Flash Flood Warning 32, NWS Houston/Galveston, TX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
310 PM CST Wed Dec 30 2020

.SHORT TERM [Thursday Through Friday Night]...

Seeing scattered showers developing this afternoon across the area
in the warm moist airmass southeast of a cold front that is
located to the southeast of College Station as of 3pm. The cold
front is expected to slow later this evening and stall across the
central counties tonight bringing widespread showers and some
thunderstorms. A Flash Flood watch is in effect for most of
southeast Texas tonight through 3 PM Thursday. The heavy rainfall
threat will increase later this evening as the front edges
southeastward and slows. High res models indicate hourly rainfall
rates of 1 to 2 inches are likely near the front overnight. This
is a good setup for training storms late tonight and into tomorrow
with storm total rainfall amounts likely in the 2 to 4 inches and
isolated amount of 6 inches possible. A surface low will develop
and deepen while moving northeast along the quasi-stationary
boundary tonight and Thursday. As this occurs, showers and
thunderstorms will increase across a broad region of southeast
Texas. Some of the storms could be severe, especially Thursday
morning southeast of the surface low as indicated by the slight
risk from SPC. Within that slight risk area, a few supercell
thunderstorms are possible with hail, strong winds and isolated
tornadoes all threats.

Expecting showers and storms to end from the southwest to the
northeast Thursday afternoon, which is a bit atypical because the
surface low is so far south with this storm system.

35/33

.LONG TERM [Friday Through Wednesday Night]...

The start of 2021 will start out our long term period. Surface high
pressure will be building over Texas in the wake Thursday's
disturbance ending the rainfall threat and bringing in cooler
temperatures through the weekend (high temperatures in the mid to
upper 50s and overnight lows in the mid to upper 30s). There will be
a couple of upper level troughs swinging across the region through
the weekend with the first one moving through Saturday afternoon,
and a second, weaker wave on Sunday afternoon. The surface high
pressure will help prevent any shower activity from forming this
weekend, but partly cloudy skies will likely persist, especially
across the north third of the area.

The surface high pressure slides to the east by Monday bringing
southerly flow back into the area that will help increase
temperatures through midweek. Weak midlevel ridging will also be
passing through Monday into Tuesday helping to increase
temperatures. Afternoon highs Monday into Tuesday will climb into
the upper 60s into low 70s. An upper level low will be moving across
the Four Corner region on Tuesday with the associated surface low
developing in the Great Plains late Tuesday into early Wednesday.
The increasing moist southerly flow will bring the potential for
rain showers beginning overnight Tuesday into Wednesday morning. The
associated cold front is currently expected to move through SE Texas
during the day on Wednesday bringing a chance of showers and
thunderstorms before exiting to the east Wednesday night.

Fowler

&&

.MARINE...

A complex weather system will be moving through the area tonight
into Thursday afternoon bringing a mixture of heavy rainfall and
strong thunderstorms. The flow through tonight will be generally
south-southeasterly with scattered showers and thunderstorms ahead
of an approaching cold front. This cold front will end up
stalling inland tonight, but a low pressure system will develop
along the cold front by Thursday morning and then move to the
northeast through the day on Thursday. There is a chance of strong
to severe thunderstorms capable of producing heavy downpours,
strong wind gusts/rough seas, and even isolated waterspouts out
ahead of the developing cold front. The cold front will move
through the area from west to east Thursday afternoon ending the
severe weather and rain threat. However, strong westerly winds are
expected to develop behind the exiting front with sustained winds
over the Gulf Waters 20 to 30kts with frequent gusts to 40kts
beginning Thursday afternoon through early Friday morning with
wave heights building to 7 to 12 feet. Because of these
conditions, the Gale Watch has been upgraded to a Gale Warning.
The Bays will have wind gusts to 30 kts for the same time period,
so have kept them in a Small Craft Advisory for the time being,
but they may also approach Gale conditions.

The strong westerly winds Thursday into Friday will bring the
potential for low water levels in the Bays at low tides Friday
and Saturday. PETSS guidance currently has tides at around a foot
below Mean Low Low Water Friday and Saturday morning, so there is
potential for the need for Low Water Advisories then.

Winds and seas lower through the weekend as high pressure builds
overhead. Onshore flow returns for the start of next week as the
high pressure slides eastward with the next potential front moving
through midweek next week.

Fowler

&&

.PREV DISCUSSION /Issued 1232 PM CST Wed Dec 30 2020/...

.AVIATION [18Z TAF Issuance]...

Quite the set up today as a boundary passes the northern
terminals and then stall in the IAH to SGR area tonight. by
tomorrow morning, a low will move into the southwestern area and
move over the SGR- CXO terminals then migrates out of the area
tomorrow night. Impacts associated with this system are lower
ceilings in the northern flying areas and areas of heavier
rainfall and thunderstorms will bring in IFR conditions or lower
tomorrow. LIFR ceilings are expected with wrap around moisture as
the low moves over the area but mainly effecting the central and
northern flying areas. Once the system moves out tomorrow evening,
stable/drier airmass will dominate and VFR conditions will
prevail into next week. 35

&&

.PRELIMINARY POINT TEMPS/POPS...

College Station (CLL)  45  56  37  54  35 / 100 100  20   0   0
Houston (IAH)          62  67  41  58  39 /  90 100  10   0   0
Galveston (GLS)        64  68  45  58  44 /  50 100  10   0   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...

TX...Flash Flood Watch from 6 PM CST this evening through Thursday
     afternoon for the following zones: Austin...Brazoria
     Islands...Brazos...Burleson...Coastal Brazoria...Coastal
     Matagorda...Colorado...Fort
     Bend...Grimes...Houston...Inland Brazoria...Inland
     Harris...Inland Jackson...Inland
     Matagorda...Madison...Matagorda
     Islands...Montgomery...Polk...San
     Jacinto...Trinity...Walker...Waller...Washington...Wharton.

     High Rip Current Risk through Thursday morning for the following
     zones: Brazoria Islands...Galveston Island and Bolivar
     Peninsula...Matagorda Islands.

GM...Small Craft Advisory from noon Thursday to 6 AM CST Friday for
     the following zones: Galveston Bay...Matagorda Bay.

     SMALL CRAFT SHOULD EXERCISE CAUTION through Thursday morning for
     the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until noon CST Thursday for the following
     zones: Coastal waters from Freeport to Matagorda Ship
     Channel TX out 20 NM...Coastal waters from High Island to
     Freeport TX out 20 NM...Waters from Freeport to Matagorda
     Ship Channel TX from 20 to 60 NM...Waters from High Island
     to Freeport TX from 20 to 60 NM.

     Gale Warning from noon Thursday to 6 AM CST Friday for the
     following zones: Coastal waters from Freeport to Matagorda
     Ship Channel TX out 20 NM...Coastal waters from High Island
     to Freeport TX out 20 NM...Waters from Freeport to
     Matagorda Ship Channel TX from 20 to 60 NM...Waters from
     High Island to Freeport TX from 20 to 60 NM.

&&

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