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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 211115

Area Forecast Discussion
National Weather Service Houston/Galveston TX
615 AM CDT Wed Aug 21 2019

.AVIATION [12Z TAF Issuance]...

Nighttime Microphysics imagery not showing a lot in the way of 
low clouds over the area this morning with a few showers 
beginnning to develop just off the coast. The expectation will be 
for a few showers and storms to form inland mainly affecting 
KLBX/KSGR and KHOU so will keep VCTS in TAFs for those terminals. 
Less certain there will be convection affecting KIAH so decided to
go with a VCSH for now with KCXO/KUTS/KCLL staying dry. VFR 
conditions are expected again overnight into Thursday AM. There 
looks like an increase in convection for Thursday as moisture 
increases from the Gulf.



.PREV DISCUSSION /Issued 350 AM CDT Wed Aug 21 2019/...

.SHORT TERM [Today through Thursday Morning]... 
Water vapor satellite imagery is picking up on a vorticity max along 
a shearing out trough over the Louisiana between 2 ridges. The 
stronger ridge has become elongated over the southern Plains so 
there will be at least some easterly flow aloft over SE Texas. There 
is still some higher moisture with precipitable water values 1.8 to 
2.0 inches over the area. This might be enough moisture to support 
an isolated storm or two with day time heating. It will take some 
convergence along the sea breeze and heating to get storms to 
develop as the atmosphere is not quite as moist or unstable as 
yesterday. Upper level ridge looks to be stronger as well with 595dm 
to 593dm 500mb heights over the area.

Latest HRRR/HREF/WRF model runs seem on track with this kind of 
evolution with showers and storms forming in the afternoon mainly 
south of I-10 and east of I-69. Like the last couple of days the 
main threats from any storms will be locally heavy rainfall causing 
street flooding, lightning and gusty winds. Stronger storms could 
produce a quick 1-2" of rain with 0.25" to 0.5" more common. With 
there being less coverage in shower and thunderstorm activity 
expected, high temperatures should be in the mid/upper 90s most 
areas. Lone exception may be along the coast with low 90s but hedged 
towards the mid 90s since storm coverage should be less and 
therefore less cloud cover.


.LONG TERM [Thursday Morning Through Tuesday]...
The upper level ridge, centered NW of SE TX begins to weaken and
retreat more to the west Thursday. As a result, more widespread 
shower and thunderstorm activity is anticipated Thursday, with 
precipitable water values (PWs) remaining just shy of 2.0 inches 
across much of the region, with the better moisture values 
concentrated along the coast. The pressure gradient also begins to
tighten slightly in response the the upper level trough that 
moves closer to the upper TX coast. Therefore, onshore flow will 
continue to advect in better moisture values through the day 
Thursday and into Friday.

Friday looks to be the best day for coverage of showers and
thunderstorms. The upper level trough should shift northward into
the NW Gulf of Mexico before pivoting east on Saturday. As this 
area of disturbed weather pushes north, very moist PWs translate 
with it, rising up to 2.2 to 2.4 inches by Friday afternoon. 
Forecast soundings keep the atmosphere uncapped, with the 
potential for some gusty winds beneath any stronger storms that 
do develop. The best rainfall accumulation totals for the 
Thursday through Saturday time frame still remains well to our 
east over LA and across the Gulf waters. At this time still 
thinking a possible one to two inches will be possible over the 
coastal counties and essentially along and south of I-10. That 
said, the main change made to the forecast in this period was to 
nudge PoPs inland down a bit, given the agreement amongst the 
global models in terms of coverage. Otherwise, high temperatures 
through the rest of the week will still reach into the low to 
upper 90s, with cloud cover over the coast maybe keeping temps a 
touch cooler.

The NAM12 is drying things out a touch faster than the GFS on
Saturday, shifting the upper level trough east quicker than the
ECMWF and GFS in the latest model runs. Regardless, we should
still have enough moisture in place for the typical summertime
like showers and thunderstorms to develop, focusing along the
sea/bay breezes. By Monday, the upper level ridge attempts to
build back in from the west, helping to suppress widespread
convection for the start of the work week.

Light to moderate onshore flow continues across the bays and Gulf 
waters this morning and should prevail most of the day. Overall 
expectation is for a typical summer time pattern with onshore winds 
increasing at night and decreasing during the day. Seas should 
remain low through the next couple of days. We are going to have to 
monitor a tropical disturbance now located over the NW Caribbean Sea 
that should move into the Gulf for the end of the week and weekend. 
The system at this point may cause a brief increase in winds and 
seas Friday into Saturday but conditions may not even increase to 
needing small craft exercise caution. We are not expecting any 
tropical development.




College Station (CLL)  99  76  98  76  97 /  10  10  10   0  30 
Houston (IAH)          96  79  94  78  93 /  30  20  30  10  50 
Galveston (GLS)        93  83  93  84  92 /  20  20  50  50  50