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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 202029

Area Forecast Discussion
National Weather Service Houston/Galveston TX
329 PM CDT Tue Aug 20 2019


Weather conditions today are a fairly good representative of what
we can expect for the next few days, until an upper disturbance
accompanied by a strong push of tropical moisture will help to
enhance rain chances even deeper into Southeast Texas this
weekend. Fortunately for our area, it still appears that the 
heaviest rainfall with this disturbance will fall well offshore 
over the Gulf and into Louisiana, with only more moderate amounts
for this area.

.NEAR TERM [Through Tonight]...

Numerous showers and storms have erupted today, and the strongest
of them have managed to produce some torrential rainfall, dropping
a little over an inch of rain in 15 to 30 minutes. Despite this,
concerns with heavy rainfall have mostly been mitigated by short 
storm lifetimes and enough motion to keep too much rain from 
piling up. As such, the worst case scenario for the rest of the
day appears to be for street/minor flooding in areas that are
heavily urbanized and/or have poor drainage.

Temperatures are being pretty effectively cooled where rain has
fallen, and locations are seeing the thermometer drop even into
the upper 70s in spots. Drier spots, though, are still getting
quite hot, with areas in the north nearly reaching 100 degrees
again today.

.SHORT TERM [Wednesday Through Thursday Night]...

Today makes for a pretty good template for the rest of the short
term period, as ridging aloft continues to drift slowly towards
the Four Corners, leaving a subtle weakness overhead. Meanwhile,
onshore flow continues, and will keep precipitable water values 
around or a shade less than two inches.

The best chances for rain will continue to be nearer the coast,
thanks to the presence initially of a weak seabreeze boundary, and
then sustained by collisions of outflow from early storms. Showers
and storms will be harder to come by farther inland, and
expectations for rainfall should be tempered somewhat in this

I don't change too much with the temperatures, perhaps a small
slide down towards more typical late summer values, particularly
in the rainier coastal areas. I do keep upper 90s in place well
inland, as the lack of convective activity will also allow
afternoon temps to rise more unchecked.

.LONG TERM [Friday Through Tuesday]...

By Friday, a midlevel vorticity max will be scraping its way up
the western coast of the Gulf through the weakness in the ridge.
It may also be drawn up some by a trough passing by the area in
the northern stream, though that is a somewhat fragile setup given
the northerly position of said upper trough. Regardless, this vort
max will be accompanied by a surge of tropical moisture, which
will help enhance the potential for showers and storms.

There are, of course, two immediate concerns that spring to mind
when we say "disturbance from the Gulf" and "surge of tropical
moisture". One is tropical cyclone development - as in the past
few days, there is very strong consensus in the guidance that this
is not really in the cards here. This feature looks to be driven
almost entirely at the midlevels, with only a weak wave/trough at
worst down at the surface. That's not to say development is
impossible, but confidence is relatively good that it should not
be a serious threat.

Now, on the surge of tropical moisture side, there may be
something to watch a bit more warily - but just a bit. Heavy rain
is being produced on the east side of this upper low in all of the
model guidance. Explicitly, this makes for the heaviest rains to
fall over the Gulf and into coastal Louisiana. This is again
something there is and has been pretty good consensus in the
guidance for this scenario. Now, I suppose, if that northern
stream tough mentioned earlier is a little slower and digs deeper
that perhaps the Gulf trough could be drawn up further west and
drag that heavy rain region over our coastal areas. But this is
entirely speculation on my part, so it's pretty unlikely. What I'd
look for this weekend instead is something much like the typical
diurnal pattern, but juiced up by the surge of moisture. This may
help showers and storms become a little more possible over the
locations well inland. And, of course, isolated cells could
produce heavy rainfall, but overall it will be a picture of more
light to moderate rain.

Similarly, I undershoot guidance a little bit on temps for the
weekend - they nudge upwards a bit, while I would expect things to
be a touch cooler with greater cloud cover and showers. But, in
general, these changes should be subtle and not deviate too far
from typical late summer temperatures.


.AVIATION [18Z TAF Issuance]...

SHRA/TSRA development within the southern and southeastern 
portion of the region has amplified over the past few hours. 
Intermittent showers and storms can be expected at coastal 
terminals until approximately 20Z. Convection will push further 
north as the afternoon progresses, reaching the metro terminals by
18Z and northern terminals by around 21Z. Beyond any temporary 
visibility reductions associated with these storms, conditions 
are expected to remain within VFR thresholds through the duration
of the TAF period. Prevailing winds should remain out of the S/SE
at 5 to 10 knots through the remainder of the day, although 
convective activity could produce some stronger, erratic gusts for
brief periods. Redevelopment of SHRA/TSRA activity is expected 
tomorrow morning over the coastal waters, but currently including 
VCSH language in TAFs with lower confidence in the timing and 
extent of this development.




Light onshore flow persists tonight, continuing through the 
remainder of the week as high pressure builds into the Gulf of 
Mexico. Winds should remain generally out of the south to 
southeast around 10-12 knots during the day, tapering off to 5-8 
knots during the overnight hours. The next several days could see 
the development of isolated to scattered showers and thunderstorms
over the coastal waters during the morning and early afternoon 
hours. This will produce some gusty and erratic winds, although 
these periods will be brief.




College Station (CLL)  76  98  76  97  76 /   0  20   0  20  10 
Houston (IAH)          78  96  78  94  78 /   0  30   0  30  20 
Galveston (GLS)        84  90  84  89  82 /  10  40  20  50  60