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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 221728

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1228 PM CDT Mon Jul 22 2019

12z sounding from LCH shows a convective temperature of 84 degrees
and with cirrus thinning, feel temperatures will warm. Convection
is expected to increase over the region with further heating. 
After collaboration with CWSU, have pulled the TEMPO for KIAH but
will watch trends carefully as the HRRR and other short tern 
guidance are very bullish with PoPs. Should be a gap this evening 
but a weak pre-frontal trough will slide through the area and 
winds will shift to the N-NW. A cold front will approach the area 
between 03-06 and slowly cross the area. Have added VCSH/VCTS for 
northern TAF sites between 03-06z and between 09-12z over the 
southern TAF sites. 43 


.PREV DISCUSSION... /ISSUED 951 AM CDT Mon Jul 22 2019/ 

Widely scattered showers and thunderstorms blossoming across the
northwestern Gulf in relation to the height weakness between the
Four Corners High and the Bermuda High. Some of this activity has
made it inland around the Galveston Bay region and dropped between
a half an inch to a inch and a half over parts of Liberty, Chambers
and Galveston County. Higher PoPs will reside across the 
southeastern CWA through the afternoon. It will not take much to 
regenerate periods of rain and isolated storms within an upper 80
F convective temperature / greater than 2 inch pwat air mass. A 
somewhat rare July cold front is on its way; the boundary is 
currently crossing the central Red River Valley. This front is 
timed to travel across the region through early tomorrow. 
Increasing drier (progged) profiles have the chance of Tuesday 
precipitation lowering with each subsequent shift. There should be
enough lower level moisture along and south of the I-10 corridor,
with the lift provided by the front, to squeeze out some daytime
southern forecast area scattered showers and storms. 31

PREV DISCUSSION /Issued 417 AM CDT Mon Jul 22 2019/...


- Best chances of showers and thunderstorms look to be today into tonight
  with models trending drier on Tuesday as the front pushes into 
  the area.
- Frontal boundary on track to push into the area early Tuesday
  with drier air behind it.
- Low temperatures Wed/Thur should be below normal but likely stay
  above records.
- Rain chances increase again for the weekend.

SHORT TERM [Today and Tonight]...

Satellite shows a veil of mainly high clouds, with surface obs
revealing lesser amounts of low clouds underneath that veil 
across Southeast Texas. GOES-East also shows an influx of deep 
moisture from the Gulf onto the Upper Texas coast with
precipitable water values ranging from 1.8 inches well inland to
2.3 inches right on the coastline.

This increase in moisture will help support higher rain chances
today for a large chunk of the area. Indeed, nocturnal showers 
are already beginning to pop up over the Gulf, a bit earlier than
24 hours ago. With the incoming cold front still a good ways off 
yet, the best potential will again focus on and near the seabreeze
boundary this afternoon while instability is maximized thanks to
daytime heating. Forecast soundings also reveal a pretty decent
looking inverted-V signature in the boundary layer, so things may
get a bit gusty in the strongest of today's storms. That said,
there's not a lot of wind in the column, and DCAPEs are rather
limited, so while gusty winds are possible, my expectation is that
things should stay beneath severe thresholds. *knocks on wood*

Other than the more enhanced diurnal rainfall pattern, today
should feel a good bit like yesterday. Temperatures are running a
touch below 24 hours ago so morning lows may manage to slip
underneath records, but will still be quite warm. The high floor
sets us up for another warm day, with expectations that highs are
more likely to be a bit cooler than yesterday, given the
expectation of more clouds/rain in the picture. The emphasis in
that last sentence is very much on "a bit".

