Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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359
FXUS64 KHGX 281204
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
704 AM CDT Sun May 28 2017

.UPDATE...
Quick update to pull mention of isolated (20 PoP) rain chances to
the coast for the remainder of the morning hours and into the
afternoon. An outflow boundary continues to push southwest into
the forecast area from decaying thunderstorms along Interstate 20
and with surface pressure rises behind this boundary on the order
of 4-5 MB over the past 3 hours, expect the boundary to continue
propagating into the northern half of the forecast area through
the morning hours. This boundary (in addition to a possible sea
breeze) may serve as a focus for shower development with
additional heating later this morning into the early afternoon
hours ahead of an approaching cold front currently stretching
from the Davis Mountains to D/FW Metroplex. No other changes made
to ongoing forecast.

Huffman

&&

.PREV DISCUSSION... /ISSUED 434 AM CDT Sun May 28 2017/

SHORT TERM (Today through Monday)...
Hot and humid conditions will again blanket Southeast Texas
today, but the arrival of a cold front this afternoon and evening
will herald in a somewhat cooler and much wetter period this week.
Temperatures as of 4 AM CDT are in the upper 70s and low 80s and
with not much change expected over the next few hours owing to
elevated onshore flow and an overcast stratus deck, this warm
start will mean not much heating is needed to push afternoon
temperatures back into the upper 80s to low 90s today. This will
help push afternoon heat index values into the low 100s again
today in many locations and residents are urged to take frequent
breaks and drink plenty of water if working outdoors. Similar to
yesterday, several record high minimum temperatures are in danger
of being tied or set again today and those records for first order
climate sites are included in the climate section below.

Regional radar mosaic early this morning shows a thunderstorm
complex slowly pushing south of the Red River towards Interstate
20 along a cold front. While this complex is expected to continue
weakening as it moves east/southeast and encounters a capping
inversion located between 950 and 800 MB, aircraft soundings out
of Dallas/Fort Worth showed that the capping inversion evident on
the 00Z FWD RAOB had begun to weaken and lift... raising concerns
that this complex may persist farther to the southeast than what
model guidance currently indicates. As a result have added 20-30
PoPs to the extreme northern counties through the remainder of the
morning. May also see a few streamer showers push onto the coast
this morning from the Gulf.

Thunderstorm chances will increase dramatically this afternoon
with the arrival of the cold front as the capping inversion in
place over the region erodes through the day. The combination of
moisture pooling along the front and diurnal heating (even with
mostly cloudy to cloudy skies in place) should be enough to
contribute to significant instability (MLCAPE values exceeding
4000 J/kg) for strong to severe thunderstorm development. In
addition, steep mid-level lapse rates evident on 00z RAOBs
(8.4-8.9 C/km) may only lessen slightly through this evening and
will only further serve to accelerate updrafts along the front.
0-6 km bulk shear during this time still appears to remain minimal
(generally 20 knots or less), which will help limit overall
thunderstorm organization. However, given a considerable dry layer
of air between 750-450 MB enhancing evaporational cooling and
downdraft acceleration, lack of thunderstorm organization does not
appear to hamper the severe weather threat for today and
concerned that thunderstorms that develop along the front will be
capable of damaging winds and possibly large hail. Best chances
for this threat to materialize appear to be north of a Columbus to
Cleveland line. Will also have to keep an eye on the possibility
for locally heavy rain and isolated minor to flash flooding issues
along the front with precipitable water values increasing to
1.6-1.9 inches and anticipated west to east storm motions
indicating the potential for training along the frontal boundary.
Through Monday, could see widespread 1-2 inch rain amounts across
the region but locally higher amounts in excess of 2-4 inches will
be possible.

