Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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508
FXUS64 KHGX 231108
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
608 AM CDT Thu Mar 23 2017

.AVIATION...
MVFR cigs a little slow to develop this morning but potential is
still there for low ceilings through 15z or so. Drier air aloft
will mix to the sfc later this morning and skies will become
mostly clear. Stronger winds aloft will mix to the sfc and gusty
south winds are expected this aftn/evening. MVFR cigs possible
again tonight and fcst soundings show cigs developing as early as
03z. Leaned a bit more toward persistence. Fcst soundings also
show low level saturation increasing Fri morning beneath a 40 kt
low level jet. Might get some streamer showers Fri AM but did not
mention in the TAFs at this time. 43

&&

.PREV DISCUSSION... /ISSUED 413 AM CDT Thu Mar 23 2017/

DISCUSSION...
Upper ridging sliding east across Texas today will promote
another day of dry and warm conditions across Southeast Texas.
Surface observations as of 4 AM show temperatures in the mid 60s
to low 70s, with a few locations reporting patchy fog.
Additionally, split channel IR satellite imagery shows stratus
gradually expanding northward from the Interstate 10 corridor.
Similar to the past few mornings, fog (some locally dense
possible) and low clouds will lift or dissipate by mid-morning
with mostly sunny skies this afternoon allowing temperatures to
rise into the upper 70s to mid 80s. Near record high temperatures
are again expected along the coast today.

Early morning water vapor imagery shows an upper trough
translating across the Great Basin, with this feature expected to
reach the Southern Plains tonight. Vorticity advection ahead of
this upper trough has already resulted in the development of a lee
surface trough over the High Plains, with surface cyclogenesis
expected to commence by this evening as the upper trough continues
to translate east. In response, southerly surface winds will
increase to 10 to 20 MPH tonight and through the day tomorrow with
stronger gusts around 25 MPH possible. Elevated winds overnight
will preclude a fog threat for the region and also allow for
temperatures a few degrees above what we observe this morning,
with overnight lows in the upper 60s to low 70s.

As the upper trough crosses the Southern Plains on Friday, it
will drag the surface cyclone east with it. Showers and
thunderstorms are expected to develop along an associated
prefrontal trough or dryline across Central Texas on Friday
(possibly leftover convection from West Texas today), with strong
lift (100-110 knot 250 MB jet streak) allowing the storms to grow
upscale into a line through the morning hours as the system moves
east. Daytime heating may encourage enough destabilization across
parts of the Brazos Valley to promote SBCAPE values around
1500-2000 J/KG, allowing rapid thunderstorm development as the
line reaches our western counties. While guidance offers slightly
different solutions with the timing for the system, this
thunderstorm line looks to reach the western counties around
midday, the Interstate 45 corridor during the afternoon to early
evening hours, and the eastern counties late Friday evening to
early Saturday morning. Depending on the strength of the cold
pool(s) associated with this line, it may end up propagating
faster than currently anticipated.

Regarding the severe weather threat for Friday, environmental
conditions still remain favorable to produce at least a few strong
to severe storms within the larger thunderstorm line. The
aforementioned instability combined with 0-6 km bulk shear values
around 50-60 knots will promote organized thunderstorms capable of
producing damaging winds, especially from any kind of downward
momentum transfer of near surface winds as a 35-45 knot low level
jet sets up across the region. This low level jet will also draw
deeper Gulf moisture back into the region, with precipitable water
values rising into the 1.2-1.5 inch range by Friday afternoon.
While locally heavy rain will be possible given the amount of
atmospheric moisture present, the overall speed of the line should
mitigate against any widespread flooding threat (mean winds in
the cloud bearing layer of 850-300 MB around 40-50 knots). Another
threat will include an isolated tornado or two along the leading
edge of the thunderstorm line as the strength of the near surface
winds will also result in 0-1 km bulk shear values 15-25 knots.
Could also see large hail given mid-level lapse rates in excess of
6.5 C/km, but have lower confidence in this hazard materializing
as forecast soundings are advertising relatively skinny CAPE
profiles (indicating the potential for updrafts to become water
loaded and not as efficient for producing hail).

The main question with Friday`s severe weather threat will
largely be where it actually materializes. While both NAM and GFS
BUFR soundings continue to erode/lift a capping inversion through
the day Friday, the strength of their mid-level southwest flow
(30-35 knots) raises concerns for enough warm air advection to
occur to limit the southern extent of the thunderstorm line as it
moves into Southeast Texas. Given how forecast parameters are
lining up across the forecast area (including lift from upper
level system and corridor of strongest 0-6 km bulk shear values),
areas north of a Brenham to Cleveland line will need to be
monitored for the greatest severe weather threat... but again
cannot rule out anywhere north of the Interstate 10 corridor.
Interestingly, inspection of the 400 MB relative humidity fields
from the ARW, NMM, NAM, and GFS (which can serve as a proxy for
stronger convection as it indicates thunderstorm tops greater than
roughly 24,000 ft AGL) shows fairly good consensus in strongest
convection remaining north of Interstate 10... with a few that
only show convection north of the aforementioned Brenham to
Cleveland line.

A weak cold front will push into the region behind the main
thunderstorm line early Saturday morning as the system exits the
region, possibly stalling near or pushing just off the Upper Texas
Coast. No impact on temperatures is expected from this front as
deepening westerly flow behind Friday`s system will encourage
temperatures to again rise back into the upper 70s to mid 80s on
both Saturday and Sunday with dry conditions prevailing across the
region.

The next disturbance to affect rain chances for the region lifts
across the Southern and Central Plains on Sunday night, sending a
weak cold front into the area on Monday. Daytime heating and weak
convergence along the front may be enough to produce a few showers
and thunderstorms along the front, with best chances along the
Upper Texas Coast where deeper moisture is forecast to reside. Dry
conditions Tuesday in the wake of the front will be short-lived
as the next storm system to affect the region arrives during the
middle of next week. Considerable timing difference exist between
medium range guidance regarding this system, but widespread
showers and thunderstorms appear possible for the region during
this time.

Huffman

MARINE...
High pressure over the northern Gulf and lower pressures in the
lee of the Rockies will maintain an onshore flow today. The
pressure gradient will tighten significantly Thursday evening into
Friday as the high moves east and low pressure over the western
plains intensifies. A Small Craft Advisory will probably be
required tonight over the gulf waters and a SCEC will be needed
for the bays. The low will move toward the mid Mississippi Valley
on Saturday and drag a cold front into Texas. It is unclear how
far the front will travel before stalling on Saturday. There are
some indications that a wind shift to the west could briefly
develop Saturday morning before winds back to the south by
Saturday evening. The gradient will weaken as the front nears the
coast and wind speeds will decrease on Saturday. Another area of
low pressure will develop over eastern New Mexico on Sunday and
the gradient is expected to tighten once again. Winds will back to
the S-SE and increase in response to the deepening low. More
caution flags possible Sunday and Sunday night. The low over NM
will move east but the associated cold front with this feature
will also stall before reaching the coast. Moderate to
occasionally strong onshore winds are expected next week with more
Caution flags Tuesday night and SCA conds Wednesday night. 43

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      85  66  79  58  81 /   0  10  80  30  10
Houston (IAH)              84  68  80  66  83 /   0  10  40  60  20
Galveston (GLS)            79  70  78  70  80 /   0  10  20  50  20

&&

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

$$

Discussion...14
Aviation/Marine...43



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