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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 181515

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1015 AM CDT Tue Jun 18 2019

Made a few updates to the near term. 12z soundings from CRP/LCH
support slightly warmer daytime high temperatures so bumped MaxT
up a degree or so across the board. Tweaked sky grids to match 
obs. Fcst soundings also suggest a bit more cloud cover this 
afternoon but there is quite a bit of dry air above 850 so not 
confident that clouds will fill in as suggested by the fcst 
soundings. That said, bumped up cloud cover for this afternoon. 43


.PREV DISCUSSION... /ISSUED 634 AM CDT Tue Jun 18 2019/ 

AVIATION [12Z TAF Issuance]...

Overall expecting VFR conditions will light winds due to the lack
of a pressure gradient. Pressure gradient tightens up tomorrow so
expect S/SW winds. There may be some deeper moisture to develop
tonight into tomorrow morning which would support some stratus
development west of KIAH and south of KCLL but confidence is low
in MVFR ceilings developing. The trends in the guidance will need
to be watched for more confidence.


PREV DISCUSSION /Issued 457 AM CDT Tue Jun 18 2019/...


As the convective weather of recent times calms down, our focus
will turn towards potential for late week heat. Temperatures will
be decidedly above normal, and a few isolated spots mainly towards
Caldwell and B/CS may make a run at triple digits. This will be
made possible by modestly drier air. So, while temps are getting
very hot, heat headlines are not necessarily a slam dunk at this 
range. We'll likely have to evaluate the potential for the heat 
index to exceed 108 each day from Wednesday on into the weekend. A
bit of a break from the heat should come later in the weekend as 
clouds and rain potential will creep back into the picture.

NEAR TERM [Through Today]...

Looking at the latest observations and then trying to do a surface 
analysis of that data proved challenging since there is practically 
no pressure gradient to analyze. Winds have decoupled inland 
allowing for temperatures in the low/mid 70s along with dewpoints in 
the low/mid 70s.

Aloft there is a short wave trough over the Texas Panhandle that 
should continue to move east. There is a weak upper level ridge over 
Mexico up into the Rio Grande that should also build east as upper 
level flow increases.

Expectation for today will be minimal impacts with a small 10-20 
percent chance at a few showers or isolated storm. This should be 
mainly for the early afternoon northeast of Houston. Shortwave 
trough axis will be moving by the area but expect there will be 
little if any lift for storms to form.


SHORT TERM [Tonight Through Wednesday]...

Jet stream flow does increase over north Texas which puts our 
forecast area on the subsident side of the jet. Boundary layer flow 
also veers to the SW which rarely produces a favorable environment 
for convection as capping should be stronger from the elevated mixed 
layer. We will need to watch for any lines of storms that form in 
north Texas that could push into the area. HREF/WRF model runs hint 
at this possibility. Otherwise the only other weather concern will 
be the building heat. High temperatures on Wednesday look to reach 
the mid/upper 90s for much of the area and heat index values could 
be in excess of 105F as well.


LONG TERM [Wednesday Night Through Tuesday]...

Heat is the name of the game for the second half of the work week.
This may be somewhat surprising if one were to look only at the
midlevel height pattern, as it does not strongly suggest big heat.
If anything, it looks fairly typical for early summer, with the
subtropical ridge in place but not overly strong. I suspect that
we have to look lower for our answer. In glancing at the 850
fields, I see flow turning southwesterly with RH below 35 percent
on Wednesday, peaking Thursday, then backing off into the weekend.
This origin and dryness of the air suggests to me we're getting
solid downsloping off Mexican terrain. The drier air should heat
up better, and the compressional heating will only boost things.

This feels fairly dependent on getting enough shift in the winds
to really turn on the torch, and if this doesn't happen, I don't
expect we'll get so hot. Still, it seems like every ensemble in
the world (literally) has been showing this, and I'm getting a bit
tired of fighting it as we draw closer. It probably also speaks
that I usually find myself adding degrees to consensus high temps
even when weak ridging is in control, but this morning's NBM
explicitly gives me a Thursday high of 100 at Caldwell. So...I 
will channel my inner Dr. Strangelove today and learn to love the 
blend. Though I just noted I like to add to blended highs in 
similar setups as it often washes the heat out a bit, I'm going to
stand pat with the higher of consensus and our previous forecast 
to keep continuity. If we really REALLY need to turn the torch on,
we've got a few days to nudge our way up there. As it is, the 
current forecast highs are generally at or above the EPS and GEFS 
means already, so I don't think we'll have to worry much about
falling way short on these highs.

Eventually flow should back some more, ending the connection with
hot, dry downsloping air. A northern stream trough over the
Canadian Rockies may pump up ridging over the Gulf, so rising
midlevel heights should keep temperatures from falling too
terribly much. However, by the back half of the weekend or early
next week, renewed moisture flow from the Gulf...and perhaps even
a connection with moisture outflow from a tropical wave (maybe a
weak tropical cyclone?) in the East Pacific should help bring 
potential for showers and storms back to the area. The GFS looks a
bit stronger with the ridging, so any Pacific moisture connection
happens more to our west. This leaves us a bit drier. The Euro, 
on the other hand, looks to pump more moisture up into our part of
Texas, and is rainier. For now, I'm leaning towards the Euro's 
solution, but am not selling out to likely PoPs (or higher) just 
yet. We'll keep things in the chance range and see how this plays 
out. For what it's worth, NHC has nothing in their 5 day outlook
and TAFB shows this as an open wave, and I'm going to trust their
forecasting for the East Pacific way before my own. As with the 
aforementioned temperature forecast, if it turns out we really 
need to crank things up, we've got plenty of time to ease our way 
to it. Chance PoPs it is.


With high pressure centered just to our east, look for light 
winds today into tomorrow. While primarily onshore, the light
pressure gradient will likely introduce some variability in
direction for today. Winds will strengthen Wednesday night both as
the surface high moves farther east and a vort max traveling
across Oklahoma spurs a surface reflection. Those winds should stay
relatively strong into the weekend. Periods of SCEC and small 
craft advisory conditions can be expected during this timeframe, 
and significant sea heights look to increase about 2 to 4 feet 
over the next several days. Showers and thunderstorms won't
completely disappear over the next few days, but they should be
restricted to being isolated and primarily during the nocturnal
period. Rain chances look stronger very early next week, but how
rainy things may get appear to be strongly dependent on how
effectively a moisture channel can be established with the Gulf,
and potentially even a portion of the Eastern Pacific.


College Station (CLL)      93  76  96  78  99 /  10  10  10  10   0 
Houston (IAH)              94  79  95  79  96 /  10  10  10   0  10 
Galveston (GLS)            90  82  89  83  90 /  10  10  10   0   0