Current timing for the cold front has it arriving into the Brazos
Valley and Piney Woods regions mid to late afternoon and the
Houston metro after midnight, but this timing is highly dependent
on outflow not driving the cold front farther south faster than
what model guidance currently indicates. As a result, the front
may push into the Houston metro tonight faster than anticipated
(possibly bringing an isolated damaging wind threat with it).
Showers and thunderstorms are expected to persist along the front
as it continues to push south during the overnight hours, with the
frontal boundary pushing near or just off of the Upper Texas
Coast by sunrise. A shortwave trough lifting out of Mexico and
moving across the region Monday morning looks to result in a
thunderstorm complex developing over South Central Texas and
moving east as well as ongoing convection flaring along the cold
front during the morning hours. This would renew the threat for at
least locally heavy rain and gusty winds. The location of the
cold front tomorrow morning will be highly dependent on how the
thunderstorms that develop along it behave today. Continuing to
advertise the front stalling somewhere in between Interstate 10
and the coast and resulting in highest rain totals over the
southwestern counties Monday morning, but if the front is able to
push off the coast as the shortwave arrives it may limit how far
inland rain chances exist. Diurnal heating and the passing
shortwave will result in rain chances persisting through the day
on Monday with high temperatures only in the low 80s.

Huffman

LONG TERM (Monday night through Saturday)...
Subsidence in the wake of the passing shortwave may be enough to
temper rain chances (at least briefly) Monday night before
increasing during the day Tuesday as a series of waves move across
the region in the west/southwest flow aloft. With little overall
change in the expected pattern through mid to late week, periods
of showers and thunderstorms capable of at least locally heavy
rain will continue through the work week. Weak mid-level flow and
deep layer saturation (1000-500 MB relative humidities in excess
of 75 percent at times) raise the concern for isolated instances
of minor or flash flooding, especially along any residual
boundaries from previous rain events. For now, have continued to
advertise 30-50 PoPs in the extended portion of the forecast with
refinements to locations and forecast rain amounts likely as
mesoscale features become evident.

Huffman

MARINE...
Southeasterly winds offshore and south-southwesterly nearshore
near 15 knots. These moderate winds will continue this morning
then gradually diminish. SCEC currently up until 6 am and will let
it expire at that point. South winds today should back to the
southeast as frontal boundary-enhanced by outflow sags into the
northern CWA. Seas changing little or diminishing slightly over
the next few days in response to lighter wind fields. Possible
showers and thunderstorms could become focused near the coast or
just offshore Monday morning...possibly strong storms as well.
Models really struggling with deep convection placement late
Monday through Tuesday and it may be so mesoscale driven that
trying to pinpoint storms as far south as coastal waters is
unreliable at best. S/W passing by to the north should lead to
increased southeasterly flow late Tuesday and probably maintain it
through Saturday. ECMWF continues to point to some sort of low
pressure system taking shape in the Western Gulf and drifting
north toward the coastal waters Sunday then just meandering
about...which of course isn`t good news this time of year. If so
stronger easterly flow possible with impacts of elevated tide
levels. Again stay tuned.
45

MARINE...
Surge of stronger winds passing through the region with MVFR
ceilings. Areas along and north of CXO seeing some breaks to VFR but
these may well fill back in toward sunrise.

MVFR ceilings should prevail through mid morning north and late
morning to early afternoon south where richer/deeper moisture is
evident. May see a few spotty showers develop this afternoon but
main focus for storms will likely be along the slowly sagging
frontal boundary (possibly outflow reinforced) late afternoon for
CLL/UTS areas. CAMs continue to highlight a band of precip extending
from well southwest of CLL to near or north of LFK at 00z. Will
likely continue to carry the VCSH/VCTS north with a TEMPO for CLL
late afternoon into the evening hours. Overnight storms and boundary
should sag slowly or stall and remain a focus for storms. Will need
to keep an eye out to the southwest for more organized circulation
to develop near SAT and shift towards the region. Could have
significant impacts Monday morning.
45

CLIMATE...
Near to record high minimum temperatures are possible at all of
the first order climate sites this morning. A listing of the
current records is provided below.

May 28
LOCATION           RECORD HIGH MINIMUM     YEAR
City of Houston           81               1996
Houston Hobby             79               1996
College Station           77               1996
Galveston                 81               1996

It was very warm yesterday with all 4 official sites either tying or
surpassing the record high minimum temperatures.

CLL 77 Tied
IAH 80 Tied
HOU 79 Record (previous 78-1996)
GLS 81 Record (previous 80-2000)

Huffman/45

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      90  70  81  69  85 /  50  80  50  30  50
Houston (IAH)              91  74  81  72  85 /  30  60  80  50  60
Galveston (GLS)            85  77  80  75  83 /  20  60  80  60  50

&&

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

$$

Discussion...14



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