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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 240418
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1118 PM CDT Sun Jun 23 2019

.DISCUSSION...

Showers developed over the western counties this morning, 
increasing in coverage and intensity throughout the day. These 
showers moved north to northeastward mainly across the central 
and northern counties of the CWA. Isolated thunderstorms moved 
across Grimes, Montgomery, Walker, San Jacinto, Trinity, Houston 
counties this afternoon producing periods of heavy rainfall, gusty
winds and frequent lightning. The shower and thunderstorms 
activity dissipated after 6 PM CDT. High temperatures in the low 
to mid 90s with heat indices of 105-112 degrees F were observed 
today. 

A line of strong showers and thunderstorms currently affecting 
the central Texas is expected to move into the northern portions 
of the CWA later tonight into Monday morning as an upper level 
low and associated trough moves eastward across the Northern and 
Central Plains. After around 1 AM CDT, the upper level divergence 
increases across the local area, particularly across the northern 
half of the CWA, which will favor the development of storms over 
this region overnight into Monday morning. The upper air sounding 
data indicate CAPE values of around 2500-3500 J/kg tonight into 
monday morning, along with K values over 30. Surface to 6 km shear
values of 24-28 knots and surface to 3 km shear values of 18-21 
knots are also seen in the soundings. PW values range from 1.5 to 
2.5 inches during this period as well. HiRes Precip data slowed 
down a bit, but are in agreement in bringing the line of showers 
and thunderstorms over the northern portions of the CWA after 
midnight, moving southward across the central portions early 
Monday morning and into the southern portions during the rest of 
the morning hours. The strongest thunderstorms are expected to 
occur mainly over the northern half of the CWA. Strong storms will
be capable of producing winds in excess 35 MPH, pea size hail, 
frequent lightning and localized areas of torrential rainfall. A 
Severe Thunderstorm Watch was issued for Brazos, Burleson, Grimes,
Houston, Madison, Trinity, Walker counties through 4 AM CDT 
Monday. Although conditions gradually improve by late Monday 
morning, environmental conditions is expected to remain somewhat 
unstable, and periods of isolated to scattered showers and 
thunderstorms can be expected throughout the day. 

Models show a weak upper level disturbance moving across the
region Tuesday, which is expected to create favorable conditions 
for another round of showers and thunderstorms across the local 
area. Drier conditions can be expected Wednesday as an upper level
ridge moves across the region. 24

&&

.PREV DISCUSSION... /ISSUED 706 PM CDT Sun Jun 23 2019/ 

AVIATION...
A cluster of storms over North Texas will race south overnight 
and will cross area TAF sites between 06-12z. The storms will
likely produce gusty winds as they move across the region. Low
level convergence will remain strong as the outflow boundary moves
south and favorable jet dynamics will also aid in lift. Have
maintained mention of wind gusts with the storms. A mix of
VFR/MVFR ceilings this evening will transition to MVFR by 06z.
Subsidence in the wake of the overnight storms should allow VFR
conditions to develop between 15-18z Monday. 43

PREV DISCUSSION... /ISSUED 341 PM CDT Sun Jun 23 2019/ 

DISCUSSION...

SHORT TERM [Rest of Afternoon through Monday]... 
Scattered showers and thunderstorms this afternoon will tend to 
die off this evening with loss of daytime heating. Storms have 
been more clustered and intense ahead of a short wave trough 
lifting northeast across the area. At this time this slightly more
intense deeper convection...with some lightning...now over 
northern parts of the forecast area and this trend should continue
as the short wave lifts northeast. 

Tonight all convective allowing models show a band of
thunderstorms moving from north to south across the area. NMM and
ARW WRF runs most aggressive in holding the line together...all 
the way to the coast. HRRR and especially TTU WRF tend to break up
the line as it progresses through the area. All 4 runs suggest 
squall line would be entering forecast area...up around 
Caldwell... Bryan and Madisonville around 06Z or 1 am...give or 
take...then reaching the coast toward daybreak. 12km NAM a little 
slower keeping storms in the area into daylight hours Monday 
morning. Some of these could be strong to severe...especially 
northern zones...with damaging winds the main threat. SPC has 
northern third of forecast area in slight risk...remainder just 
one level down in Marginal.

Monday...will likely see a break once the line of storms push
through with just some isolated showers and thunderstorms 
possible after line pushes off the coast. Also not expected to be
as hot as recent days. This afternoon we saw several sites exceed
heat index levels...and issued a heat advisory for southern half 
of forecast area. Don't expect that to be an issue tomorrow.
Reilly

LONG TERM [Monday Night through Sunday]...
We may have to keep an eye on things to the NW once again for Mon 
night as models are hinting at the possibility of another MCS 
developing over North Central TX overnight and moving it SSE 
toward our CWA through early Tue morning. However, there is not a 
lot of confidence with this as there still a lot of questions 
concerning how worked over the airmass will be (from Mon's 
activity). At any rate, there will be plenty of heating/some 
lingering moisture for perhaps widely scattered activity by Tue 
afternoon. 

With a surface high pressure/upper ridge building into the region 
from the east by Tue, rain chances should be decreasing Weds/Thur. 
Any activity will likely be during the afternoon and isolated. By 
the end of the week, long-range guidance seems to be keeping with 
the idea of a slightly increased POPs as the Gulf opens up (via a 
weakness region between the main upper ridge). 41

MARINE...
Elevated winds/seas will continue over the coastal waters tonight 
with slightly lower wind speeds across the bays. Models are still 
indicating the approach/passage of strong storms into SE TX 
overnight then reaching the Upper TX coastal waters by early Mon
morning. Winds/seas will be higher in and near these 
thunderstorms. 

With high pressure building into the region from the east by Tue, 
we'll begin to see increasingly decreased winds/seas at that time. 
This weak surface high is expected to linger over the NW Gulf for
much of the week with generally light/moderate onshore winds 
prevailing over the marine waters. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      75  91  74  91  73 /  60  50  40  50  20 
Houston (IAH)              78  92  77  91  76 /  60  60  30  40  20 
Galveston (GLS)            81  87  80  89  80 /  50  60  20  30  20 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...High Rip Current Risk until 10 AM CDT Monday for the following 
     zones: Brazoria Islands...Galveston Island and Bolivar 
     Peninsula...Matagorda Islands.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 10 AM CDT Monday for 
     the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until 10 AM CDT Monday for the following 
     zones: Coastal waters from Freeport to Matagorda Ship 
     Channel TX out 20 NM...Coastal waters from High Island to 
     Freeport TX out 20 NM...Waters from Freeport to Matagorda 
     Ship Channel TX from 20 to 60 NM...Waters from High Island 
     to Freeport TX from 20 to 60 NM.

&&

$$

Discussion...24
Aviation/Marine...43