Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 282339
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
639 PM CDT Fri Apr 28 2017

.AVIATION...
Look for ceilings to drop into MVFR territory this evening.
There`s actually some sea fog out there too w/ 77 degree dewpoints
moving over what`s already above normal water temps. Winds should
remain gusty overnight - and actually increase in speed - as the
low level jet strengthens to 40-50kt by sunrise. I did throw the
mention of low level wind shear into the UTS TAF later tonight
given portions of the runway are somewhat wind blocked. Otherwise,
continued breezy on Saturday. Not expecting much improvement in
regards to ceilings as moisture remains trapped under low level
inversion. May see some scattered WAA shra, but tstm chances
shouldn`t enter the picture until Sat night. 47

&&

.PREV DISCUSSION... /ISSUED 318 PM CDT Fri Apr 28 2017/
SHORT TERM...
Today resembles more mid/late June than late April
with temperatures in the upper 80s to low 90s. Surface dewpoints
are in the low 70s so combined with the temperatures, heat index
values have reached the upper 90s this afternoon. The only thing
that is making it feel "cooler" are the winds which have been
gusting to 30 mph at times. Gusty southerly winds will continue
tonight which means temperatures will not drop too much. Low
temperatures in the upper 70s look on track. This may break a few
record high minimum temperatures for Saturday but that may depend
upon cooling Saturday night as a cold front approaches. The strong
winds will certainly drive up tide levels and increase the threat
for rip currents through tomorrow.

Severe weather threat Saturday night...
Water vapor imagery and upper level analysis show a pretty strong
upper level low over the Great Basin which should drop south
towards the Southern Rockies by 12Z Saturday. A warm front should
extend from the Ozarks SW through Oklahoma to the Trans Pecos of
Texas with perhaps a couple small centers of low pressure along it
depending upon the evolution of convection tonight.

Upper level low should then move towards Tx/Ok Panhandles by 12Z
Sunday allowing for a broad area of large scale lift and height
falls to spread over the region. With this there will also be an
elevated mixed layer support capping over much of SE Texas
Saturday going into Saturday night. Upper level forcing may be
slow to spread over the area depending upon the track of the upper
low. Yesterday this lift looked out of phase with surface forcing
and suggest capping may hinder thunderstorm development. Cold
frontal forcing should be strong enough to at least initiate
linear convection with a squall line moving through the area.
Models differ in instability but overall looks like CAPE over 2000
J/Kg will be possible Saturday night over SE Texas. Deep layer
shear near 40-50 knots may be increasing with strong low level
shear around 20-30 knots during the overnight time frame. This
suggests at least a small tornado threat if a discrete cell can
develop which with the frontal forcing look unlikely since the
cap will be eroding all along the front. Storm model looks linear
as suggested by several of the WRF models so feel confident with a
squall line support hail/damaging wind threats. Any tornado may
be brief and along the squall line. Once again this all depends
upon there being enough instability and that capping weakens.

Basic timing of the line of storms looks to be near midnight
Sunday for the College Station area, 4AM Sunday for Houston area
and off the coast 7-10AM Sunday. Temperatures will be dropping
behind the front into the 60s/50s for Sunday.

Last but not least, the flood threat does not look at great for SE
Texas. While there will be high precipitable water values near 2
inches, the axis of moisture looks to move east Saturday afternoon
and night which would support heavy rainfall more over NE Texas
into Arkansas. The line of storms may produce brief heavy rainfall
but the storms will be moving quickly through the area which will
limit the flood threat.

Overpeck

LONG TERM...
12Z model runs are showing both the ECMWF and GFS pushing higher
values of positive vorticity associated with the frontal passage,
over SE TX through late Saturday into early Sunday. Precipitation
out ahead of the front will push into the forecast area late
Saturday, and should slide across the CWA and push off the coast
by 15Z Sunday. Models are currently in agreement regarding the
timing of the precip associated with the cold front. Best chance
for showers and thunderstorms will be between 00-12Z Sunday.

As the front pushes off the coast, heights should rise in its
wake.  High pressure will remain over SE TX through early Wednesday.
Temperatures will feel more like Spring than Summer, with high
temperatures expected to reach into the low to mid 80s through
Wednesday.  Moisture conditions will be on the drier side with
dewpoints staying in the upper 40s to lower 50s through Monday
evening. The dry air will not last for long though, as humid
air filters back in with dewpoints into the mid to upper 60s by late
Tuesday. A weak ripple in the 500mb pressure fields show a
disturbance propagating across southern TX late Tuesday into early
Wednesday, which could aid in the chance of precipitation before the
next frontal system.

Late Wednesday, the next frontal boundary will enter the SE TX area
and is expected to clear the coast by early Thursday morning. At 500
mb, the pressure field shows a upper level trough plunging into SE
Texas late Thursday and into early Friday. Forecast soundings show
PWs on the rise beginning late Tuesday and into early Wednesday,
toping out between 1.40 to 1.55 inches late Wednesday. With CAPE
values reaching between 200 to 4600 J/kg Wednesday afternoon
as indicated by the forecast soundings, the possibility of showers
and some stronger thunderstorms can not be ruled out.  PW values
lower to less than a half inch, after the passing of the front and
should hold through Saturday afternoon. Again, high pressure will
build in, this time over the central US, keeping conditions warm and
dry through the end of next weekend.

Hathaway

MARINE...
Onshore winds are continuing to build over the bays/coastal waters
this afternoon and expected to pick up significantly tonight. Near
gale force gusts will be possible overnight into Sat. Winds should
diminish briefly Sat night just ahead of the cold front...but will
be picking up once again behind the boundary. Strong N/NW winds in
the wake of the front will reach SCA criteria over the Gulf waters.
High pressure moving quickly into the region will lower winds back
below Advisory/Caution levels late Sun night/early Mon morning. We
could see the return of light S/SE winds over the marine waters by
Mon afternoon. These onshore winds will strengthen Tues/Weds ahead
of yet another cold front that is forecast to move into the coast-
al waters early Thurs.

In the meantime, the Coastal Flood Watch will remain in effect for
the Upper TX coast through tomorrow afternoon. The deep/persistent
SE fetch will result in long-period swells and high surf. Forecast
models are indicating the possibility of water levels reaching at/
near 3.8 feet at Pleasure Pier on Galveston Island and 3.5 feet at
Freeport late tomorrow morning at high tide. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      77  88  58  73  49 /  20  40  80  10  10
Houston (IAH)              78  86  62  76  52 /  20  40  80  20  10
Galveston (GLS)            78  83  66  77  64 /  20  30  80  40  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Coastal Flood Watch from 1 AM CDT Saturday through Saturday
     afternoon for the following zones: Brazoria...Chambers...
     Galveston...Matagorda.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION through Sunday afternoon for
     the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until 2 PM CDT Sunday 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 7 PM CDT this evening
     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...99


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