Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 181139

Area Forecast Discussion
National Weather Service Houston/Galveston TX
639 AM CDT Tue Jul 18 2017

SHRA/TSRA are developing in and around Galveston Bay and also near
the coast, so have included mainly VCSH to start out the 12Z TAFs
in that area. Expect activity to develop further inland during the
day today, so have included VCTS at some sites near the coast this
morning and inland TAF sites this afternoon. Light winds this
morning become SE and S this afternoon, all expected to increase
to around 5 to 10 knots. VFR outside of the rain areas and
MVFR/IFR near the storms.  42

.PREV DISCUSSION... /ISSUED 426 AM CDT Tue Jul 18 2017/

Weather will be very typical of mid to late July for the next
several days, with high temperatures near average in the 90s and
lows in the lower to middle 70s. Heat indices will be easily above
100 and many locations look to see max values above 105 if the
current forecast holds deep into the week, so trends there will
need to be watched in the coming days. Scattered showers and
thunderstorms can be expected each day before dissipating in the
evening as the sun goes down. Though convection will be possible
area-wide, especially today, the best potential will be along and
near the incoming seabreeze each afternoon. Seemingly as always,
localized heavy rainfall and potential flooding issues may crop up
with the strongest shower/storm cells for at least the next couple
of days.

NEAR TERM [Through Today]...

Water vapor imagery shows an amplified, but narrow upper trough
focused in far southern Quebec, but with impacts snaking down the
Appalachians, then southwesterly into Texas. Though the Euro puts
the trough axis just south of us, theoretically giving all of
Southeast Texas potential for subsidence, WV imagery really shows
substantial impact ending around the northern edge of our area of
responsibility. Indeed, using the "multi-layer" WV imagery from
GOES-16, this pattern largely seems to hold at all three levels
G-16 samples. For this reason, I`ve tried to keep a relatively
tight PoP gradient around this boundary, with higher precip
potential south of the line than north of it. In other words, to
carry my mid-shift predecessor`s words forward, your odds of
hitting the lotto jackpot will be higher farther south towards the
seabreeze. Storm motion doesn`t bad as it has
recently, and model soundings do show a bit of a dry spot around
500 mb, which may help mitigate heavy rain potential. Despite
that, we`re still looking at relatively slow movers and heaps of
CAPE with a generally saturated profile, so I would still expect
that if you get the wrong cell over the wrong spot, heavy hourly
rain rates could cause some very localized flooding issues. Additionally,
I`ve allowed temperatures farther north where subsidence could be
expected to have a greater impact to drift into the middle 90s,
while the rest of the area stays in the lower 90s.

SHORT TERM [Tonight Through Thursday]...

As you`d probably expect this time of year, look for any rain to
wind itself down through the evening as the sun goes down, with
shower and storm potential shifting offshore overnight with the
slower to cool waters holding onto some of the heat of the day.

Tomorrow looks pretty similar to today, but would focus even more
on the seabreeze for the best rain chances, as high pressure
aloft builds stronger over Oklahoma, and heights corresponding
rise over our area. This should be the start of rain chances
gradually clamping down towards the late week. This also means we
should probably look for modestly warmer temperatures, and heat
indices look to push into the 103-108 range.

LONG TERM [Thursday Night Through Tuesday]...

Through the rest of the workweek, we should look for that ridge
aloft to peak in strength, but then begin to moderate some. 850
flow also begins to modestly veer a little towards southwesterly.
In general, you`d think things would start to dry out in the
scenario, and to some extent, the forecast indicates this.
However, the GFS and the Euro both to some extent cut off a chunk
of vorticity from the upper trough mentioned up top and drag it
back over Texas along the edge of this ridge (more the GFS than
Euro). This will likely keep midlevel heights from rising quite
as high, and dampen subsidence aloft. Because of this, I`m loath
to go totally dry, but continuing to focus the best chances right
on the seabreeze boundary as I gradually taper PoPs down.

Another thing I`ve got my eye on going into the weekend will be
yet another pinched off vort max from this trough - this time off
the Georgia coast, will move back into the northern Gulf. The GFS
is similarly much more enamored of this blob of vorticity than
the Euro, but both show the feature in some way or another. Key
thing to remember: neither model does a whole lot with it other
than drag it across the northeast Gulf coast, and enhance shower
potential in that area. But, I`m also not going to completely
trust midlevel vorticity hanging out over the Gulf in July with
only some modest shear, so I`ll watch things so you don`t have to.
Likely there will be little worth mentioning anyway.


Light to moderate south to southeast winds will persist
for the remainder of the week (higher winds and seas are
possible in/around shower and thunderstorm activity).
Mainly moderate south winds and slightly higher seas
(both expected to remain under caution levels) are
anticipated over the weekend in response to a tightening
pressure gradient. 42


Don`t want to waste a ton of space here, but did want to address
today`s new Tropical Storm Don. It`s formed just east of the
Windward Islands, and is expected to reach the Windward Islands by
tonight. It is also very low in latitude, and I could understand
simply from its status as a named storm and location that it could
cause some consternation for us in Southeast Texas.

But, the eastern Caribbean is typically a graveyard for tropical
cyclones in July, and it appears to be the case here for Don. The
official NHC forecast dissipates it in the Caribbean by Friday,
which looks reasonable. Some guidance does actually show Don
becoming a hurricane in the short term while conditions are still
relatively good - possibly a function of the ability for small
cyclones to undergo fairly wild swings in intensity. But even
these aggressive models largely tear the storm apart in the
Caribbean. The Euro and GFS do bring Don`s remnant vorticity up
across the Yucatan after some time, so I suppose potential impacts
for us aren`t zero, but it`s small enough to be functionally
indistinguishable from zero at this time.

To make a long story short (too late!), Don poses no notable
threat to Southeast Texas at this time.


College Station (CLL)      95  75  96  76  98 /  30  20  20  10  20
Houston (IAH)              92  76  94  77  94 /  50  20  40  10  30
Galveston (GLS)            88  81  89  82  89 /  40  20  40  20  30




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