Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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784
FXUS64 KHGX 171148
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
548 AM CST Fri Feb 17 2017

.AVIATION...
Showers were moving into the area from the southwest and should
affect most sites through at least mid morning. There was enough
instability for thunder already at 1130Z near LBX. Latest model
data showed the best potential for thunderstorms to be near the
coast -- between KHOU, KSGR, KLBX, and KGLS. There is some
uncertainty as to when the rain chances will end. For now took a
consensus between the 10Z run of the RAP13 and HRRR and ended the
rain threat between 18Z/19Z inland and 20Z at the coast.

VFR is then expected into the evening. Chances for fog increase
near the coast at KGLS by around 03Z mainly because of sea fog
formation off of the coast drifting inland. The fog may get into
KLBX and KHOU overnight. Both the NAM and GFS were not as bullish
on MVFR ceilings and was not as confident about these forming;
however, there is still an outside chance after 06 to 08Z tonight
due to the temperature profile on the model sounding forecasts.

40

&&

.PREV DISCUSSION... /ISSUED 359 AM CST Fri Feb 17 2017/

DISCUSSION...
At 2 AM, surface high pressure was located over the eastern Gulf
of Mexico and a trough of low pressure was located over high
plains. A weak area of low pressure was noted at 850 mb in north
central Mexico but 850 moisture looked sparse. Moisture was deeper
at 700 mb and a 700 mb trough extended from the central Rockies
into northern Mexico. At 300 mb, a well defined upper low was
spinning near the Big Bend and this feature shows up very well in
water vapor imagery. Upper level diffluence was noted over East
Texas and the western Gulf. The upper low over the Big Bend will
move east today and lift will increase over the eastern half of SE
TX today.Pw values will increase to through the day and reach
about 1.30 inches near the coast. Radar is already showing precip
and much of the rain is evaporating before reaching the ground
since the lower levels of atmosphere remain very dry. It`ll
moisten up with time and rain will begin to reach the ground
later this morning. Not sure how far rain will move inland today
but the higher rain chances will be near the coast and maybe a
tier of counties inland. Further inland, just expecting cloudy
skies and maybe a sprinkle.

Saturday looks nice with SE TX between high pressure to the east
and low pressure out west. There could could be some morning fog
and added patchy fog to the grids. 850 mb temps support sfc temps
in the lower 80`s with morning clouds giving way to aftn
sunshine. Not as clear cut on Sunday as fcst soundings show PW
values increasing to 1.50 inches during the afternoon and clouds
hanging around for much of the day. GFS and NAM both support
shra/tsra developing in the late afternoon and have raised PoPs
for the central and western zones. Temps are a bit tricky as
clouds and rain could keep temps lower than currently fcst, but
considering the warm start to the day, even nominal heating should
bump the temp to near 80. The SREF ensemble shows some potential
for sea fog near Galveston Bay and the Gulf waters off of
Galveston. Have added patchy fog to marine zones for Sat night
into Sun morning.

Made a few changes to the previous forecast and increased PoPs
for Sunday night as models want to speed the next system up. There
are some timing differences between the GFS and the slower ECMWF
and Canadian. PW values will increase to around 1.80 inches and
fcst soundings look saturated from the sfc to 300 mb. Upper level
winds split over SE TX and the region will lie in a 300 mb jet
couplet. Have increased rain chances Sunday night to 80 percent.
There will be some heavy rain but the system is looking more
progressive with each new model run so not quite ready to add
heavy rain to the grids. FWIW, WPC has placed SE TX in a marginal
risk for excessive rain for Day 3. The rain will likely linger
into Monday but the GFS dries things out pretty quick with PW
values falling to 1.40 inches by noon and to 1.13 inches by 00z.
The ECMWf/Can are slower but still progressive. Can`t completely
ignore the timing differences so have carried higher PoPs into
Monday afternoon but confidence is not terribly high for the rain
to continue through Monday aftn.

Rest of the week ahead looks quiet with dry conditions with above
normal temperatures though the end of the week. 43

MARINE...
Onshore winds will help increase the moisture enough for patchy sea
fog to develop later tonight and Saturday morning. The high
resolution models are showing the best potential east of Freeport
within 20 nm of the shore and over Galveston Bay. The statistical
model output is then showing the potential for at least patchy sea
fog in the same locations Saturday night and Sunday morning.

Another impact to the marine areas will be chances for showers and
thunderstorms today as a storm system moves across the area. A
strong storm system will then move across the state early next week
and bring additional chances for showers and thunderstorms Sunday
night and Monday. Onshore winds will slowly strengthen over the
weekend ahead of this second storm system. Small craft advisories
may be needed by Monday over the Gulf waters.

40

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      72  57  82  62  80 /  30  10  10  10  40
Houston (IAH)              70  57  82  63  80 /  50  10  10  10  30
Galveston (GLS)            67  63  73  66  73 /  70  10  10  10  20

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&

$$

Discussion...43
Aviation/Marine...40



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