Special Weather Statement, NWS Houston/Galveston, TX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
313 PM CDT Mon May 17 2021

...Locally Heavy Rain Possible Through Thursday...

.SHORT TERM...[Through Tuesday Night]

An outflow boundary moving into the area from North Texas coupled
with another boundary over LA combined with deep moisture (PWs
near 2.00 inches) and favorable jet dynamics will create a
favorable environment for additional SHRA/TSRA for the remainder
of the afternoon. A Flash Flood Watch has been issued for all of
SE TX as this line of storms has already produced 7 inches of rain
in Liberty county and will have the potential to produce 2-4
inches of rain as it moves south toward the coast. The boundary
should reach the coast by 01z with some stabilization behind it so
the rest of the night is looking fairly tranquil with partial
clearing this evening before clouds roll back in by 06z. MinT
values will remain on the warm side (low 70's) as low level
moisture remains high and winds don't decouple and will aid in
mixing.

Very late tonight, another upper level disturbance rotating
around a broad upper trough located over New Mexico will bring an
area of showers and storms toward the western half of the CWA
between 09-12z. Jet dynamics still looking impressive with a split
in the jet noted between 12-18z especially over the northern half
of the region. PW values will be around 1.70 inches which is a
bit less than today but still sufficient to generate pockets of
heavy rain. Will again go with likely PoPs for Tuesday but meso-
scale forcing will likely dictate who receives heavy rain. Will
maintain the Flash Flood Watch through the short term forecast as
area rivers are high and rain over the last week or so has left
soils saturated. Parts of Wharton County received 12-14 inches of
rain yesterday and 7 inches of rain fell in Liberty County today
and with conditions remaining favorable for heavy rain, feel the
best course of action is to hoist an FFA and keep it going until
the threat for heavy rain wanes toward the end of the week. 43

&&

.LONG TERM [Wednesday Through Monday]...

No noticeable changes in the long term regarding the potential for
heavy rainfall through the remainder of the week. An upper-level
cutoff low is expected to hover over N TX and the OK Panhandle,
driving up onshore flow across Southeastern Texas. Forecast
soundings are very warm and saturated as PWATS climb to around 2.0
inches. A low-level jet will continue to strengthen, advecting more
heat and moisture across our region. An upper-level meridional jet
streak will aid in divergence aloft and therefore convergence at the
surface for convection to initiate and persist. Small lobes of
vorticity from the lingering upper-level low will also help
invigorate and sustain convection.

Understandably so, global models are having a rough time trying to
resolve mesoscale features. These features will ultimately determine
where the heaviest precipitation will occur, but for now a broad
swath of 4 to 6 inches of rain is expected to fall between Wednesday
and Friday for areas around I-10 and north of I-10 as well as areas
west of I-45. Localized higher amounts are possible. Elsewhere, 2 to
4 inches are expected with isolated higher amounts possible.

Towards the weekend, rainfall chances will persist despite that
cutoff low meandering towards The Great Lakes since onshore flow
will remain and upper-level vorticity advection will still occur. A
high pressure and upper-level ridge over the Southeast US will begin
to push west and into our area this weekend, helping to suppress the
chances for rain slightly. However by "suppress" I mean, rather than
a deluge, it will return to a more seasonable pattern of afternoon
showers and thunderstorms.

KBL

&&

.MARINE...
Lower pressures over New Mexico and high pressure over the eastern
Gulf will maintain a moderate to strong onshore flow tonight. Seas
are between 5 and 6 feet so will maintain a SCEC for all waters
tonight. The pressure gradient remains tight and the SCEC will
likely need to be extended into Tuesday Night with a possible SCA
required Wednesday into Thursday. A moderate to strong onshore
flow will persist into next weekend.

Strong Rip Currents will persist as well so use extreme care if
heading to any of the Gulf facing beaches. The strong and
persistent onshore flow will also drive water levels up and there
could be come coastal flood issues by early Wednesday into
Thursday. Water levels could reach 3.8 feet and a Coastal Flood
Advisory or Warning may be needed by Wednesday morning.  43

&&

.HYDROLOGY...

Heavy rain over the next couple of days will likely impact area
watersheds.It's still not completely clear which watershed will
bear the brunt of flooding but the San Bernard and Lavaca-Navidad
systems remain high and it won't take much additional rain to
cause flooding. The Brazos river basin can probably take some
rain but heavy rain on the Trinity basin today will likely lead
to some rises on that watershed. As for the Harris County Bayou
system, your guess is as good as mine. It depends when, where and
how quickly rain falls over Harris County. 43

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      72  85  70  80  68 /  50  60  70 100  90
Houston (IAH)              73  85  73  83  71 /  60  60  60  90  90
Galveston (GLS)            75  83  76  83  74 /  70  40  50  70  90

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Flash Flood Watch through Thursday morning for the following
     zones: Austin...Bolivar Peninsula...Brazoria Islands...
     Brazos...Burleson...Chambers...Coastal Brazoria...Coastal
     Galveston...Coastal Harris...Coastal Jackson...Coastal
     Matagorda...Colorado...Fort Bend...Galveston Island...
     Grimes...Houston...Inland Brazoria...Inland Galveston...
     Inland Harris...Inland Jackson...Inland Matagorda...
     Madison...Matagorda Islands...Montgomery...Northern
     Liberty...Polk...San Jacinto...Southern Liberty...Trinity...
     Walker...Waller...Washington...Wharton.

     High Rip Current Risk through Tuesday evening for the following
     zones: Bolivar Peninsula...Brazoria Islands...Galveston
     Island...Matagorda Islands.

GM...NONE.
&&

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