For tonight, the diurnal will again fall off with the loss of
solar heating towards sunset. However, rain chances will begin to
creep up in the far north as our very heavily advertised cold
front finally starts to work its way into the picture. Of course,
now that we're at the moment of truth, the convective-allowing
guidance is suddenly beginning to get cold front about rain and
storms along the front, while the parameterized models are still
fans of rain. Both approaches have their pros and cons, so I'm not
exactly sure which - if either - story to buy into on here. One
thing that does look to be hinted at in the guidance is a loose
pre-frontal trough. This may shift winds a little early, reducing
surface convergence. Without a very dynamic setup aloft and
relatively poor lapse rates, I wonder if this is enough to cause
the CAMs reticence to fire widespread rainfall. So, in the end, 
I'm going to hedge my bets, and roll with a blend here that takes
down PoPs, but not as aggressively as a CAM-only consensus might 
suggest. Luchs

LONG TERM [Tuesday Through Monday]...

As expected, upper level ridge has formed over the 4 Corners 
Region with a trough digging into the Plains/Midwest. Surface 
analysis at 08Z has frontal boundary through central Oklahoma into
the Texas Panhandle. Overall basic forecast is on track with the 
front reaching the area tonight and pushing through the area 
Tuesday morning. Models had been fairly aggressive with rain 
chances along/ahead of the front but not seeing that trend any 
more. In fact, the trend looks to be for lower rain chances so we 
have had to tap the breaks a bit with our PoP forecast. We still 
think there could be some spots with chances in the 50-60 percent 
range but this is lower than the 70s we had before. The hi-res 
models have come in fairly dry and even the synoptic models show 
lower QPF amounts on Tuesday. The main question now is why and 
what has changed in the models. Looking at various model 
soundings, it looks like the models are now picking up on some 
drier air at 500mb and above in the 09Z to 15Z Tuesday time frame 
that moves into the area quickly as the trough axis passes. This 
drier air looks to be a bit subsident and once it entrains into 
any updraft it will likely dissipate the convection. So the better
chances of showers and storms looks to be more Monday night now 
instead of later on Tuesday. Chances do look higher along the 
coast and out into the Gulf.

Wednesday through Friday looks to be relatively cooler and drier 
as high pressure moves from the Plains towards the Mid-Atlantic. 
We are still on track for dewpoints in the 50s/60s and low 
temperatures in the upper 60s for most of the area. Looking at 
the record low temperatures for Wednesday and Thursday, I think 
we will stay above those levels by a couple of degrees.

It is still looking like moisture return occurs Friday into 
Saturday with precipitation chances increasing accordingly. We 
went up to 50 PoPs for Saturday which seems a bit ambitious. Still
this is in line with GFS/ECMWF PoPs so not a bad place to start 
with adjustments likely through the next few forecast cycles. 


With high pressure south of New Orleans, and weak low pressure
over the Mexican mountains, the general pattern of southerly winds
continues. However, tonight it's a more S/SW pattern, and since
the pressure gradient is not terribly tight, we may even see a bit
more veering with a very small nocturnal landbreeze feature. As
the sun rises through the morning, though, look for things to snap
back to a more sustained southerly wind.

As one might expect, with all this talk of "weak" features and
discussion of nuance, wind speeds will be light to moderate
through tonight. That begins to change tomorrow as an infrequent
summer cold front drops towards the waters tomorrow. Watch for
winds to veer to northwesterly and northerly through the day into
early evening, and continue veering right on through to
northeasterly by Wednesday morning. Wind speeds will also pick up,
and a SCEC may be needed Tuesday night into Wednesday for at least
some of the waters.

With this front lifting back north later in the week, winds will
take on a greater onshore component in the back half of the work
week, reaching southeasterly for the end of the week and next
the weekend.


Hobby Airport in Houston and Scholes Field at Galveston both
managed to tie record warm minimum temperatures yesterday with the
continued warm morning temperatures. Galveston tied its record
from 1995, while Hobby's min temp matched a record from 1953.


College Station (CLL)      96  76  90  68  90 /  10  30  10  10   0 
Houston (IAH)              94  76  90  71  90 /  30  40  40  10   0 
Galveston (GLS)            90  82  88  77  87 /  40  50  50  50  10