riverine flood warnings necessary. Still, folks should be
cautious around the water, particularly smaller, flashier streams
in the area as the current may be faster than usual.
&&
.TROPICAL...
The Atlantic Basin is busy, with advisories being issued for
hurricanes Florence and Helene, as well as Tropical Storm Isaac.
Additionally, there are two areas being monitored for possible
development by NHC - one of them being the aforementioned Invest
95L.
- 95L currently has a 30 percent (low) chance of developing within
the next two days due to entanglement with the Yucatan
Peninsula, but a higher 60 percent (medium) chance of
development once it emerges in the Gulf of Mexico. We'll have to
monitor this one closely as there is high confidence in this
area of showers and storms making its way to the Texas coast,
and bringing us another round of tropical moisture and high rain
chances at the end of the week.
- Tropical Storm Isaac is still forecast to move roughly due west
over the next 5 days. It may briefly become a hurricane again,
but increasing shear should knock it back down to a tropical
storm as it approaches the Lesser Antilles. There are no
indications in the guidance that Southeast Texas will see direct
impacts from this storm, but neither can we completely rule it
out, either. This one also bears watching just to ensure it
stays away.
- None of the other tropical features are expected to have any
impact on Southeast Texas.
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 80 71 87 72 89 / 70 40 60 30 60
Houston (IAH) 77 74 83 73 85 / 60 50 60 40 60
Galveston (GLS) 78 78 82 78 82 / 90 80 80 60 70
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Flash Flood Watch through this afternoon for the following
zones: Brazoria Islands...Chambers...Coastal
Brazoria...Coastal Galveston...Coastal Harris...Coastal
Jackson...Coastal Matagorda...Fort Bend...Galveston Island
and Bolivar Peninsula...Inland Brazoria...Inland
Galveston...Inland Harris...Inland Jackson...Inland
Matagorda...Matagorda Islands...Northern Liberty...Southern
Liberty...Wharton.
GM...None.
&&
NEAR TERM...Luchs
SHORT TERM...Luchs
LONG TERM...Luchs
AVIATION...42
MARINE...45
HYDROLOGY...Luchs
TROPICAL...Luchs