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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 201548
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1048 AM CDT Sat Jul 20 2019

.UPDATE...
Early day showers around Galveston Bay dissipated by 10 AM. Greater
than 1.6 inch pwat air resides across the southeastern forecast 
area. Temperatures warming into the upper 80s should allow these 
rows of towering cumulus to grow enough to produce heat of the day
showers. Prior outflow boundaries will also assist in providing 
the needed focus to generate afternoon showers and isolated 
thunderstorms. any precipitation will occur primarily across the 
central and and eastern CWA. Upper ridging will provide another 
day of lower to middle 90 F heat that, when coupled with lower to
middle 70 F dew points, will produce near 105 F maximum heat 
indices. 

Afternoon weather conditions around JSC for the Apollo 11 50th 
Anniversary celebrations will be partially cloudy, hot and muggy 
with a near 10 mph southeast breeze. 31

&&

.PREV DISCUSSION... /ISSUED 630 AM CDT Sat Jul 20 2019/ 

AVIATION [12Z TAF Issuance]...

Sporadic MVFR conditions have been observed so far at CLL, UTS,
and CXO - this is expected to continue through sunrise and briefly
into the morning before joining the rest of the area in VFR.
Slightly better chances of isolated showers today, with onshore
winds increasing to around 10 knots. For now only have a short
VCSH mention at IAH - rain chances look better, but not THAT much
better. We may see more widespread MVFR towards dawn tomorrow, but
mostly stick with persistence for now. Keep an eye on that in
coming forecast issuances.

PREV DISCUSSION /Issued 406 AM CDT Sat Jul 20 2019/...

DISCUSSION...

- Moisture should increase today and Sunday with an inverted
  trough moving into the area this weekend. This will cause an
  increase in shower and thunderstorm activity especially for
  Sunday.
- Upper level ridge should build over the Rockies Monday and
  Tuesday with a trough over the Great Lakes. This allows for a
  front to move into the area Tuesday with higher chances of
  showers and thunderstorms.
- Drier air should move into the the area behind the front
  Wednesday into Thursday which should decrease rain chances.

SHORT TERM [Today and Tonight]...

Ridging continues to dominate the region, though we are a bit 
more towards the fringe of that ridging now. This should provide 
at least a slightly better opportunity for isolated showers and 
possibly a thunderstorm to crop up, mostly off the convergence 
provided by the seabreeze boundary. GOES-East still shows 
precipitable water values in the 1.3-1.5 inch range, but deeper 
moisture lurks offshore, and may move in enough to provide a more 
plentiful source of moisture for diurnal convection. Of course, 
with the diurnal pattern continuing to command weather trends, 
potential should fall off pretty rapidly after sunset.

With the persistent onshore flow, dewpoints will remain elevated,
as will the temperature floor in anomalously high overnight lows.
Like the last couple days, suspect we'll slide safely below 
record values, but expect that both this morning and overnight 
tonight that we'll be closer to record warm minimum values than 
normal minimum values. On the flip side, temperatures this 
afternoon will be hot, but high temps do not look as anomalously 
warm as the low temps do. Look for pretty stable temperature 
trends, with highs around 90 right at the coast, rising into the 
mid 90s deeper inland. Similarly, expect maximum heat indices to 
be around or above 100 degrees, but we're not looking for any 
widespread 105+ readings today. The plus to this is that 
conditions will be no more severe than they have been in recent 
days. The drawback is that heat stress is cumulative, and with 
such warm nights, you can find yourself in trouble more rapidly 
without time to recover in air-conditioned spaces.

Luchs

LONG TERM [Sunday Through Saturday]...
Inverted trough along the northern Gulf should push into the area
on Sunday increasing thunderstorm chances. There will also be a 
surge of higher moisture to further support the need for higher 
thunderstorm chances. the 00Z Tx Tech WRF has more scattered 
activity than the other WRF model runs, but all point to scattered
activity from Houston eastward into east Texas. It will be quite 
possible for a few of these storms to drop some locally heavy 
rainfall, gusty winds and lightning.

A trough over the Pacific NW today should move across the 
northern Plains and into the Midwest/Great Lakes by late Monday. 
Upper level ridge should develop over the Rockies in response to 
this developing trough. Northerly flow aloft will support a front 
pushing into the area Tuesday into Wednesday. By in large, model 
data has become fairly consistent with this pattern developing so 
there is overall growing confidence in a front pushing off the 
coast Tuesday night.

We did bump up thunderstorm chances along the front to 60-70 
percent mainly on Tuesday when the front moves through the area 
slowly. We will need to watch for locally heavy rainfall as 
precipitable water values climb to 2 to 2.2 inches ahead of the 
front but storms should be moving fast enough with no training 
that the flood potential looks rather low. Also antecedent 
conditions have been dry for the most part so the general 1 inch 
rainfall amounts should not cause any problems. Even isolated 
higher amounts should not cause any flooding issues.

Ridge/trough pattern over the US should continue through the week
so with the weakness in the ridge, it looks like we could have a 
wet pattern set up. However it looks like from Wednesday through 
at least Friday much drier air will be in place. In fact the 00Z 
NAEFS show precipitable water values under 1 inch for 12Z Thursday
and this correlates to 0.5 to 1 percent of climo or close to a 
record minimum for PWAT for this time of year. The front also 
pushes a little further into the Gulf than previous runs which 
also indicates that drier/cooler air will be in place behind the 
front. Not only that, but the 00Z NAEFS has 850mb temperatures at 
or near climo minimums for this time of year. All that to say, the
airmass behind the front is anomalously cooler and drier than 
climo norms.

It looks like now we might not see a return flow event until 
Friday into Saturday when precipitable water values climb back 
above 1.5 inches. So we will include some increased rain chances 
for the end of the week but confidence still pretty low now that 
there will be more dry air behind the front.

Overpeck

MARINE...

Typical summertime conditions continue on the waters through the 
weekend and into early next week, with generally light to moderate
onshore flow. Similarly, seas are expected to be 3 feet or less. 
The next significant change will be likely to come Tuesday into 
Wednesday as a cold front looks like it will manage to bring a 
increased showers and storms, and also briefly shift winds to 
northeasterly. Onshore flow is expected to return before the week 
ends.

At the coast, the persistent light to moderate onshore flow 
should support moderate rip current strength as reported by beach 
patrols, and boost tides above astronomical norms. Despite coming 
in higher, winds are unlikely to be strong enough for water levels
to reach critical thresholds along the coast.

Luchs

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      77  95  76  96  75 /  10  10   0  20  40 
Houston (IAH)              78  91  77  94  77 /  10  50  10  30  50 
Galveston (GLS)            82  90  81  88  80 /  30  40  30  30  50 

&&

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

$$

Discussion...31