Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KHGX 161639
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1039 AM CST Sat Dec 16 2017

.UPDATE...
Morning surface analysis shows a coastal trough beginning to
develop from near the Texas Coastal Bend south towards
Brownsville, with further deepening into a closed surface low
expected this morning as height falls associated with an upper
trough lifting into Southwest Texas spread across the northwest
Gulf. Current forecast with regards to this coastal low remains
largely on track as steering flow pushes this feature up the Texas
coast late this afternoon through tonight.

Morning soundings showed precipitable water values of 0.93 inches
and 1.15 inches at Lake Charles and Corpus Christi, respectively,
and as this coastal system strengthens today forecast
precipitable water values surge into the 1.5 to 1.8 inch range.
This is much above normal for this time of year (average values
usually 0.75-0.9 per SPC sounding climatology). This moisture,
combined with mid-level lapse rates steepening to 6-6.5 C/km as
the upper trough crosses Texas, will mean the atmosphere is
supportive of numerous to widespread showers and thunderstorms as
the coastal low skirts the Upper Texas coast tonight. High
resolution guidance and convection allowing models are all showing
a fairly consistent signal for light showers to spread into the
western counties later this morning as divergence aloft increases
(which is already fairly well supported by ongoing activity across
the Hill Country seen on regional radar mosaic), spreading east
across the region through the afternoon. Increased surface
convergence associated with the development of the coastal low
looks to result in the development of a line of thunderstorms this
evening that quickly moves across Southeast Texas west to east
between the 6 PM to 1 AM timeframe.

As was noted in the overnight forecast package, the track of this
coastal low will be crucial for what hazards materialize for the
region as wind energy associated with this system is very strong.
Preliminary analysis of 12Z guidance shows that at least some of
the warm sector (and best potential for surface-based
thunderstorms) may be able to push into Matagorda, Brazoria,
Galveston, and Chambers. This means that an isolated tornado or
waterspout threat will exist for a brief window tonight generally
along the immediate coast, but best chances will likely be over
the coastal waters. North of this warm sector, elevated
thunderstorms will preclude this threat. As was noted earlier,
above normal moisture will mean most of the region receives much-
needed rainfall from this system. Unidirectional wind profiles in
the cloud-bearing layer will encourage some training of storms and
allow for rain totals in some areas to accumulate into the 1-3
inch range. However, the speed of this system and fairly low rain
rates due to limited instability will help mitigate against any
widespread or significant flooding threats.

Long story short: most of the region will receive much-needed
rainfall tonight from a fast-moving system that will keep the
widespread flooding risk low. An isolated tornado or waterspout
threat is possible along the immediate coastal areas between 6
PM-1 AM but best chances are over the Gulf. The main update for
this forecast was to simply update based on latest observations.

Huffman

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      54  45  65  53  66 /  80  90   0  20  50
Houston (IAH)              58  48  68  56  70 /  20  90  10  30  60
Galveston (GLS)            60  56  64  60  69 /  20  80  30  50  70

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory until midnight CST tonight for the
     following zones: Coastal waters from Freeport to the
     Matagorda Ship Channel out 20 NM...Coastal waters from High
     Island to Freeport out 20 NM...Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.

     SMALL CRAFT SHOULD EXERCISE CAUTION until noon CST today for the
     following zones: Coastal waters from Freeport to the
     Matagorda Ship Channel out 20 NM...Coastal waters from High
     Island to Freeport out 20 NM...Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.

&&

$$

Discussion...14



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.